BOTANY, VOLUME 1: CELL BIOLOGY, ANATOMY, MORPHOLOGY — 2007

4. MORPHOLOGY AND ANATOMY OF CORMOFHTES (VASCULAR PLANTS)

4.2. The Shoot

As mentioned previously, the SHOOT consists of three main Organs: the stem, the leaf, and the ROOT¹. In extant plants, these organs are not homologous to one another and perform different primary Functions. (This assessment is not invalidated by the concept that leaves evolutionarily arose from dichotomously branched lateral axes of early land plants; see Fig. 11.129.) Typically, cylindrical stems and roots are unifacial—possessing a single uniform surface (from Greek facies, appearance)—radially symmetrical in cross-section, and theoretically capable of indeterminate longitudinal growth due to the presence of specialized apical Cells at the tip. In contrast, Foliar Organs (phyllomes) are generally flattened and bifacial in Structure, meaning their upper and lower sides differ: they may exhibit Different types of pubescence or bear varying numbers of Stomata. Furthermore, phyllomes exhibit determinate growth driven by The activity of two-sided apical cells or a linear marginal meristem. External bifaciality of the leaf typically corresponds to dorsiventrality in its internal Anatomical Structure.

¹ The root is not a component of the shoot. — Editor's note.

The fundamental correspondence in organ Organization across all cormophytes is especially evident in young sporophytes (where the sporophyte is defined as the diploid generation developing from a zygote). This can be illustrated using the embryo found within the seed of a seed plant (Fig. 4.8; cf. Fig. 3.1). A typical embryo consists of an embryonic root (radicle) and an axis bearing one, two, or several embryonic leaves, or cotyledons (from Greek kotyledon, a cup-shaped hollow). It is characterized by bipolarity (the presence of shoot and root poles), which is maintained throughout the plant's subsequent development. The transition zone between the shoot and root is called the root collar (hypocotyl-root junction). Between the root collar and the point of attachment of the cotyledon(s) lies the hypocotyl, while the region of the axis between the attachment points of the cotyledon(s) and the first true leaf is termed the epicotyl. The shoot terminates at the apex in a terminal bud (plumule). Cotyledons, like all foliar organs, are lateral outgrowths of the axis surface that originate exogenously (see Fig. 3.3). Like all subsequent leaves formed on the shoot, the cotyledons curve over and protect the shoot apex. The upper side of the leaf base forms an acute angle with the stem. Within the space between them—the leaf axil—there is at least one axillary bud capable of resuming growth to form a lateral branch. This arrangement of buds is characteristic of all flowering plants, a phenomenon known as axillary branching. The spatial arrangement of branches often reflects the phyllotaxis (leaf arrangement) on the parent shoot. Many pteridophytes branch differently. Even in seed plants, under certain circumstances—such as regeneration following wounding—new shoot and root apices can arise anywhere on any primary organ through the embryonization of Tissues, developing into adventitious shoots or roots.

Fig. 4.8. Typical structure of a dicotyledonous plant (after J. Sachs and W. Troll)

A—mature embryo with cotyledons Co, embryonic root Ra, and hypocotyl Hy; B—seedling with primary root Rw; C—plant in the vegetative stage with lateral and adventitious roots w, foliage leaves, and terminal bud Gk

Seeds are the typical dispersal units of seed plants, in which the embryo represents a temporarily dormant stage of the sporophyte. Upon germinating under favorable environmental conditions, it develops a system of shoots and roots. As the shoot grows, distinct series of leaves are produced, since leaf Morphology changes systematically along the stem. The simplest cotyledons are followed by juvenile leaves, which exhibit transitional structures bridging the gap between embryonic forms and the mature leaves specialized for assimilation and Transpiration. In the inflorescence region, simplified bracts (upper leaves) develop, in the axils of which flowers or lateral inflorescence branches may arise. Within the flower itself, leaf Structure and function undergo dramatic modification, culminating in The formation of stamens and carpels¹. Flower Formation "consumes" the apical meristem of the branch: the shoot terminates in a flower.

¹ Stamens and carpels may not be homologous to leaves. — Editor's note.

During germination, shoots and roots not only elongate but also increase in thickness. This is known as primary thickening. In annual and biennial herbs, longitudinal growth and thickening eventually cease, and after fruiting and seed dispersal, these plants die due to internal physiological factors. In perennial plants (shrubs and trees), by contrast, longitudinal growth continues for many years or even centuries. This is primarily driven by the activity of root and shoot apices, which in large trees may number over 100,000 within the crown alone. Furthermore, woody plants possess an even greater reserve of inactive, so-called "dormant" axillary buds. These buds are activated and begin to grow if the terminal apices cease functioning. The longitudinal growth of axial organs is accompanied (chiefly in perennials) by secondary thickening resulting from the activity of lateral Meristems, or cambia (see 3.1.2). The morphological and Water/140.html">Anatomical structure of a plant formed prior to the onset of cambial activity is termed Primary Structure. As the cambium functions, the Introduction/11.html">Secondary structure of the axial organs develops.

4.2.1. Longitudinal STRUCTURE OF THE Shoot

All shoots (including rhizomes) bear leaves, although these may sometimes be inconspicuous, such as the scale-like lower leaves on rhizomes. In woody plants, leaves are absent on multi-year sections of shoots because they are less durable than stems: older leaves are shed following the formation of a specialized abscission zone (see Fig. 7.61); in deciduous trees, this occurs at the end of each growing season.

The region of the stem where leaves attach is often thickened and is termed the node (from Latin nodus), while the segments of the shoot between nodes are called internodes. The alternation of nodes and internodes reflects the metameric structure of shoots. The repeating structural unit is the phytomer (node with its leaf + internode).

Typically, internode length is measured in centimeters, but within the bud, young leaf primordia (leaf primordia) are densely packed. Internodes elongate later through Cell expansion and frequently via intercalary growth, driven by the time-limited activity of intercalary meristems, which represent typical residual meristems (see Box 3.1).

Internode length often varies significantly along the shoots of the same plant. It is common to find shoots with shortened or greatly elongated internodes (compared to typical long shoots). The former give rise to dwarf (short) shoots, rosette shoots, and bulbs, whereas the latter form scapes (flower stalks) or stolons.

Short shoots are typically lateral branches whose nodes and leaves are closely crowded together. A well-known example is the fascicles (bundles) of needle leaves on two- and multi-year short shoots of larch (Fig. 4.9, B, 4.20). In pines, green needles occur exclusively on short shoots—in fascicles of 2 in Scots pine (Pinus sylvestris) and 5 in Swiss stone pine (Pinus cembra). Functionally, such short shoots are equivalent to leaves and, like them, eventually shed entirely as a unit. Short shoots also occur in many deciduous trees (e.g., beech and various fruit trees). In cherries, such shoots initially bear only leaves, in the axils of which additional short shoots subsequently develop, though these bear flowers instead (inflorescences, or "spur shoots"; Fig. 4.9, C, D). These reproductive spurs die off after fruiting, whereas vegetative short shoots continue to grow for many years (for example, in larch).

Fig. 4.9. Long and short shoots

In European larch (Larix decidua), current-year shoots are long shoots (A), whereas on older branches, densely foliated short shoots arise from axillary buds (B). In cherry, the long shoots display bud scale scars (arrows) that mark the boundaries of annual increments and are spaced far apart (C, 0.9x), whereas on short shoots (D, 2x) these scars are closely spaced (cf. Fig. 4.20).

Extremely shortened internodes are characteristic of many inflorescences (particularly capitula or heads, typical of Asteraceae); morphologically, most flowers represent typical short shoots.

Rosette shoots (see Fig. 4.15, A; 4.16, D) are formed in many rhizomatous perennials, such as species of primrose (Primula), cushion plants (see Fig. 4.21), as well as in annual and biennial herbs. In these plants, germination produces a root system and a rosette of basal leaves flattened against the ground, from which a flowering stem arises (in biennials, typically In the second year), as seen in mullein and foxglove.

Underground shoots (rhizomes) frequently serve a storage function for nutrients and, accordingly, become tuberously thickened (for ex-

ample, in lords-and-ladies, Arum). Often, reserve substances accumulate not in the axial organs, but in chlorophyll-free, thickened ("fleshy") lower foliage leaves (scale leaves). If the internodes fail to elongate in the process, a bulb is formed (morphologically equivalent to a bud), which is characteristic of many Liliaceae representatives, including onion (Fig. 4.10), garlic, as well as hyacinths, daffodils, and amaryllises.

Fig. 4.10. Bulb of the common onion (Allium cepa) in longitudinal (A) and cross-sections (B)

Basal plate (*)—heavily shortened stem of the scape—shoot apices. The bulk of the bulb is formed by fleshy scale leaves or the tubular bases of foliage leaves. When the bulb sprouts, a hollow false stem is formed, through which the flowering scape ultimately pushes its way.

While the leaves of shortened shoots are closely crowded together, on elongated shoots they are set far apart. In primroses, an upright scape grows out from the basal rosette of leaves; it appears leafless and terminates in bracts and flowers, essentially representing an elongated internode. In many other plants (such as strawberries, creeping bugleweed, creeping buttercup, and reed), stolons develop—lateral shoots featuring slender, highly elongated internodes. They creep along the ground or arch downward under their own weight, root at the node, and, by forming a leaf rosette, can grow into a new plant. Because the stolons between the parent and daughter plants subsequently wither away, Vegetative Reproduction takes place—a process of vital importance in horticulture (known as "layering"). The thickened tips of stolons can accumulate reserve substances. The most familiar example of this is the potato (Fig. 4.11): the stolon tips located in the soil develop into starch-storing tubers, while their "eyes" correspond to buds capable of sprouting into a new plant (vegetative propagation via "seed potatoes").

Fig. 4.11. Potato plant Solanum tuberosum (A — after H. Schenck; B — after Percival; C, D — after W. Troll): A—fully grown specimen with the dark parent tuber from which the plant developed; on the thickened stolon tips ("potatoes"), the scars of scale leaves and axillary buds ("eyes") are clearly visible. B—seedling, with shoots in the axils of the cotyledons already bearing small apical tubers. C, D—Cytology/cytology/16.html">Early stages of tuber formation at the stolon apex.

In some plants, elongated and shortened internodes alternate along the shoot. This leads to the formation of false whorls of leaves, which can be observed in Lilium martagon.

4.2.2. Phyllotaxis

There are three MAIN TYPES OF leaf arrangement on the stem (phyllotaxis, from the Greek taxis meaning arrangement): whorled, distichous (two-ranked), and spiral (alternate). In a whorled leaf arrangement, each node bears more than one leaf, most commonly two (opposite leaf arrangement). In distichous and spiral arrangements, each node bears only a single leaf. In whorled phyllotaxis, two or more leaf primordia are initiated simultaneously at the shoot apical meristem, whereas in distichous and spiral arrangements, all primordia form successively. Phyllotaxis is typically depicted schematically using diagrams (Fig. 4.12) in which concentric circles represent successive nodes, with the lowermost node having the largest diameter. These circles correspond to imaginary cross-sections through the nodes.

Fig. 4.12. Types of leaf arrangement

A—whorled leaf arrangement in mare's tail (Hippuris vulgaris), showing both shoot and diagram; B—decussate arrangement, exemplified by lilac (here, as well as in C and D, leaf primordia are shown in black alongside inhibition zones where no new leaf primordia can form, with transverse sections and bud diagrams shown below); C—distichous arrangement, exemplified by thoroughwax (Bupleurum rotundifolium) from the Apiaceae family; D—alternate spiral arrangement, exemplified by blessed thistle (Cnicus benedictus) from the Asteraceae family; E–G—in many plants, different leaf arrangements occur on different shoots of the same individual, illustrated here by purple loosestrife (Lythrum salicaria) showing whorled, decussate, and alternate spiral leaf arrangements. The apparent disruption of the alternation rule in E and F is merely an optical illusion caused by a slight twist of the stem within a single internode (0.5x).

Whorled phyllotaxis is governed by two fundamental principles:

— The angles between the attachment points of leaves within the same node, and typically between the leaves themselves, are always identical; that is, the leaves are positioned equidistantly (the rule of equidistance).

— The leaves are positioned in the gaps between the leaves of the nodes directly above and below them (the rule of alternation). Consequently, the leaves of every second node lie directly above one another, creating distinct longitudinal rows of leaves along the stem known as orthostichies (from the Greek orthos meaning straight, and stichos meaning row). The number of orthostichies is twice the number of leaves per node.

The rules of equidistance and alternation manifest regardless of the number of leaves at a single node. In opposite leaf arrangement, the leaves of adjacent nodes are positioned crosswise (decussately) relative to one another (Fig. 4.13), a pattern characteristic of all Lamiaceae (mints), as well as nettles, maples, ashes, and horse chestnuts. The number of orthostichies in a decussate arrangement is 4, which is the minimum number of orthostichies found in whorled phyllotaxis.

Fig. 4.13. Decussate leaf arrangement

A—shrubby veronica (Hebe pinguifolia) (2x); B—vertical shoot of sycamore maple (Acer pseudoplatanus), top view (1.3x); C—horizontal shoot of wire netting vine (Lonicera pileata); the leaves are opposite but appear distichous due to the twisting of their petioles (cf. Fig. 4.16, E).

Distichous phyllotaxis also features orthostichies, but there are only 2 of them, because the leaves—one per node—lie in a single plane on opposite sides of the stem (Fig. 4.14). The divergence angle between leaves of adjacent nodes is 180°. Distichous leaf arrangement is typical of many monocots (grasses, and the genera Iris and Gasteria), and among dicots, of elms and many legumes, such as Vicia. It frequently occurs on horizontally growing shoots of numerous trees and shrubs that otherwise typically display spiral phyllotaxis (such as hazel, linden, and beech). In ivy shoots that climb tree trunks and walls using adventitious roots (Box 4.5, Fig. A), the leaf arrangement is distichous, whereas the later-forming, upright, flower-bearing shoots exhibit a spiral arrangement. In spiral phyllotaxis, true orthostichies are absent; the attachment points of leaves on successive nodes lie along a helical line. When internodes are heavily shortened (as in leaf rosettes, pine cones, or capitate inflorescences), this appears as a flat spiral, known as the genetic or fundamental spiral (Figs. 4.15; 4.16). The angle between successive leaves is typically slightly greater than 1/3 of 360°, most commonly 135°. In a spiral arrangement, leaves of certain nodes are positioned roughly above one another; however, true orthostichies are absent, and instead spirostichies are formed. The direction of the genetic spiral can vary even on shoots of the same plant.

Fig. 4.14. Examples of distichous leaf arrangement

A—Solomon's seal (Polygonatum multiflorum) (0.4x); B—cotoneaster (Cotoneaster), one-year-old branch (0.4x); in the second year, lateral branches emerge fan-like from axillary buds (such a branch "fan" can be observed, for example, in elms, Fig. 4.28, B); C—Aloe plicatilis, the stem becomes visible only after the fleshy leaves have fallen (0.4x). In other aloe species, as well as in many lilies, grasses, orchids, etc., both orthostichies become helical lines due to stem twisting—a condition known as spiral distichy; D—flowering wheat spike (2x).

Fig. 4.15. Alternate spiral leaf arrangement (A — after W. Troll; C–E — after P.H. Richter and H. Dullin): A—leaf rosette of medium plantain (Plantago media): successive leaves are arranged along the primary genetic spiral; the divergence angle is approximately 135°, corresponding to a phyllotaxis of 3/8 (0.7x); B—scales of a pine cone (cf. Fig. 4.16, B), numbered in the order of their emergence (1–56); solid lines (1–13) and dashed lines I–VIII represent the numerous parastichies characteristic of alternate spiral leaf arrangement (oblique rows, not to be confused with the fundamental spiral or spirostichy). Orthostichies are not formed; the fine dashed lines 1–21 are distinctly arcuate; C–E—computer-simulated flower HEAD (receptacle): two successive leaves are separated by a specific divergence angle corresponding to the "golden angle" of 137.5° in D, 136.5° in C, and 138° in E. Comparison with B or Figs. 4.16, B and C demonstrates that in an undisturbed alternate spiral arrangement of leaves or flowers, the angle corresponding to the "golden ratio" is precisely maintained.

Fig. 4.16. Examples of alternate spiral leaf arrangement

A—shoot of spurge Euphorbia myrsinites, with leaves arranged in prominent oblique rows (parastichies); B—woody scales of a pine cone—modified leaves; here too, the alternate spiral leaf arrangement is discernible through numerous parastichies (1.4x); C—flower head of the common sunflower (Helianthus annuus); more than 1000 tubular florets of the disc inflorescence bloom from the outside inward (morphologically from "bottom" to "top"); they sit in the axils of alternate spiral leaves and therefore are also arranged in an alternate spiral pattern along numerous parastichies (0.25x); D—leaf rosette of Aeonium manriqueorum (1.2x); E—branch of silver fir (viewed from below, showing characteristic wax bands on the needles) with a distichous arrangement of needle-like leaves, though this is not true distichy, but rather the result of the twisting of spirally arranged needles (2x).

It was formerly believed that true orthostichies also exist in spiral leaf arrangement, and distichy was viewed as an extreme case of a spiral arrangement. Accordingly, the angle α between leaves of adjacent nodes was calculated using a mathematical fraction D, where the numerator indicates the number of turns of the spirostichy between leaves lying on the same orthostichy, and the denominator represents the number of leaves or nodes along the shoot segment between such leaves. From this, it follows that D × 360° = α (for distichy: D = 1/2, α = 180°; for D = 1/3, which is characteristic of sedges with their three-angled stems, the angle α = 120°; fractions frequently take values of 2/5 or 3/8, yielding angles of 144° and 135°). If distichy is set aside, the aforementioned values for D form a numerical sequence (the Schimper-Braun series) in which the numerator and denominator values are members of the so-called Fibonacci sequence. In such a sequence, each number is the sum of the two preceding ones: 1, 2, 3, 5, 8, 13..., and α approaches an irrational limiting value: α_L = 137°30'. This angle divides a circle According to the "golden ratio," which played a significant role in architecture even in antiquity: a line segment a is divided into two unequal parts such that the smaller part c relates to the larger part b in the same way that the larger part relates to the entire segment a. From this it follows that b² = c/a, b/c = (360° - α_L)/α_L = 1.618... The proportionality of the golden ratio is particularly pleasing to the eye, which is perhaps why we find the flower head of an asteraceous plant (such as the sunflower), in which the golden ratio is manifested a thousandfold, to be especially beautiful. In general, however, it must be noted that theoretical constructs like the Schimper-Braun series tended to obscure rather than clarify the true nature of phyllotaxis and its underlying causes.

The various types of leaf arrangement are linked to the positioning of leaf primordia on the shoot apical meristems. Young primordia are uniform in size (see Fig. 3.3, C, D) and position themselves within accessible morphogenetic zones On the surface of the apical meristem as closely together as possible, which results in their hexagonal "packing" (Fig. 4.17, 1). All known types of phyllotaxis can be reduced to this hexagonal surface packing of leaf primordia. Additional determining parameters are, firstly, the size ratio between the leaf primordium and the circumference of the apical meristem and, secondly, the direct or oblique orientation of the hexagonal primordia pattern. When the pattern is oriented directly—meaning one of the three coordinate axes runs parallel to the shoot axis—orthostichies are formed. This condition is fulfilled in whorled and distichous leaf arrangements. When the pattern is oriented obliquely, the two simplest leaf arrangements occur, roughly corresponding to the 2/5 and 3/8 arrangements of classical phyllotaxis theory.

Fig. 4.17. Leaf arrangement can be reduced to the dense, hexagonal "packing" of leaf primordia on the shoot apical meristem.

Assuming for simplicity that all leaf primordia are equal in size and rounded, and that the apical meristem has the shape of a cylinder whose surface is cut open longitudinally and flattened into a plane, the individual diagrams represent the following scenarios: 1—hexagonal pattern, corresponding to 4-membered whorls across five successive nodes; 2—distichous leaf arrangement; 3—decussate leaf arrangement; 4—3-membered whorls (e.g., oleander, balsam); 5, 6—alternate spiral leaf arrangement in 3/8 and 2/5. Orthostichies (solid lines) occur in whorled and distichous arrangements (1–4), but not in alternate spiral arrangements, where the hexagonal primordia pattern runs obliquely relative to the shoot axis (dashed lines in 5 and 6 represent the primary genetic spiral).

Pattern Formation of the type described above is frequently encountered in both plants and animals. A familiar example at THE TISSUE LEVEL is the distribution of stomata or trichomes within the leaf epidermis. Most commonly, however, pattern elements do not Touch one another, and their arrangement—though similarly orderly and symmetrical to that of leaf primordia—is rarely observed. Nevertheless, such patterns are invariably governed by a fundamental principle: an established element inhibits the Formation of other similar elements in its immediate vicinity (within the boundaries of its inhibition field). New elements can only form outside these inhibition fields, which functionally means they appear at the minimum possible distance from an existing element. Ultimately, this results in the tightest possible packing of inhibition fields and, consequently, a geometrically regular pattern of roughly equidistant elements (the lattice effect), easily distinguished from a chaotic aggregation, such as a pile of stones (Box 4.1, Fig. E; see also Section 7.4.2).

Such a model clearly demonstrates that leaf primordia on apical meristems can, firstly, form at the minimum distance from the growing apex and, secondly, adjoin pre-existing primordia very closely. This pattern continues to develop further in accordance with the rules of equidistance and alternation. Many biological patterns, even extremely complex ones, can be simulated on a computer based on these simple assumptions.

When the youngest leaf primordia emerge, the initial field (the zone of the shoot apex lacking primordia) changes its shape, but after a strictly determined time, it resumes its original form. This periodic change in shape, repeating rhythmically between successive appearances of leaf primordia, is termed the plastochron.

4.2.3. Rhizome

Many herbaceous plants possess subterranean shoots known as rhizomes. They generally grow horizontally within the substrate. Rhizomes can be distinguished from roots both by the Development and Structure of the shoot apex, and by the peripheral arrangement of vascular bundles and the presence of leaf organs (or leaf scars). Leaves on a rhizome are most commonly scale-like and/or represented by deciduous cataphylls. Rhizomes overwinter in the soil and therefore frequently function as storage organs for nutrient reserves (iris, Solomon's seal: Fig. 4.18, B, C, E). Rhizomes produce adventitious roots and branch from time to time. As older sections of the rhizome die off, vegetative Propagation of the plant takes place; a single rhizomatous plant can give rise to a clone that covers a large area and persists for a very long time, while the aerial PARTS OF THE plant die back annually. Examples include Herb Paris, lily-of-the-valley, reed, anemone, dog's mercury, many primroses, and bracken.

Fig. 4.18. Rhizomes (A—after A. Braun)

A—in Herb Paris (Paris quadrifolia), the green aerial shoots are lateral shoots of the rhizome; Paris is a monopodial rhizomatous herbaceous perennial (see 4.2.4); a—b are flowering shoots of three successive years; B, C—in angular Solomon's seal (Polygonatum multiflorum), conversely, the terminal bud of the rhizome annually produces an aerial flowering shoot, which subsequently dies back, leaving a characteristic scar (C, 1.5x), to which the plant owes its common name. The rhizome continues to grow sympodially, that is, through the growth of a lateral bud; D—rhizome of sweet violet (Viola odorata) with dark remnants of cataphylls and distinct metamerism of nodes and internodes (2x); E—branched storage rhizome of Iris with densely spaced transverse leaf scars in which traces of vascular bundles are still discernible (0.6x)

4.2.4. Life Forms

In all natural zones with pronounced seasonal climate variations, plants face The Challenge of surviving unfavorable periods. Depending on geographical latitude, different factors play a decisive role, primarily humidity (see Section 13.5) and/or Temperature (see Section 13.3). In temperate climates with wide differences between summer and winter temperatures, plants have evolved a range of adaptations collectively termed "life forms." In this context, primary importance is attached to the mechanism by which delicate renewal buds survive winter frosts. The following life forms are distinguished (Fig. 4.19).

Fig. 4.19. Life forms (after H. Walter).

The highlighted plant parts overwinter, while the rest die off in autumn. A, B — chamaephytes (periwinkle Vinca and lingonberry Vaccinium), C — phanerophyte (beech), D—F — hemicryptophytes (D — dandelion Taraxacum as an example of a rosette plant; E — creeping herbaceous perennial creeping buttercup Ranunculus repens, F — loosestrife Lysimachia); G, H — cryptophytes (G — rhizomatous geophyte Anemone, H — tuberous geophyte Crocus); I — therophyte (corn poppy Papaver rhoeas)

— Phanerophytes (from Greek phaneros — visible, open) are trees and shrubs whose renewal buds are situated well above both the soil surface and the snow cover. The apical meristems of phanerophytes are frost-hardy and protected from desiccation by tightly appressed bud scales. These dry, leathery, very simply structured leaf organs are often sealed with resin or sticky or mucilaginous secretions from glandular hairs. In spring, the scales fall off, leaving closely spaced annular scars (bud scale scars) that mark the boundaries of annual growth (Fig. 4.20).

Fig. 4.20. Bud scale scars on a beech branch that grew as a short shoot for 7 years and then continued growth as a long shoot (with a lateral short shoot, 2.4x) (cf. also Fig. 4.9, C, D)

Depending on whether the leaf organs are frost-hardy or not, evergreen and deciduous phanerophytes are distinguished. Among perennial plants native to frost-free regions, such as the Mediterranean, evergreen forms predominate.

— Chamaephytes (from Greek chamaiphyes — growing low to the ground) are dwarf shrubs and subshrubs whose renewal buds are located directly above the soil surface, where they are protected from frost by a blanket of snow (snow has low thermal conductivity due to its high air content). This group includes many prostrate and creeping (espallier) woody plants, as well as cushion plants of tundras and high mountains, and, for example, Erica carnea and Calluna (heather) (Fig. 4.21; see Fig. 15.3, G; 15.11, A).

Fig. 4.21. Cushion plants (A — after A.F.W. Schimper; B — after W. Rauh): A — Azorella selago, Apiaceae, from Kerguelen Island in the storm belt of the southern Indian Ocean (0.2x); B — sympodial shoot system of cushion plants

— Cryptophytes (from Greek kryptos — hidden), or geophytes, have subterranean shoots, meaning their renewal buds are located within the soil. (Note that the similar term Cryptophyta refers to a division of Algae; see Fig. 11.68.) They are most commonly represented by bulbous and rhizomatous plants. Their aerial shoots with leaves and flowers develop annually utilizing nutrients stored in underground organs, but die back by winter (sometimes only a basal leaf rosette remains).

— Hemicryptophytes occupy an intermediate position between chamaephytes and cryptophytes. Their renewal buds are situated at the soil surface within the litter layer and are protected from low temperatures by snow cover, leaf litter, or herbaceous detritus. Hemicryptophytes include many grasses (including winter cereals), rosette plants (plantain, snapdragon), stoloniferous plants (strawberry, creeping buttercup), and fairly tall herbaceous perennials whose renewal buds are located at the base of dying aerial shoots (stinging nettle — Urtica, yellow loosestrife — Lysimachia vulgaris).

— Therophytes (from Greek theros — summer) lack perennial organs entirely and survive the winter as seeds, which are highly resistant to cold due to their low water content. In addition, seeds contain all the nutrients required for germination, stored either in the embryo (cotyledons) or in specialized nutritive tissues—the endosperm or perisperm. Therophytes are herbaceous plants whose intrinsic developmental program dictates their complete death after fruiting. They include annuals and biennials. Annuals most commonly grow as weeds, rapidly colonizing secondary habitats (arable fields, fallow land, etc.), whereas biennials are also found within stable plant communities.

4.2.5. Shoot Branching

4.2.5.1. Dichotomous and Phyllogenous Branching

In contrast to seed plants, axillary branching is relatively rare in pteridophytes; however, even in these plants, lateral buds are regularly positioned relative to leaf bases, for instance, below and to the side of the point of leaf attachment. Thus, the buds here are not strictly axillary, although they are associated with phyllomes (phyllogenous). Axillary branching represents a specialized case of phyllogenous branching.

A fundamentally different type is dichotomous branching, resulting from the equal division of the apical meristem (see Fig. 5.12). While phyllogenous branching occurs in the zone of leaf primordia, i.e., lateral to the shoot apex, dichotomous branching takes place directly within the initial zone of the apical meristem. Dichotomy predominates in lycophytes (see Fig. 11.134, G) but is occasionally encountered in ferns as well. Shoot systems arising through dichotomy are termed dichotodia (from Greek dichos — in two; kladion — branch).

4.2.5.2. Axillary Branching Systems

In seed plants, branching is typically axillary: lateral shoots develop from the leaf axils. Such leaves in the vegetative region of the plant are termed foliage leaves or subtending leaves, while those in the inflorescence region are called bracts. Whereas in conifers axillary buds develop in the axils of only relatively few leaves, in angiosperms lateral buds are present in the axils of all leaves in the vegetative region. Frequently, multiple buds are initiated within a single leaf axil; these are referred to as accessory buds (Fig. 4.22). The outgrowth of axillary buds, as well as the degree of development and branching of the resulting shoots, is under strict hormonal control (see Section 7.6). Intraorganismic interactions, known as correlations (see Section 7.5), also govern whether branch growth is vertical (orthotropic), oblique, or horizontal (plagiotropic). The result is a plant-specific shoot system that determines its overall form, or habitus (from Latin — appearance).

Fig. 4.22. Serial accessory buds: A—C — accessory buds positioned one above the other on the stem; the largest is typically either the uppermost ("descending series": A — Forsythia, 3.5x; B — blackberry, 2x) or the lowermost ("ascending series": C — Lonicera xylosteum, 5x). Accessory buds situated side by side (such as banana fruits or cloves in garlic bulbs) are termed lateral. (Their correct botanical term is collateral. — Ed. note)

In many shoot systems, lateral shoots lag behind the main axis in growth. Such systems are hierarchically organized (main axis, lateral axes of the 1st, 2nd, 3rd, and subsequent orders) and are termed monopodial (Fig. 4.23, A). The most familiar example of a monopodial shoot system is the spruce: the orthotropic, radially symmetrical trunk represents the leading main shoot (monopodium); the lateral branches also branch monopodially and grow plagiotropically. Due to the dominance of the apical shoot, the overall shape of the tree crown is conical. Most conifers branch in this manner.

Fig. 4.23. Types of branching: A—monopodial axis structure with lateral (racemose) branching: H—main axis; 1–4 — lateral axes (from the first to the fourth branching orders). Sympodial branching: B—monochasium; C—dichasium; 1—primary shoot axis; 2–5 — lateral axes. (It is necessary to distinguish branching from elongation. In both monopodial and sympodial elongation, the branching type is the same—lateral. — Ed. note)

Monopodial branching also predominates in the crowns of many broad-leaved tree species, despite their different habit, for example, in poplars, ashes, and maples.

In other cases, lateral shoots develop more strongly than the main one. Here, the terminal bud either withers or forms a flower/inflorescence, etc. Thus, further longitudinal growth of the shoot becomes impossible. The elongation of the axial system is carried out by new shoots developing from lateral buds; a sympodial shoot system is formed (Fig. 4.23, B, C).

The Greco-Latin word *podium* in this context means 'axis segment'. Sympodial shoot systems consist of equally developed segments (phytomers) of various orders. In monopodial shoot systems, There is a main axis, and the lateral branches, corresponding to their order, develop progressively weaker: this type of elongation is maintained throughout the entire shoot System of the plant.

The most common type of sympodium is the monochasium, in which a single lateral shoot overtops ('over-summits') the growth-stopped main axis and thus continues the elongation of the entire shoot system. This shoot in turn soon ceases growth, and is overtopped by its own lateral shoot, and so on (Fig. 4.23, B). Usually, the overtopping lateral shoots grow in the same direction as the parent ones. Therefore, a sympodium can be distinguished from a monopodial system only upon careful examination.

The trunks and branches of most broad-leaved trees (linden, beech, elm, chestnut, hazel) are sympodiums. The buds at the ends of their overwintering shoots appear to be terminal, but in reality, they are subterminal lateral buds; actually, the terminal buds usually drop off. Another example of a sympodial-monochasial shoot system is the grapevine (Fig. 4.24). It should be noted that rhizomes can also grow monopodially or sympodially-monochasially (see Fig. 4.18, A–C).

Fig. 4.24. Monochasium of the grapevine *Vitis vinifera* (after A.W. Eichler, modified).

The successive members of the sympodium, shown alternately as light and dark, terminate in tendrils. In the axils of the terminal leaves of the sympodium members, there are serial (accessory) buds—'side shoots' or 'suckers', the removal of which ('desuckering' or 'pruning') is an important and labor-intensive Procedure in grapevine cultivation.

Rarer types include dichasia and pleiochasia, where two or, respectively, several lateral branches overtop the parent one (see Fig. 4.23, C). Well-known examples of a dichasium are the shoot systems of lilac, mistletoe (see Fig. 11.241), and many Caryophyllaceae, in which terminal buds are regularly consumed in the formation of a flower (see Fig. 11.239, H). The close relationship between phyllotaxis and shoot branching, characteristic of axillary branching, is emphasized by the fact that dichasia arise in plants with decussate leaf arrangement.

If the terminal bud in monopodial systems is damaged by some external influence, its role usually passes to the nearest axillary bud, and elongation proceeds (as an exception) as in a monochasium. However, many plants change their branching type (from monopodial to sympodial and vice versa) regardless of external factors, especially when transitioning to the reproductive stage of development.

In rare cases, the primary shoot of a seedling grows monopodially up to a terminal flower (as in poppies). Much more commonly, flowers are formed on higher-order lateral shoots according to the plant's characteristic 'axiality'. For example, in the greater plantain (*Plantago major*), the main shoot bears only a basal rosette of leaves, first-order lateral shoots develop barely noticeable upper leaves, and only in their axils do short branchlets terminating in a flower develop. Plantain is a so-called 'triaxial' plant. In many trees, the first flowers appear on shoots of a very high branching order that develop many years after seed germination.

Box 4.2. Morphology of Inflorescences

Simple inflorescences (see Fig. 4.25): a spike differs from a raceme by having sessile flowers (lacking pedicels) in the axils of bracts; an umbel differs by a shortened inflorescence axis, which is compensated for by elongated pedicels of flowers attached at almost the same height. In the spike-like spadix, the inflorescence axis is greatly thickened; in the head (capitulum), it is thickened, more or less shortened, and often bears an involucre of rosette-like leaves at its base, which should not be confused with the floral bracts inside the head.

If, in simple inflorescences (raceme, spike, umbel), individual flowers are replaced by entire inflorescences, compound inflorescences are formed (see Fig. 4.26)—a compound (double) raceme, compound spike, compound umbel. In a panicle, the inflorescence axis terminates in a terminal flower, as do all lateral axes, the degree of branching of which gradually increases from top to bottom, starting from the topmost individual flowers located just below the terminal flower. The conical outline of the panicle can change because all flowers end up at the same level due to the corresponding elongation of the lateral axes (corymbose panicle, example: rowan *Sorbus aucuparia*). Due to the strong elongation of the basal inflorescence branches, the panicle can even become saucer-shaped (as in meadowsweet *Filipendula*), and is referred to as a 'spire' (an anagram of *Rispe*, panicle)¹.

¹ A more common term is *anthela*. — Ed. note

Fig. A. Leaf series of lateral shoots in inflorescences (after D. von Denffer).

The series also begin with one or two prophylls, differing in shape and position from other leaves. If There are two of them (1, 2 in the fig.) (as in most eudicots), they are usually located transversely. In monocots, there is most often a single prophyll, which is located not in a lateral position, but on the side facing the stem of the parent shoot. It is possible that it originated from the fusion of two prophylls. Left—side view, right—diagrams

Fig. B. Some forms of cymose inflorescence branching (after W. Troll and F. Weberling).

Dichasium—side view and diagram. Cincinus (scorpioid cyme)—e.g., bugloss, petunia. Bostryx (helicoid cyme)—e.g., St. John's wort

In contrast to a panicle, a thyrsoid (possessing a terminal flower) and a thyrsus (lacking a terminal flower) are characterized by cymosely branched lateral branchlets—partial inflorescences. Cymose branching is understood to be branching exclusively from the axils of prophylls (Fig. A), which represent the only leaf organs below the flower and in eudicots (and some monocots) usually develop as a pair in a transverse position (opposite or alternate). If branching always occurs from the axils of both prophylls, a dichasium arises (Fig. B), the branches of which continue to branch further from the axils of their own prophylls, with the lateral branches overtopping the parent one. Instead of dichasial branching, monochasial branching occurs when one of the prophyll axils remains sterile each time. If, in this process, lateral shoots are initiated alternately in the left and right axils, a cincinus (scorpioid cyme) arises. If always only the left or right prophyll is 'fertile' (relative to the median plane passing through the covering leaf and the daughter branch), a bostryx (helicoid cyme) is obtained (Fig. B). If both Branches of the dichasial partial inflorescence are represented by bostryces or cincini, one speaks, respectively, of a double bostryx (deadnettle and other Lamiaceae) or a double cincinus. If, as in many monocots and some eudicots, there is only a single prophyll located on the side facing the stem of the parent shoot, then branching of the cincinus type gives rise to a special form of cymose partial inflorescence called a ripidium (fan-shaped cyme) (e.g., in *Iris* species), all axes of which lie in a single plane.

Monotelic and polytelic inflorescences. From a comparative-morphological perspective, Two Types of inflorescences are distinguished—monotelic and polytelic. In monotelic inflorescences, as in a panicle (see Fig. 4.26), the main and lateral axes terminate in terminal flowers. All lateral flower-bearing axes departing from the main axis below the terminal flower—whether branched or not—represent a system of equivalent, homologous elements. They are called repeating shoots or paracladia, since they equally repeat The structure of the main axis to a certain extent, and their branches—respectively, paracladia of the first to the *n*-th order. This system of paracladia can be transformed in various ways according to THE PRINCIPLE OF varying proportions, i.e., by increasing the number of flowers or reducing them down to a single terminal flower, by varying internode development in different parts of the inflorescence, by changing the branching pattern (e.g., thyrsoid), or due to more vigorous development of upper or lower branches (acrotonic or basitonic enhancement).

Inflorescences constructed according to the polytelic type terminate not in a terminal flower, but in a multi-flowered florescence (main florescence, Fig. C, II) composed of individual lateral flowers or—in a thyrsus—of cymose partial florescences. In contrast to monotelic inflorescences, the apex of their axis remains open (sometimes it even later reverts to vegetative growth, e.g., in pineapples). Below the main florescence are lateral branches which, like the main axis, also terminate in a florescence (co-florescence). They are called paracladia (of the polytelic type). Polytelic inflorescences arise from monotelic ones in two stages: 1) loss of the terminal flower; 2) specialization of lateral axes (some of them, as individual flowers or partial florescences, become elements of a higher-order unit—the florescence, while others become paracladia, which in turn terminate in a florescence).

Fig. C. Closed and open inflorescences (after W. Troll and F. Weberling).

I—closed inflorescence with terminal flower E, lateral flowers blooming from bottom to top; II—compound open synflorescence: main florescence HF and co-florescence CoF, enrichment zone BZ with partial florescences PF. The enrichment zone is formed by lateral shoots (paracladia Pc and Pc'). In the suppression zone HZ/HZ', the activation of axillary buds is hormonally blocked; JZ—regeneration zone. The main florescence is usually separated from the enrichment zone by a long main internode GJ

The terminal flower of a monotelic inflorescence and, correspondingly, the main florescence of a polytelic inflorescence are always preceded by a shoot section on which paracladia develop, enriching the inflorescence with additional flowers. Below this enrichment zone, a more or less abrupt inhibition of paracladia development occurs (suppression zone). In herbaceous perennials, buds in the axils of the basal leaves of the main shoot act as renewal buds, from which new above-ground shoots develop in the next growing season (regeneration zone). Together, these three zones form the so-called Unterbau (this term has no equivalent in other languages. — Ed. note), which encompasses the entire predominantly vegetative part of the main shoot. A similar segmentation can usually be observed in paracladia, with the exception of the regeneration zone, which is absent in them. In flower-bearing shoots of woody plants (trees, shrubs), the zone of renewal buds and the repeating shoots developing from it are often directly adjacent to the enrichment zone.

Monotelic inflorescences, as closed structures, have frequently been contrasted with polytelic inflorescences as open ones. However, even among monotelic inflorescences, there are those in which the apex of the main shoot remains open and even continues to grow indefinitely (many lianas and perennial rosette plants). Yet, the monotelic structure of the paracladia clearly indicates that this is a monotelic inflorescence. The pairs of concepts monotelic/polytelic and open/closed designate different characteristics of inflorescences.

4.2.5.3. Inflorescences

Inflorescences (inflorescentiae; from Latin florescere — to bloom) provide particularly clear examples of various branching types. Their diversity is immense, surpassed only by The Diversity of the flowers themselves. Inflorescences can be classified as follows.

— Simple and complex (compound) inflorescences: these differ in their degree of branching (Figs. 4.25; 4.26). In simple inflorescences, the order of axes differs by no more than one, whereas complex inflorescences possess axes of several successive branching orders.

Fig. 4.25. Simple inflorescences (after F. Weberling and O. Schwantes)

Familiar examples of racemes are found in many Liliaceae and Brassicaceae, as well as in willowherbs and barberries. Spikes are seen in the flowers of evening primroses, plantains, rampion, and most orchids. Spadices occur in maize and arum. Umbels are formed by astrantia, ivy, and primroses. Heads, along with their flat counterparts, capitula, are characteristic of scabious and knautia, as well as Asteraceae.

Fig. 4.26. Complex inflorescences (after W. Troll and F. Weberling)

Double racemes occur in many clover species, and double umbels are typical of most Apiaceae (Umbelliferae). Examples of panicles include the inflorescences of lilac, privet, and grapevine. A corymbose panicle arises when all flowers of a panicle reach approximately the same level due to the elongation of the lower lateral branches (elderberry, rowan, hydrangea). In the panicles of grasses (Poaceae), small partial inflorescences—spikelets—are gathered into a panicle (e.g., in oats and smooth meadow-grass). Similarly, in the "spikes" of such cereals as wheat, rye, barley, Lolium, and Elymus, simple spikelets are arranged into a (compound) spike. A corymb-like panicle (spirre) is best developed in Filipendula (meadowsweet). Thyrses correspond to the inflorescences of the horse chestnut, great mullein, borage, and many Lamiaceae (e.g., sage). Dichasia are typical of Caryophyllaceae (especially prominent in chickweed, mouse-ear chickweed, and sandwort; cf. Fig. 11.239, D), as well as strawberries and lindens (cf. Box 4.2, Fig. B).

— Racemose/cymose inflorescences: these differ in the same way as monopodial and sympodial branching. Simple inflorescences are constructed according to the monopodial (racemose) type.

Open/closed inflorescences (Box 4.2, Fig. C): if all inflorescence axes (their number may vary) terminate in terminal flowers, it is a closed inflorescence. In open inflorescences, a terminal flower is lacking. However, even in an open inflorescence, the terminal bud gradually ceases development, but under certain circumstances it can be reactivated, allowing the open inflorescence to grow out into a vegetative shoot.

Extensive comparative studies have led to the typological distinction between monotelic and polytelic inflorescences (from Greek telos — end). Details of inflorescence morphology are provided in Box 4.2.

4.2.5.4. LIFE FORMS OF woody plants: shrubs and trees

The habit of a shrub—a multi-stemmed plant—is determined by the fact that buds located at the Base of the shoots, or respectively the lateral branches, grow more strongly than those positioned higher up: basitony (Fig. 4.27). Due to this, shrubs are capable of rejuvenating themselves from the base in every vegetative period by producing vigorous new shoots in the basal regeneration zone. The branches, especially at their ends, branch weakly and in most cases have a limited lifespan and height. The woody base of the bush, from which new renewal shoots emerge annually, gradually develops into a short, knobby xylopodium. The growth system of shrubs is fundamentally sympodial.

Fig. 4.27. Growth and branching forms in shrubs (after W. Rauh): A—hazel Corylus avellana; B—elderberry Sambucus nigra; The Root System with the main root W is shown greatly simplified; P—main shoot; 1–4—individual annual shoots; o—vigorous branches of the upper side; u—weakened branches of the lower side; E—dead shoot tips of various generations; Ek—buds in the regeneration zone of the xylopodium; C—cotyledonary nodes; H—hypocotyl

In the axial system of trees—whether monopodial or monochasial—acrotony predominates: here, in contrast to shrubs, the terminal bud and the lateral buds closest to it grow the most strongly (Fig. 4.28) (from Greek akros — topmost, outermost; tonos — tension, enhancement). Annual growth thus takes place primarily in the peripheral part of the crown, which is supported by a single trunk.

Fig. 4.28. Growth and branching forms in trees (A, B—after W. Rauh; C—after W. Troll): A—monopodial; B—sympodial branching; C—two-year-old branch of the field elm Ulmus minor with pronounced acrotony; the uppermost lateral branch s continues the main axis as a new sympodial member; flowers are present on the basal parts of the lower and middle lateral branches (0.1×)

The different habits of coniferous (gymnosperm) and deciduous (angiosperm, specifically dicotyledonous) trees are due to the fact that in deciduous trees, the earliest lateral branches located in the lower part of the trunk develop poorly; they eventually wither and fall off. As a result of this "upward-spreading acrotony," after several years or decades, a trunk stripped of branches emerges, supporting a broad crown of rounded contour. In contrast, in monopodial conifers, even low-lying old lateral branches continue to grow, giving rise to the familiar pyramidal crown shape. In overly dense conifer stands, however, the lower branches do not receive enough light and die off for this (external) reason. Even then, they are not shed, but remain as a rigid, needle-less tangle of branches. In modern forestry, this process is induced artificially through dense planting without subsequent thinning in order to obtain industrially more valuable timber stands with straight, long trunks and, consequently, higher profits. The gloomy deformity of such stands—where a slow-decaying, thick layer of acidic needle litter covers the ground and green undergrowth is entirely absent—is a clear symptom of an artificially mutilated ecosystem.

A peculiar intermediate state between a shrub and a tree is represented by the lilac (Syringa). Its axial system is acrotonous, but growth is sympodial and, in accordance with the decussate leaf arrangement, dichasial. Therefore, it always produces a fork of two equally strong continuation shoots at the ends of its annual shoots, making the formation of a single trunk impossible.

4.2.5.5. Metatopy, cauliflory, adventitious shoots, brood buds

In some flowering plants, the principle of axillary branching appears to be lost because axillary buds or the attachment sites of lateral branches are shifted onto the mother axis as a result of fusion (concaulescence; from Greek kaulos — stem) or are located on the bract (recaulescence). In such cases, one speaks of metatopy (displacement; Fig. 4.29). Concaulescence is widespread in The Nightshade family (Solanaceae), which includes the potato.

Fig. 4.29. Metatopy (after W. Troll, modified): A—typical structure for comparison: lateral branch in the axil of a bract; B—recaulescence; C—concaulescence; D—inflorescence of the potato Solarium tuberosum: concaulescence of two lateral shoots with cincinnus inflorescences

When flowers form directly on the trunks—cauliflory—axillary branching also appears to vanish: shortened shoots bearing flowers or fruits emerge directly from branches or massive trunks. They owe their origin to long-dormant buds (Figs. 4.30; 11.266, D).

Fig. 4.30. Cauliflory (photos A—W. Barthlott; B—D. Zissler): A—in Goethea cauliflora (Malvaceae), flowers are located in the axils of already fallen leaves, whose former position on the stem is still marked by a leaf scar; B—in the Mediterranean Judas tree Cercis siliquastrum, flowers are located on an older stem where bark has already formed, so leaf scars are no longer visible

In some plants, correspondingly modified axillary buds are shed as brood buds (Fig. 4.31), which root in the soil and give rise to a new plant.

Fig. 4.31. Axillary buds transformed into brood buds in the bulbous coralroot Cardamine (Dentaria) bulbifera

However, flowering plants also possess buds and shoots that do not originate in the axils of leaves. This applies primarily to The Development of the embryonic bud, as well as to specialized adventitious buds and shoots that arise on roots or leaves (Fig. 4.32). The formation of adventitious shoots is frequently triggered by plant injury. Familiar examples include the vigorous sprout growth commonly seen on freshly cut tree stumps¹ or the formation of new vegetative apexes in callus tissue, a technique utilized in plant propagation via cell culture (see Fig. 7.47).

¹ Stump sprouting is driven by the outgrowth of dormant buds rather than the de novo formation of adventitious buds — Ed. note.

Fig. 4.32. Brood buds in Kalanchoe daigremontiana, a succulent from the Crassulaceae family.

Adventitious buds (A) formed on the margins of the leaf blade develop into young plantlets (B, 2x), which subsequently detach and fall off. While this plant does possess axillary buds, they remain externally inconspicuous.

4.2.6. Special Functions and Adaptive Forms

Tendrils—which serve for vegetative propagation and dispersal—as well as stolons with potato tubers (see Fig. 4.11), have already been mentioned as examples of shoot metamorphosis. An unusual lifestyle and/or adaptations to specific environmental conditions lead to a range of further stem modifications.

Storage stems. In all stems, the fundamental parenchyma functions primarily in substance storage. In certain plants, this function is particularly pronounced: the fundamental parenchyma proliferates significantly, causing the stem to become more or less thickened in places, thereby forming stem tubers.

The hypocotyl frequently undergoes a similar transformation (hypocotyl tubers, such as those in Cyclamen, radishes, and red beets). In plant morphology, the term root tuber (or root crop) is generally used when the taproot also participates—sometimes even predominantly—in tuber formation (Fig. 4.33, Box 4.5, Figs. D, E). In some cases, leafy parts of the shoot are also converted into tubers, as seen in kohlrabi. In herbaceous perennials with an annual, sequentially replaced subterranean tuber (such as autumn crocus and Crocus), the underground-concealed base of the shoot swells into an overwintering tuber. The following spring, a lateral bud gives rise to a new renewal shoot, the base of which subsequently develops into a new tuber.

Fig. 4.33. Participation of the taproot and hypocotyl (shaded) in root crop formation across different races of Beta vulgaris (after W. Rauh).

A—sugar beet, B—fodder beet, C—garden (red) beet.

Stems with leaf function. The cortical parenchyma of an herbaceous stem contains METABOLISM/14.html">Chloroplasts and is thus green, enabling Photosynthesis. This function, which is already clearly evident in twiggy plants (such as broom), can be further amplified in flattened, leaf-like stems known as platycladodes (from Greek platys, flat). Platycladodes may take the form of either a shortened shoot (phylloclade, see Fig. 4.3) or an elongated shoot (cladode, Fig. 4.34). In such cases, leaves are reduced to scales or spines, or are shed early.

Fig. 4.34. Flattened cactus stems as an example of cladodes (after Schumann, modified).

A—Christmas cactus (Zygocactus truncatus, 0.5x); B—prickly pear (Opuntia) bearing a flower and two fruits; the helical leaf arrangement is reflected in the regular pattern of parastichies along which areoles are aligned (0.3x).

Stem succulents. Plants inhabiting extremely arid environments (xerophytes) are primarily forced to minimize water loss (transpiration). Because leaves are sites of both intensive photosynthesis and high transpiration, xerophyte leaves are frequently modified into spines, which simultaneously protect the plants against herbivory. Photosynthesis is thus relocated to the stem. In plants actively resistant to water deficit, the green stems are additionally succulent—meaning they are transformed into bulky water-storage reservoirs with a high volume-to-surface-area ratio (from Latin succus, juice).

Among stem succulents, cacti are undoubtedly the most widely known. As observed with fascination by Johann Wolfgang von Goethe, their seedlings bear a striking resemblance to those of other dicotyledonous plants. During subsequent development, the cortical parenchyma expands into water-storing tissue, leaves are transformed into spines, and lateral buds develop into Hair bundles, spines, or areoles. Globose and columnar cacti develop pronounced, protruding longitudinal Ribs; due to differential solar exposure, the flanks of these ribs maintain varying temperatures, thereby generating a cooling convective air current via this thermal gradient.

Fig. 4.35. Stem succulents as an example of phyletic parallelism driven by an arid climate with short periods of heavy rainfall (after D. von Denffer).

A—Cereus iquiquensis (Cactaceae), B—Euphorbia fimbriata (Euphorbiaceae), C—Huernia verekeri (Asclepiadaceae), D—Kleinia stapeliiformis (Asteraceae), E—Cissus cactiformis (¹) (Vitaceae) (all 0.5x).

Stem succulents are not restricted to cacti; they also evolve as a convergent adaptation in plants from entirely different orders (Fig. 4.35). Despite striking external similarities, internal anatomy can vary considerably. For instance, in some species, the pith rather than the cortex becomes water-storing, resulting in succulent stem vascular bundles that are positioned peripherally rather than centrally, unlike in cacti.

Fig. 4.36. Shoot thorns and prickles.

A—lignified short shoot of firethorn (Pyracantha coccinea); B—short, leafy, spine-modified shoots of sea buckthorn (Hippophae rhamnoides); C—spine-modified short shoots of blackthorn (Prunus spinosa; Latin spina = thorn, spine) bearing flower buds; D—fully lignified, branched thorns originating from lateral shoots on the trunk of Gleditsia triacanthos (this species also includes an unarmed race, inermis, which lacks thorns because it does not form the corresponding shoots); E–H—prickles: E—emergences on the trunk of Chorisia, whose extremely sharp, lignified tips deter tree-climbers; F—rose, with prickles positioned independently of nodes (indicated by arrows); G—raspberry (1.5x); H—teasel (Dipsacus fullonum, 1.5x).

Fig. 4.37. Schematic longitudinal sections of a thorn (A) and a prickle (B).

The wood of a thorn is a direct continuation of the wood of the supporting branch, and the thorn is situated in the axil of a bract or its corresponding leaf scar. By contrast, a prickle, being an emergence, is formed exclusively from cortical tissue and therefore easily snaps off.

Shoot thorns. Not only leaves (see Fig. 4.7) but also lignified short shoots can be modified into thorns (Fig. 4.36). Well-known examples include the unbranched thorns of blackthorn, hawthorn, and Japanese quince, as well as the branched thorns of honeylocust. Prickles found on roses and blackberries are analogous in function but structurally non-homologous to thorns; they represent surface outgrowths known as emergences (see 3.2.2.1; Fig. 4.37). The piercing and wounding action of thorns and prickles relies on the same principle as claws, Teeth, etc.—namely, that minimal applied force generates high pressure (pressure = force / area) when concentrated on a rigid, pointed tip.

Table 4.1. Climbing plants (lianas) and their supporting organs

¹ The authors make major errors: the tendrils of cucurbits have a complex and not yet fully resolved nature (axillary shoots or parts of inflorescences), but are not of leaf origin; clematis plants twine around Supports using their petioles, whereas vanilla, although climbing with the aid of roots, lacks tendrils. — Trans. note.

Stem tendrils. Like leaves (see Fig. 4.69), stems can transform into tendrils to serve as anchoring structures for climbing plants¹. Both stem and leaf tendrils grow while performing searching movements and are highly sensitive to touch (thigmonasty, see 8.3.2.4). Stem tendrils are always modified tips of lateral shoots: axillary shoots of a monopodium (e.g., in Passiflora) or members of a monochasium, as in the grapevine (see Fig. 4.24). In Boston ivy (Parthenocissus), the tips of the tendrils are modified into attachment discs (see Fig. 4.69, C).

Fig. 4.38. Cuscuta europaea (0.5x) (after F. Noll)

A—seedlings, the longest growing along the soil surface and dying off at the rear end; B—flowering dodder twining around a willow branch

Fig. 4.39. Haustoria (photos: I. Dorr)

A—Cuscuta odorata on Pelargonium zonale. The parasite (top) has penetrated the host leaf petiole with its haustorium H and is developing "searching hyphae" S within its parenchyma, one of which is already in contact with the phloem (arrow) (30x); B—haustorium of European mistletoe (Viscum album), a hemiparasite (left), which has developed on an apple tree. An opening providing water conduction has formed between the host vessel (*) and the short vessel (**) originating from the parenchymal Cells of the haustorium (arrow) (250x)

Climbing plants root in the soil and use slender stems to ascend other plants, rocks, walls, or similar supports. This allows them to secure better illumination for their leaves without the need to develop massive trunks. Given the pivotal role that light availability plays in plant life (along with water supply and temperature), it is unsurprising that climbing can be achieved through various means and that plants possess numerous structures analogous to tendrils (Table 4.1; Figs. 4.37; 4.38).

¹ It is more accurate to refer to such plants as scramblers or clinging plants. — Ed. note.

Haustoria. By means of these absorbing organs (from Latin haurere to draw/absorb), stem-and-leaf parasites, for instance, gain access to the Vascular Tissues of their host plants. These are dominated by root parasites that "hook up" to the roots of their victims. Their haustoria are modified roots. Some stem parasites, such as mistletoe, are also connected to the host via their own roots (Fig. 4.39, B). However, there are parasites with haustoria borne on aerial shoots. These include various species of dodder (Fig. 4.39; 11.281, D).

Cuscuta belongs to the holoparasites—total parasites (from Greek holos meaning whole, entire). The pale yellow or red stems of most species almost completely lack chlorophyll and are therefore incapable of photosynthesis; the leaves are reduced to tiny scales, and the seedling root withers away early on. The stem of the seedli

ng grows exclusively in length, performing revolving movements (nutations) until it encounters a suitable host, around which its stem can twine. At the point of contact, the cortical parenchyma of the parasite forms papilla-like outgrowths and ultimately penetrates the host tissues with the help of its own haustoria. Connections with the sieve tubes of the victim are established via so-called "searching hyphae" (Fig. 4.39, A).

4.2.7. Stem anatomy: primary structure

4.2.7.1. Development

In the shoot apical meristem, right after the apical initial zone (which is only 10 — 50 µm long) and the organogenic region (zone of differentiation or determination),

where leaf primordia emerge, lies the histogenic zone (see 3.1.1.1). It begins at a distance of 50–150 µm from the apex. Within the histogenic zone, the flank meristem—which surrounds the ring of the central pith meristem—differentiates into procambium and cortex meristem. Procambial cells rapidly acquire a prosenchymatous shape. These narrow, longitudinally oriented, Cytoplasm-rich cells stand out clearly from the isodiametric, already visibly vacuolated neighboring cells of the ground meristem (Fig. 4.40). Procambial strands approach the leaf primordia at a particularly early stage, later developing into leaf traces. (A leaf trace is defined as any vascular bundle that branches off from the stem's Vascular System and connects it to the leaves¹. Similarly, a branch trace refers to any bundle connecting The vascular system of the parent shoot with that of a lateral bud or lateral shoot.)

¹ The term leaf trace is also frequently used to describe the entire set of bundles supplying a single leaf. — Ed. note.

Fig. 4.40. Procambium (A after K. Esau, B after Helm)

A—Longitudinal section of a flax (Linum) shoot apex below a leaf primodium; a procambial strand is differentiating (107x). B—transverse section of the shoot apex of meadow buttercup (Ranunculus acer) near the apex; the cells of the procambial ring are marked with dots, and the onset of vascular bundle differentiation is visible in 4 places (90x). C—transverse section of the shoot apex of Veronica traversii, showing a distinct procambial ring between the pith and cortex (60x).

Starting from the determination zone, the future fate of the cells—and thus the subsequent structure of the stem tissues—is established: the outermost dermatogen gives rise to the epidermis, the cortical meristem to the primary cortex, the procambium to the vascular tissues, and the pith meristem to the pith. Further down, the histogenetic zone transitions into the elongation zone, where Cell Division activity subsides and cells attain their final size and shape. Here, derivatives of the procambium give rise to the first elements of the phloem and (usually somewhat later) the xylem—the protophloem and protoxylem. Their conducting elements do not participate in the elongation of the young branch, but are passively stretched and typically torn or crushed (see Fig. 4.42). As soon as longitudinal growth and primary thickening are complete, the longer-lived, larger, and more efficient conducting elements of the metaxylem and metaphloem begin to function.

4.2.7.2. Primary Structure

Fig. 4.41 shows a transverse section of the stem of a typical eudicot plant. It is nearly radially symmetrical. From the inside out, the following tissues can be distinguished (Figs. 4.41, 4.42).

Pith parenchyma. This tissue fills the center of the stem. It functions as a storage tissue or eventually dies off, with its cells becoming filled with gas (for instance, in sunflower and elderberry). In other cases, tearing or dissolution of the tissue results in the formation of a pith cavity.

Fig. 4.41. Transverse section of a herbaceous eudicot stem

A—diagram: parenchymatous pith rays lie between the vascular bundles; within the vascular bundles, the xylem is located on the inside and the phloem on the outside. B—portion of a transverse section of a white dead-nettle (Lamium album) stem (35x).

Fig. 4.42. Distribution of vascular bundles

In monocots, the vascular bundles are scattered throughout the cross-section of the stem (part of a corn stem cross-section, 50x) (cf. Fig. 3.26, A); however, the xylem within the bundles always faces inward (downward in the photo). The protoxylem is often destroyed due to stretching.

Vascular tissue (see 3.2.4.3). In herbaceous dicots, individual vascular bundles are arranged around the pith. Open collateral bundles (xylem inside, phloem outside) are separated from one another by parenchymatous medullary rays ("interfascicular parenchyma"). The phloem region is frequently accompanied on the outside by a strand of tightly packed protophloem fibers. Because of the characteristic shape of this mechanical and protective tissue in the stem cross-section, it is often referred to as sclerenchymatous caps.

The ring of vascular bundles is frequently surrounded by a specialized cell layer—the stem endodermis. The cells of this single-layered internal barrier tissue (see 3.2.2.3) fit closely together and often contain numerous amyloplasts (starch sheath). In some plants (e.g., primroses, composites), Casparian strips are visible on the anticlinal walls of the endodermal cells. In others, by contrast, the stem endodermis is practically absent.

Primary cortex parenchyma. This tissue is located between the ring of vascular bundles and the epidermis. It is typically chlorenchyma. The peripheral Regions of the primary cortex are frequently composed of collenchyma.

Epidermis. Together with the cuticle, this tissue forms the outer protective layer of the stem (see 3.2.2.1); it almost always contains idioblasts. Typical Components of the stem epidermis also include stomata and trichomes, which are frequently glandular.

The primary cortex and epidermis constitute the cortex¹ (from Latin cortex – bark).

¹ In modern botany, primary cortex and cortex are synonymous. – Ed. note.

This cross-sectional pattern can vary considerably. In woody true dicots and gymnosperms, where the primary structure undergoes significant changes over time due to secondary thickening, rings of vascular tissues develop instead of a ring of discrete vascular bundles, interrupted at intervals by narrow, low medullary rays (see Fig. 4.45, C).

The stem anatomy of monocots differs more markedly. Their closed collateral vascular bundles are not arranged in a ring, but are instead scattered throughout the entire stem cross-section (see Fig. 4.42), meaning that neither the pith nor the cortex is differentiated as clearly demarcated regions.

The vascular bundles of the stem and roots of a single plant form an anatomically and functionally interconnected system known as the stele (Box 4.3).

Box

The stele (from Greek στήλη – pillar) refers to the entire vascular system of the axial organs (stem and root) at the primary growth stage. The structure of the stele varies among different groups of cormophytes, showing particularly high diversity in pteridophytes. Nevertheless, distinct stele types were already identified in the 19th century, though their evolutionary significance and The Theory of stelar evolution (stelar theory) were developed only later. It is customary to distinguish the following stele types (Fig. A).

Fig. A. Types of vascular tissue arrangement in stems: top, cross sections, xylem in grey, protoxylem in black¹; bottom, 3D diagram (after D. von Denffer)

P—protostele, A—actinostele, S—siphonostele, D—cylinder of vascular bundles with leaf gaps (dictyostele, cf. Fig. B), E—eustele

¹ In a radial root bundle, protoxylem is always located at the very tips of the xylem ridges (exarch²). — Ed. note.

² By definition, xylem in a protostele always occupies a central position. — Ed. note.

Protostele — a central, concentric vascular system, often (though not always) with xylem in the center¹. The protostele is considered the most ancient type; it was typical of the earliest land plants (see Fig. 11.130, C) and is still found today in the juvenile forms of many ferns.

¹ By definition, xylem in a protostele always occupies a central position. — Ed. note.

Actinostele — a robust central strand whose (inner) xylem appears star-shaped in cross section, with phloem located between its rays¹ (from the Greek actinotos, radiant). The actinostele already appeared in prepteridophytes² and is currently found in Psilotopsida (see Figs. 11.144, B; 13.131, B) and clubmosses. The central cylinder of roots also corresponds to this stele type³ (see 4.4.2.1).

In steles of both aforementioned types, the center of the axis is occupied by vascular tissue, and a pith is normally absent. In all Other types of steles, by contrast, the center of the axial organ lacks vascular tissue, allowing for the formation of pith tissue ( parenchyma) or a central cavity.

Polystele — a system of parallel, longitudinal, most commonly concentric vascular bundles distributed throughout the Cross section of the stem. The polystele can be derived from the actinostele through enhanced longitudinal splitting of the latter. The plectostele can be distinguished as an intermediate form.

Plectostele is characteristic of most clubmoss species⁴ (see Fig. 11.134, L; from the Greek plectos, braided or twisted).

¹ In an actinostele, the phloem surrounds the xylem on all sides rather than being situated between its lobes. — Ed. note.

² This group is now classified into the Zygopteridopsida and Cladoxylopsida, and the term "prepteridophytes" is no longer used. — Ed. note.

³ The root stele corresponds to a radial bundle rather than an actinostele. — Ed. note.

⁴ The polystele arises from the actinostele independently of the plectostele. Its elements—the meristeles—are not scattered across the stem cross-section, but are arranged in a ring. An indispensable attribute of the polystele is the radial branching of leaf traces from the meristeles. — Ed. note

Siphonostele—a tubular strand of conducting tissues with a central pith, characteristic of certain fern families (see Fig. 11.151, A; from Greek siphon, tube). The hollow cylinder of conducting tissues almost always has gaps (leaf gaps) where conducting bundles (leaf traces) branch off from the stele into the leaves. The dictyostele is very closely related to the siphonostele¹.

¹ A distinction is made between the ectophloic siphonostele, in which the phloem is located outside the xylem (a very rare variant!), and the amphiphloic siphonostele (solenostele), in which, alongside the outer phloem, there is also an inner phloem between the xylem and the pith. The inner phloem is separated from the latter by an inner pericycle and an inner endodermis. — Ed. note

Dictyostele—the typical "bundle tube" of most ferns (Fig. B). This reticulate system of bundles (from Greek diktyon, net) is composed of concentric conducting bundles¹, each surrounded by sheaths of pericycle and endodermis. This distinguishes the dictyostele from the eustele.

¹ More precisely, meristeles. — Ed. note

Fig. B. A—bundle tube of the male fern Dryopteris filix-mas, isolated by maceration as an example of a dictyostele; the obliquely departing leaf trace strands are cut; B—bundle tube of the cactus Trichocereus pasacana, used as a waste basket (A—after J. Reinke; B—photo by W. Barthlott)

Eusteleis characteristic of Magnoliidae and eudicots (see Fig. 4.41). The eustele generally corresponds to a single concentric conducting system with a pith, whose rays divide the conducting tissue into several seemingly independent conducting bundles; each of these is therefore no longer concentric, but collateral¹. The entire stele is surrounded by a common endodermis. In woody plants among eudicots (which are phylogenetically older than shrubs and herbs), the Separation of the concentric conducting system into individual collateral bundles has either not yet occurred or is only weakly developed.

¹ Unlike the dictyostele, the eustele is composed of leaf traces. Despite its external resemblance, it has little in common with the dictyostele; it originated from the polystele through A change in the branching direction of leaf traces from the meristeles—from radial to tangential—followed by the reduction of the meristeles and their replacement by a system of fused leaf traces. — Ed. note

Atactosteleof monocots (Fig. 4.43; Greek atactos, disorderly) can also ultimately be reduced to a single concentric conducting system. Since the individual bundles here are also collateral, their xylem pole is oriented toward the center of the stem; a common stele sheath is sometimes discernible. Thus, the similarity to the polystele is purely external and non-essential¹. However, here (as in the conducting bundles of ferns), the procambium is completely consumed during the formation of phloem and xylem, resulting in closed conducting bundles.

¹ The atactostele is a derivative of the eustele. — Ed. note

Fig. 4.43. Cambial initials as stem cells (A—after L. Jost; B—after Holman and Robbins): A—diagram of The sequence of divisions (cross-section), initials are dark; a—before division, b—after, the arrow points to the stem periphery; B—differentiation of cells laid down by the initial; x—wood cells; r—bast cells; K—initial

4.2.7.3. Primary Thickening

Due to cell divisions and their subsequent growth, the shoot apex grows not only in length, but also in thickness: primary thickening. The interplay of longitudinal and transverse growth determines the structure of the meristematic cone, which can vary greatly. If longitudinal growth predominates, the shoot apex is narrow and pointed (see Fig. 3.3, A, C); if thickening growth predominates, it is blunt or flat. In extreme cases—palms, cacti, rosette plants—an apical pit may even form.

In large palms, which nevertheless reach heights of over 50 m without secondary thickening, primary thickening of the trunk through the prolonged activity of the meristematic mantle leads to the formation of a saucer-shaped apex up to 30 cm in diameter. The trunk thus acquires its final diameter, which does not change during subsequent longitudinal growth; the palm trunk maintains a uniform thickness throughout, resembling a slender Column. As a rule, it does not branch, and therefore does not form a typical crown, instead bearing a tuft of large leaves at the summit.

Various eudicots also exhibit strong primary thickening, affecting primarily either the primary cortex (cortical form: cacti) or the pith (medullary form: celery, kohlrabi; potato tubers). In both cases, storage parenchyma proliferates.

During the development of a shoot-bearing plant, the size of the shoot apex also changes. In the embryo, the initial zone of the shoot apex is usually tiny, but during germination it enlarges through cell division of the primary meristem. Correspondingly, the circumference of the stem also increases (with constant primary thickening): Amplification growth. Finally, the diameter of the apex reaches a maximum and then decreases again as the plant transitions to the flowering phase. As a result of these changes, the stem takes on the shape of a double cone. This is particularly noticeable in annual monocots, as it is not masked by subsequent secondary thickening (see Fig. 11.220).

4.2.8. Secondary Stem Structure

4.2.8.1. Functional Significance of Secondary Thickening

Old conifers and deciduous trees are the largest terrestrial living organisms; the tops of giant sequoias and eucalyptus trees can tower more than 100 m above the ground. In most cases, the trunks support a crown weighing over a ton and can withstand immense bending loads during storms. Just as the shoot system branches out in the air, the root system branches in the soil—a reflection of the bipolar organization of all leafy plants. However, all Metabolic exchange between the crown and the root system must take place through the trunk, which connects both systems and becomes the true central organ (usually the only one) of an otherwise open, decentralized plant organization¹. This dual function—support and transport—requires a thickening trunk that must correspond to the size of the root system and the mass of foliage or needles. Trunk thickening is achieved through secondary thickening, which in turn is driven by cambial activity (see 3.1.2). This produces predominantly secondary xylem—wood. In later stages of thickening, wood constitutes more than ⁴/₅ of the secondary increment and, consequently, of the trunk as a whole.

¹ All Living organisms are open systems. The authors conflate The concepts of open and modular organization. — Ed. note

Naturally, roots also undergo secondary thickening (see 4.4.2.3).

The special properties of wood, based on its cellular structure, chemistry, and the texture of lignified cell walls, have since ancient times made wood critically important to humanity as a building material and an energy source (firewood, charcoal). The economic value of wood remains high today. According to FAO data, more than 3.3 billion m³ of roundwood was harvested worldwide in 1998.

4.2.8.2. Cambium, Wood, and Bast

In a fully developed tree trunk, the cambium is represented by a hollow cylinder one cell layer thick. It develops from the procambium of the shoot apex and consists of two types of cambial initial cells: isodiametric ray initials and elongated fusiform initials. Ray initials form the parenchyma of the Primary and secondary rays, i.e., the transverse (horizontal) conducting system of woody stems¹. Fusiform initials (from Latin fusus, spindle, due to the shape of these cells) divide to form the strand conducting system². These initials are elongated, tapered at the ends, but otherwise flattened cells that are stretched longitudinally and flattened tangentially (periclinally) within the stem. They are vacuolated and relatively large, reaching up to 5 mm in length in conifers.

¹ The conducting function of ray parenchyma is very limited. It primarily controls processes occurring in the dead elements of wood and oak, stores nutrients, and serves as a reservoir for Metabolic waste products. — Ed. note

² They also form fibrous mechanical elements. — Editor's note.

Cambial initials divide predominantly in such a way that the newly formed wall is oriented periclinally. This means that new cells are cut off from the cambium (which appears as a ring in transverse section) alternately inwards and outwards in the radial direction (see Fig. 4.43). This gives rise to radial rows of cells that are characteristic of tissues formed by the cambium. The aggregate of cells cut off inwards constitutes the wood, which histologically corresponds to secondary xylem with primary and secondary wood rays¹. All cells cut off outwards constitute the secondary phloem, or bast ². Derivatives of the cambial initials differentiate very rapidly. This is possible because the fusiform cambial initials are already strongly vacuolated, so that the postembryonic elongation growth phase is omitted³. As a consequence, unlike primary meristems, stem cells here have the highest division frequency, while their derivatives rarely divide even once more⁴. Auxin concentration is also maximal in the cambium (see Fig. 7.38), especially at the beginning of the vegetative period when "early wood" is formed (see 4.2.8.5).

¹ Rays are not a separate tissue, but a component of the secondary xylem. — Editor's note.

² Like secondary wood, secondary bast consists of axial (system) elements and rays that represent a continuation of the wood rays. — Editor's note.

³ In many cases, elongation growth does occur, albeit to a minor extent. — Editor's note.

⁴ This statement is incorrect. — Editor's note.

The circumference of the cambial cylinder increases as a result of secondary thickening; this is known as dilatation growth. In European trees, the cambial circumference increases about 1,000-fold compared to the initial state, and in exotic giant trees it is even greater. The size of the cambial initials remains approximately constant, which is why their number at a given height of the stem inevitably increases over time.

The necessary increase in the number of initials occurs through their longitudinal divisions, which form anticlinal (radial) longitudinal cell walls. In this case, a storied cambium arises, which is typical of many tropical trees. In trees of temperate and cold zones, by contrast, the cambial initials first undergo transverse division, after which the upper and lower daughter cells grow with their tips longitudinally between adjacent initials — an example of intrusive growth (see 3.2.3)¹. In this way, a cambium is formed whose cellular pattern in the tangential plane is less ordered than that of a storied cambium; such a cambium is called non-storied.

¹ These divisions are not transverse, but oblique. — Editor's note.

Although the procambium develops as a continuous hollow cylinder, the cambium at the primary stem structure stage is often restricted to vascular bundles (fascicular cambium) separated by parenchymatous pith rays. When secondary thickening begins in such a stem, the formation of interfascicular cambium is first induced, resulting in a continuous cambial layer (Fig. 4.44). This process is associated with the re-embryoniation of already differentiated parenchyma cells of the pith rays.

Fig. 4.44. Origin of the interfascicular cambium (arrows) on the sides of the vascular bundle cambium through re-embryoniation and resumption of divisions in the parenchyma cells of the pith rays of pipevine, Aristolochia durior (80×) (after E. Strasburger)

In lianas, whose woody stems show little thickening (their mechanical support function is not prominent in these plants), initials that arise secondarily in the parenchyma of the pith rays subsequently form only ray parenchyma. Therefore, the primary (pith) rays remain clearly visible and separate clearly defined individual vascular bundles in each internode (Aristolochia type, Fig. 4.45, A; cf. also Fig. 4.50, C; 3.21, L). Individual vascular bundles surrounded by elastic parenchyma act here like the strands of a cable, making the liana stems resistant to tension yet simultaneously flexible.

However, in many woody stems, the majority of newly formed cambial initials in the pith rays become fusiform and cut off prosenchymatous cells of the conducting and mechanical tissues. In this case, the pith rays are restricted to narrow bands of parenchyma (Ricinus type, Fig. 4.45, B). Finally, in typical trees, the procambium differentiates into a cylinder of closely packed vascular bundles and a continuous cambial layer (Tilia type, Fig. 4.45, C). Only above the branch points of leaf and branch traces are leaf and branch gaps initially formed, which are later closed by conducting tissues.

Fig. 4.45. Types of secondary thickening in dicotyledons (after D. von Denffer)

A — Aristolochia type, B — Ricinus type, C — Tilia type, a–c — Formation of primary structure, d — secondary thickening

Primary (pith) rays, which extend from the pith to the primary cortex, are pushed further and further apart at the periphery of the wood cylinder and especially in the bast during secondary thickening, so that they are no longer capable of performing the function of a transversal (radial) transport system and storage function. This necessitates the Formation of secondary wood-bast rays by the transformation of some fusiform initials into ray initials. Secondary rays (formerly often erroneously called "secondary medullary rays") do not extend from the pith to the primary cortex, but end blindly in the wood and bast. They are shorter the later the transformation of the initials took place. The sites of such initial transformations are arranged in such a way that the rays form a regular pattern in tangential section (see Fig. 4.51): wherever the distance between rays exceeds a certain limit due to secondary thickening, a new ray is laid down (see 4.2.2).

In conifers, the rays are usually a few cells high and only a single cell row wide; their volume accounts for less than 1/10 of the total wood volume. In deciduous trees, the rays are often several cells wide and up to 100 cells high, and their specific volume is noticeably higher than 10%, occasionally reaching 1/5 of the wood volume (Fig. 4.51, D).

4.2.8.3. Secondary thickening in monocotyledons

Monocotyledonous plants have an atactostele and closed vascular bundles, and therefore they lack both fundamental Prerequisites for the formation of a cambial cylinder. In fact, they lack cambial secondary thickening. It is therefore not surprising that almost all species of trees and shrubs belong to gymnosperms, magnoliids, or eudicots. We have also seen that in palms the final stem diameter is established during primary thickening (see

4.2.7.3). Only some arborescent lilioids (e.g., Dracaena, as well as certain species of Yucca and Aloe) exhibit secondary thickening, which, however, proceeds in a completely different manner than in gymnosperms and dicotyledons (Fig. 4.46): here, a secondary thickening meristem functions as the cambium, surrounding the entire stele and producing parenchyma inwards with secondary vascular bundles.

Fig. 4.46. Secondary thickening in the arborescent monocot Dracaena (dragon tree, cf. Fig. 11.229) (A, B — after W. Troll; C — after G. Haberlandt): A — Primary structure of the stem in transverse section, vascular bundles in black; B — secondary structure; the cambial ring (the secondary thickening meristem of monocotyledons is incorrectly called a cambium — Editor's note) has produced parenchyma inwards with secondary vascular bundles (light); C — enlarged section of the slice in the cambial region, with concentric vascular bundles at various stages of maturity in the secondary parenchyma (90×)

4.2.8.4. Wood

The wood in a living tree or shrub performs three main functions that determine the specific structure of its Cells and Tissues: a mechanical system fulfills the support function, water and mineral salts are transported by the water-conducting system, and assimilation products are stored by the storage system. Four forms of wood cell elements can be distinguished (Fig. 4.47) and assigned to these functional systems.

Tracheids — dead tubular cells 1 – 5 mm long (up to 8 mm maximum) with strongly thickened, lignified walls and tapering ends, where bordered pits are crowded¹ (see Fig. 2.75, C–H). Tracheids belong to both the mechanical and the conducting system. The maximum water flow velocity in tracheids is about 0.4 mm/s.

Vessel elements — also dead, water-filled tubular cells with bordered pits (the latter are generally specific to cells of the water-conducting system)²; however, they are significantly shorter and wider than tracheids; their lignified walls are moderately thickened, and the transverse cell walls between successively arranged vessel elements dissolve (forming simple or scalariform perforation plates, respectively; see Fig. 3.24). Thus, consecutive axial vessel elements form a long tube — a vessel. They belong exclusively to the conducting system. Their diameter can exceed 0.7 mm. The resistance to water flow is correspondingly reduced, and the flow velocity reaches 15 mm/s, and in extreme cases 40 mm/s (see 6.3.5; Table 6.14).

¹ In the vast majority of cases, bordered pits are more or less evenly distributed along the entire length of the tracheid. — Ed. note.

² Found only in vascular plants (Tracheophyta). — Ed. note.

Рис. 4.47. Cell types in hardwood (150x) (after E. Strasburger): A—wood parenchyma, B, C—non-septate and septate cambiform elements; D—libriform fiber; E—fiber-tracheid; F, G—pitted and spiral tracheids, H, I—vessels: H—scalariform perforation plates, I—vessel with pitted perforation and lysed end walls between vessel elements (cf. Fig. 3.24)

Libriform fibers are similar in shape and size to tracheids, but their walls are even thicker and lack bordered pits. The secondary wall Cellulose is deposited as spiral fibrils (see Fig. 3.20, D). Transitional forms—fiber-tracheids—occur between tracheids and fibers. Intermediate forms are also found between fibers and axial parenchyma: living "cambiform" elements that can be unicellular or multicellular. Libriform fibers are often, but not always, dead; in the former case, they belong exclusively to the mechanical system, while in the latter, they belong to both the mechanical system and the storage system.

Tracheids, vessels, and libriform fibers in stems and branches are oriented longitudinally (the only exception being ray tracheids in conifer wood, see below).

Axial parenchyma cells are living. They serve to store starch and/or oil, and when necessary, also for The transport of organic nutrients.

4.2.8.5. Gymnosperm Wood

Conifer wood consists mainly of tracheids (Fig. 4.48) and is fairly homogeneous and uniform. Tightly packed tracheids simultaneously perform the Functions of the water-conducting and mechanical systems. Vessels are absent, and parenchyma is represented by wood rays and the epithelial cells of resin ducts (if present).

Half-bordered pits are formed between tracheids and the parenchymatous cells of wood rays; these are particularly large in pines—just one per cell contact, forming a "fenestrate pit" (see Fig. 4.51, C). Along the upper and lower margins of the wood rays run strands of ray tracheids—elongated, dead cells with bordered pits on their walls, which facilitate radial water transport.

Resin ducts (see Fig. 3.29), partly longitudinal and partly radial within the rays, form an interconnected system of tubes throughout the conifer stem. Resin oozing out upon injury forms an antiseptic "bandage" over the wound. Accordingly, additional resin ducts form in response to damage, whereas in firs, whose undamaged wood lacks resin ducts, traumatic resin ducts are produced upon injury.

Secondary thickening in woody plants of temperate latitudes is restricted to the period from late April to early September, occurring discretely. Specifically, so-called earlywood is formed until July, followed by latewood while cambial activity continues. Latewood tracheids have thicker walls and consequently narrower lumina than earlywood tracheids. However, the transition from earlywood to latewood is gradual. The sharp, naked-eye-visible boundaries of growth rings, which determine the grain patterns of wood cross-sections, result from the fact that the final latewood tracheids are particularly thick-walled and narrow, whereas the first earlywood tracheids formed in the next growing season, by contrast, have very thin walls and wide lumina.

Fig. 4.48. Schematic representation of a conifer stem section in the cambial region (after K. Magdefrau).

Directions of sections: top—transverse; right—radial; left—tangential. 1—latewood with vertical and horizontal (in the wood ray) resin ducts, secretory cells (these are epithelial cells. — Ed. note) shown in black, 2—earlingwood; large bordered pits between tracheids are located exclusively on radial walls (cf. Fig. 2.75, C–F); C—cambium; S—conducting phloem with axial parenchyma; P—obliterated outer sieve cells. Bottom, on the radial section: a longitudinally sliced pith ray, bounded above and below by a single row of ray tracheids, which continue into the phloem as Strasburger cell rows; between them lie 4 rows of ray parenchyma cells (250x)

Tree growth rings also form outside temperate latitudes where seasonal precipitation fluctuations occur (such as alternating rainy and dry seasons). In permanently humid tropical regions, tree growth rings do not form.

The age of a tree can be determined quite accurately by counting its growth rings (up to 3,500 years for sequoias with trunk diameters up to 6 m; over 4,800 years for the bristlecone pine Pinus longaeva; up to 3,600 years for the South American conifer Fitzroya cupressoides). Growth rings also make it possible to reconstruct climatic fluctuations over the lifespan of a tree; in dry years they are narrow with a high proportion of latewood, while in wet years the rings are wider. Such a chronology provides a way to precisely date prehistoric wood finds going back to periods long before the oldest living trees today (dendrochronology). Using the method of cross-dating (Fig. 4.49), researchers in the United States have achieved precise dating back to the seventh millennium BC. In Europe, chronologies spanning the last 2,500 years have been established using oak and fir. Known errors of this method are associated with the formation of "false" additional growth rings resulting from a second flush of foliage and shoot growth caused by sudden temperature drops during a single growing season, or due to pest damage; counting difficulties can also arise from indistinct boundaries between growth rings. Nevertheless, dendrochronology has successfully revealed drastic climate fluctuations over a period of some 50,000 years.

Fig. 4.49. Archaeological dating using the dendrochronological cross-dating method (after Glock).

The oldest tree rings of a living tree are synchronized with the outermost rings of logs from recent historical buildings, and so on, extending back into prehistoric times.

4.2.8.6. Angiosperm Wood

The wood of deciduous trees and shrubs has a much more complex structure than that of conifers. The differentiation of libriform fibers¹ and vessels has led to a functional division between the water-conducting and mechanical systems.

The Increasing complexity of wood composition reflects evolutionary development. Woody gymnosperms appeared in the Permian, approximately 260 million years ago. This occurred rather in the cold climate that currently dominates the taiga and mountain forests of the temperate zone, which serve as (albeit very extensive) refugia for conifers today². Deciduous trees arose only about 100 million years ago, in the warm climate of the mid-Cretaceous, which today prevails in the tropics and subtropics and is especially pronounced in tropical rainforests. Once they emerged, deciduous woody species spread very rapidly, a process that continues to this day. Alongside other features, their complex wood—featuring diverse combinations of histological elements—proved better adapted to varying environmental conditions than the uniform tracheidal wood of gymnosperms.

The ongoing evolution of angiosperm wood can be reconstructed by studying extant representatives. Alongside relatively "primitive" wood types whose bulk still consists of tracheids³ (such as in the sweet chestnut), there are all transitions to woods in which tracheids are partially (e.g., oak, elm, walnut, horse chestnut) and ultimately completely replaced by libriform fibers interspersed with storage parenchyma (apotracheal parenchyma in ash and maple).

¹ A great many plants have fiber-tracheids rather than libriform fibers. — Ed. note.

² This is a completely fantastic claim by the authors. Gymnosperms arose in the Early Carboniferous under humid tropical climatic conditions. — Ed. note.

³ In some angiosperms, the wood is tracheidal, just as in gymnosperms. — Ed. note.

Рис. 4.50. Diffuse-porous and ring-porous woods in cross-section. A, B—Small-leaved lime (Tilia cordata) features diffuse-porous wood with relatively narrow vessels (100 µm diameter): A—with three growth ring boundaries (25x), B—with one (70x). Ring-porous wood: C—Aristolochia sipho, a liana with pores restricted to the earlywood of each annual increment (actually, vessels are present in both earlywood and latewood, but they are wide-lumen in earlywood and narrow-lumen in latewood — Ed. note); wide wood rays, with calcium oxalate druses appearing as dark dots within them; asterisks mark the origins of new wood rays (25x); D—three growth ring boundaries in pedunculate oak (Quercus robur). Large earlywood vessels (up to 500 µm in diameter) are surrounded by axial parenchyma, while narrow latewood vessels are embedded among tracheids. Dark zones correspond to densely packed wood fibers (25x). Oak wood is classified as a hardwood due to the high density of its Cell wall material.

The vessels are not strictly parallel to the axis of the stem, but are slightly sinuous; consequently, within the annual growth increment, they converge at certain points, creating the visual impression on a transverse section of being grouped together. In the areas of contact, bordered pits (in hardwoods, most often scalariform with oval borders — see Fig. 2.75, G, H) are particularly numerous, functionally forming a vascular network.

In many European hardwoods, the wood is diffuse-porous: narrow-lumen vessels (diameter <100 µm) are distributed in large numbers throughout the annual growth increment (examples include beech, birch, alder, willow, poplar, maple, horse chestnut, and linden; Fig. 4.50, A, B). In oak, elm, and sweet chestnut, by contrast, earlywood contains a small number of wide-lumen¹ vessels (diameter >100 µm) that are visible to the naked eye — this is ring-porous wood (Fig. 4.50, C, D). The vessels are associated (especially in ring-porous woods) with paratracheal parenchyma, referred to as contact parenchyma due to the numerous pits connecting its cells to the vessel elements. The cells of this parenchyma are of a glandular type.

Indeed, when transpiration-driven suction ceases under high air humidity and the supply of mineral salts to rapidly growing shoots is interrupted, the glandular parenchyma cells can secrete sugars and other organic substances into the vessels. The sugar in the xylem osmotically draws in water, which can only move upward through the vessels (downward flow is prevented by the root endodermis). Within the crown, the sugar-containing water can be excreted through hydathodes via guttation (see 6.3.4.2), after which the leaf cells receive the salts they need. These functional connections make it clear why paratracheal parenchyma is so robustly developed in the trunks of large trees in tropical rain forests, forming a multilayered sheath that surrounds individual vessels. Wide-lumen vessels in ring-porous woods are likewise surrounded by a sheath of paratracheal parenchyma (Fig. 4.51, D). Tree and shrub species with contact parenchyma are particularly adapted to the Mediterranean climate with its short growth periods between a mild, wet winter and a hot, dry summer. In diffuse-porous woods, contact parenchyma is poorly developed. Such wood is typical of regions where soils are moist while the air is rarely saturated with vapor. However, even in these trees (for example, in Central European species), the paratracheal contact parenchyma becomes active in spring, immediately before leaf unfolding. At this time, sap flow begins, organic reserves in the wood parenchyma are mobilized, and they pass into the vessels even before leaf transpiration is initiated. The liquid that generates high pressure in the vessels is an aqueous solution of various organic substances, primarily sugars and Amino Acids. When the stem is damaged, it exudes from the vessels (sometimes in large quantities) as sap (see 6.3.5). Later, during the growing season proper, the transpiration of the expanded leaves provides the energy required to lift the liquid against gravity and frictional forces, at which point negative pressure develops within the vessels.

Fig. 4.51. Primary and secondary rays (A — after E. Strasburger)

A–C of pine *Pinus sylvestris*. A — piece of a four-year-old branch: top — transverse section, right — longitudinal radial section, left — longitudinal tangential section (6×). c — cambium, SR — secondary cortex, B — periderm, M — pith, 1–4 — successive annual rings, pm — primary ray, sm — secondary ray, m — ray in tangential section, r — ray in phloem, h — resin ducts. B — tangential section showing numerous uniseriate rays (cut across) between longitudinally sliced tracheids, whose oblique walls bear bordered pits; two wide rays contain a resin duct H (75×). C — radial section of wood showing tracheids with large bordered pits; at the bottom — a longitudinally sectioned ray with a central row of parenchymatous cells P connected to the tracheids via large, square window-like pits; at the top and bottom — horizontal ray tracheids with small bordered pits (150×). D — tangential section of the wood of pedunculate oak (*Quercus robur*) with a single vessel (*), a region of paratracheal strand parenchyma K, and numerous uniseriate rays within a dense mass of libriform fibers; to the right are several multiseriate, aggregated rays formed as a result of fusiform initials dropping out of the cambium between adjacent rays. This process, which is very common in oak, gives rise to tall and wide aggregated rays (75×).

The wood rays of angiosperms are generally larger — i.e., taller and wider — than those of gymnosperms, and consequently consist of a significantly greater number of cells (Fig. 4.51, B). Ray parenchyma is connected to the vessels at specific sites. Furthermore, together with paratracheal and (where present) apotracheal parenchyma, it forms a loose network of living elements that permeates the wood in all directions and accounts for 1/4 to 1/3 of its total volume.

In the wood of angiosperms growing in regions with a pronounced seasonal climate, just as in conifers, well-defined annual rings are formed, corresponding to yearly growth zones. Therefore, dendrochronological studies can also be conducted on the stems of deciduous trees (see Fig. 4.49).

4.2.8.7. Sapwood and Heartwood

Wood is predominantly dead tissue from the very beginning due to the multitude of dead cells (tracheids, vessel elements, wood fibers). The lifespan of wood parenchyma is also limited; it dies off in older annual rings. Consequently, during prolonged secondary thickening, no living cells remain at all in the central region of a tree trunk — a condition known as heartwood. The "living" (functioning) wood of the outer parts of the trunk is called sapwood. While in many diffuse-porous woods the water-conducting capacity of vessels is retained for more than 20 years, in ring-porous woods (ash, sweet chestnut, elm, robinia) it declines after just a few years, and in oak, as early as the second year. Even in these trees, however, heartwood forms later, so a distinction must be made between conducting sapwood, which belongs to the still-active water-conduction system, and storage sapwood, whose functions are limited to substance storage and mechanical support.

Trees with narrow conducting sapwood are particularly vulnerable to external damage, such as severe stem heating from prolonged exposure to sunlight, as well as mechanical injury and fungal attack. This often manifests as continent-scale epidemics (epiphytotics) (such as oak and chestnut declines in North America, and Dutch elm disease caused by ascomycetes spread by bark beetles).

The transformation of wood into heartwood is not a simple, slow dying-off process, but rather an active one. Gradually, the vessels fill with air, and adjacent wood parenchyma cells grow into them through pits (formation of tyloses, Fig. 4.52; from the Greek *thyllis* — sack), blocking them up¹. The reserve substances still remaining in the parenchyma are mobilized and transported away from it, though they are often utilized for the formation of tyloses and special substances characteristic of heartwood (primarily Tannins and resins). Similarly, valuable mineral nutrients (P, K, S) are relocated to the sapwood, whereas excesses of elements such as Ca or Si are instead deposited in the heartwood.

¹ Desiccation of inner tracheids, but without the formation of tyloses, also occurs in the wood of gymnosperms. However, such non-conducting wood differs less markedly from the sapwood in these plants than angiosperm heartwood does, which is why it is termed ripe wood. — *Ed. note.*

Рис. 4.52. Occlusion of a vessel by tyloses (A — after H. Schenck; B — SEM photo by S. Fink): A — wood parenchyma cells U grow through pits into the vessel lumen and occlude it (transverse section of robinia heartwood, 250×); B — tyloses growing into a vessel in the tropical hardwood *Nectandra pichunum*, Lauraceae (170×)

For instance, the exceptional durability of teak wood and its resistance to all kinds of damage are due to heavy silica deposition.

The heartwood of many conifer and hardwood species is the most technically valuable part of the timber. As the vessels fill with air, the deposited tannins, which provide resistance against pests, can undergo further oxidation into colored phlobaphenes. This yields a beautiful, naturally colored wood that is simultaneously distinguished by high durability. The heartwood of such trees as mahogany (*Swietenia mahagoni*), rosewood (*Dalbergia*), teak (*Tectona grandis*), and ebony (*Diospyros* species) is particularly prized.

4.2.8.8. Phloem (Bast)

Like wood, secondary phloem is microscopically heterogeneous (Fig. 4.53) in accordance with its diverse functional requirements: long-distance Transport of Assimilates (sieve elements — sieve cells or sieve tube elements, see 3.2.4.1); accumulation of assimilates and radial short-distance transport (phloem parenchyma and rays); support and mechanical protection (sclerenchyma — bast fibers and stone cells; crystal-bearing cells).

Рис. 4.53. Phloem (B, C — after D. von Denffer)

A — transverse section of a multi-year branch of linden (*Tilia platyphyllos*): 1 — primary cortex and primary phloem (marked with an asterisk in two places), 2 — secondary phloem, in which tangential layers of hard phloem (dark) and soft phloem (light) alternate; triangular light parenchymatous zones in the secondary phloem are phloem rays narrowing centripetally to the width of the wood rays; numerous black dots in the cortical parenchyma are crystal-bearing cells containing calcium oxalate druses (23×). Bo — periderm, sPh — secondary phloem. B, C — secondary cortex of grapevine (*Vitis vinifera*) in longitudinal and transverse sections (200×): s — sieve tubes, spl — sieve plates, g — companion cells, p — strand parenchyma, M — ray parenchyma with reserve starch, kr — crystal-bearing cells, wB — soft phloem (sieve tubes + companion cells) (soft phloem also includes strand parenchyma — *Ed. note*), hB — hard phloem (bast fibers), C — cambium, H — wood.

Sieve elements of the phloem replace the sieve elements of the primary phloem, thereby maintaining continuous conducting pathways from shoot apices and leaves down into the roots. Anucleate sieve elements remain alive through contact with glandular-type parenchymal cells via numerous perforations in adjacent cell walls. These cells maintain the functioning of sieve elements as transport pathways by unloading and loading the assimilate solution (companion cells in hardwoods; Strasburger cells in gymnosperms).

Phloem rays are the continuation of wood rays; they link the wood and phloem together, passing through the cambium. The parenchymal cells of phloem rays are most frequently packed with reserve substances (starch, oil). The same is characteristic of the cells of strand phloem parenchyma.

Bast fibers often become very long (see 3.2.3); during development, their cells intrusively elongate by growing their tips between hundreds of other cells. They gave their name to the entire tissue complex; mature bast fibers from willow and linden branches — bast — are used by gardeners for tying up saplings and branches, as well as for wrapping graft unions.

In accordance with THE POSITION OF the cambium between the wood and the phloem, the woody cylinder grows in thickness at its periphery, whereas the phloem grows from the inside outward. While the oldest parts of the wood are located on the inside and the youngest on the outside, the oldest regions of the phloem are situated at the periphery and the youngest on the inside.

Living sieve elements and phloem parenchyma together constitute the soft phloem. Its layers alternate with tangential layers composed of bast fibers, known as hard phloem. This lamination generally masks the indistinctly delimited annual growth Zones of the phloem, each of which contains several layers of soft and hard phloem.

Sieve elements typically function for only one year. This means that all assimilate transport in a thick tree trunk is concentrated in a layer of conducting phloem only 1 mm thick, which lies directly adjacent to the outside of the cambium and accounts for no more than 5 % of the trunk's volume. In older phloem regions — the non-conducting phloem — the sieve elements and their companion cells die and are crushed by neighboring cells. Here, individual strand parenchyma cells abruptly begin to grow, increasing significantly in size. As a result, they not only fill the space previously occupied by the sieve elements, but also effect diffuse dilatation of the phloem, allowing its circumference to expand in accommodation with the ongoing secondary thickening of the stem. In most woody plants, both stretching and proliferation of phloem parenchyma cells take place. Both processes occur exclusively in the living soft phloem. Some of its parenchymal cells transform into stone cells and Supplement the fiber strands if the latter are ruptured as a result of dilatation¹.

¹ Sclerification of bast parenchyma cells frequently occurs independently of any disruption in the integrity of bast fiber strands. — Editor's note.

4.2.8.9. Rhytidome (Bark)

As the stem circumference increases due to secondary thickening, many peripheral tissues undergo dilation. This is also typical of the epidermis in the stems of certain plants (such as Ilex, Cornus, Kerria, roses, and cacti) whose branches remain green for a long time. Usually, however, the epidermis does not dilate; instead, it ruptures and is replaced by periderm (see 3.2.2.2). Because the periderm is impermeable, the tissues located outside of it are cut off from the supply of water and mineral salts coming from the stem, causing them to die and dry out. Externally, this appears as a brown or gray coloration of the stem surface.¹

¹ This coloration is due to the color of the suberized walls of the phellem cells. — Editor's note.

The first periderm, which functionally replaces the epidermis, originates in the outermost part of the cortex and is called the superficial periderm¹. In some trees, the phellogen of this initial periderm remains active for many years and is capable of expanding circumferentially through dilation to match the increasing surface area of the trunk. This gives rise to the smooth trunks of beech and hornbeam, as well as young birches. In most trees, however, under the pressure of the internal Tissues of the continually thickening trunk, the periderm ruptures primarily with longitudinal cracks, since the trunk and branches—having initiated secondary thickening—no longer grow in length. These cracks are closed from the inside through the formation of new periderms in deeper, still-living zones of the cortex and, eventually, in the phloem (inner periderm). The phellogen of the inner periderm is usually active for only a short time, but increasingly deeper periderms continue to develop. An ever-thickening mantle of dead tissue accumulates on the trunk surface, penetrated by numerous thin, periclinal layers of phellem that gradually crack inward. This dead tissue complex, continually replenished from the inside (Fig. 4.54), constitutes the tertiary protective tissue known as the rhytidome² (commonly referred to as "bark" or "tree bark," whereas in botanical terminology, bark tissue refers to the parenchyma located outside the vascular bundles or cambium³). The youngest periderms are laid down close to the cambium in the non-conducting phloem, enclosing the living phloem within a very narrow pericambial zone.

¹ In many species, the very first periderm forms deep within the cortex or even in the phloem. — Editor's note.

² Rhytidome represents an anatomical-topographical zone rather than a distinct tissue. — Editor's note.

³ In botany, a distinction is made between primary and secondary bark. Primary bark is the complex of tissues outside the stele, including the epidermis, cortical parenchyma, endodermis (starch sheath), and more rarely chlorenchyma, collenchyma, or sclerenchyma. Secondary bark is the secondary phloem (bast) of trees and shrubs, covered by the periderm, primary bark, or rhytidome. — Editor's note.

In contrast, the rhytidome on larger tree trunks often reaches a thickness of several centimeters. It is sufficiently elastic to fully or partially protect the sensitive and vital conducting phloem from mechanical damage. The rhytidome is also low in water and therefore exceptionally lightweight. Due to its thickness and deposited tannins or phlobaphenes, which give it a dark color, the rhytidome provides reliable protection against parasitic Fungi and insects. For instance, a trunk covered with rhytidome is completely immune to attacks by sap-sucking aphids. When dry, rhytidome is difficult to ignite and practically non-flammable (cf. Fig. 13.7, B; 13.9). Finally, the rhytidome plays an important protective role against radiation and acts as a thermal insulator, owing to its high air content and pigmentation—since soft phloem can be irreparably damaged both by prolonged frosts and by heating above 50 °C under direct sunlight.

Fig. 4.54. Formation of periderm and rhytidome: A—cork ridges on a branch of field maple (Acer campestre, 1.6×); B—cross section through the trunk of a 96-year-old Douglas fir; between the two asterisks lie the cambium and a very thin layer of living phloem, with the dark periderm on the outside and light, wavy scales of rhytidome (0.2×); C—phellem layers (dark) interspersed with dead cortical tissue in the scaly rhytidome of ponderosa pine (Pinus ponderosa, 2.6×)

Particularly sensitive are trees that do not form a rhytidome, whose trunks are protected throughout their entire lives solely by the superficial periderm. These primarily include the beech (Fig. 4.55, D). If these trees are suddenly exposed to open conditions due to thinning, road construction, or similar activities, they can succumb to sunscald. Conversely, "sun-loving trees" of south-facing slopes, such as oaks, are exceptionally well protected by a thick rhytidome and shade-casting branches at various heights along the trunk. Within the crevices of the rhytidome, varying exposures to sunlight create sharp temperature differences even across tiny spaces, which maintain a cooling air Circulation.

In most cases (Fig. 4.55), the inner periderms are relatively short, concave, and abut older phellem layers at their edges (see Fig. 4.54, C; 4.55, E, F). Such periderms isolate scale-like tissue segments from the cortex/phloem, forming a scaly rhytidome. Older rhytidome scales are shed, a process that occurs in pine, plane tree, and sycamore along a distinct separation layer. Less commonly, the inner periderms are convex and parallel to the surface, resulting in closed cylindrical periderms known as ring rhytidome (found on young trunks and branches of juniper and cypress). In many lianas (honeysuckle, clematis, grapevine), ring rhytidome initially transitions into strip-like rhytidome through longitudinal cracking (Fig. 4.55, B, C).

Fig. 4.55. Rhytidome (G — photo by W. Barthlott): A—ring rhytidome (this is a ring-exfoliating periderm rather than a true rhytidome. — Editor's note) on the trunk of a young birch; lenticels are arranged in horizontal strips; the bright white color of the trunk is caused by multiple light reflections from betulin crystals in the air-filled cells of the periderm (0.25×); B, C—strip rhytidome in clematis (Clematis, B) and grapevine (C) (0.74×); D—beech forms a thick periderm with numerous lenticels rather than a rhytidome, which is why carvings made in it persist for a long time; its uneven stretching is related to trunk thickening in the absence of longitudinal growth (0.25×); E, F—typical scaly rhytidome in plane tree (E, 0.25×) and Scots pine (Pinus sylvestris, F, 0.12×). Deeply fissured rhytidome: G—West African savanna tree Butyrospermum paradoxum, fire-resistant thanks to its rhytidome (0.11×); H—pedunculate oak (Quercus robur, 0.25×); J—Amur cork tree (Phellodendron amurense, 0.25×)

Under natural conditions, wounds on woody trunks and thick branches are common because, unlike flexible twigs or herbaceous stems, they cannot bend under impact. If a wound reaches the wood, cell proliferation begins at its margins, forming an unstructured callus tissue (Lat. callus — thick Skin, Swelling). The slow-growing, gradually lignifying wound callus, whose surface is protected by a periderm, eventually heals the wound and, if not excessively large, can cover it completely. Normal wood, phloem, and rhytidome typically regenerate over such callus-healed wounds.