MEDICAL BOTANY - A.G. Serbin - 2003

CHAPTER 1. ANATOMY

VEGETATIVE ORGANS

STEM

The stem is the axial, supporting part of the SHOOT that develops from the embryonic stem, possessing unlimited apical growth and positive heliotropism. The stem connects the aboveground and underground Organs; Supports the entire aboveground mass in space; ensures the movement of ascending and descending substance fluxes; assimilates while green; stores nutrients; and participates in vegetative propagation. The stem is metameric, meaning it consists of repeating parts — a node with leaves, buds, shoots, and an internode. The skeletal-Vascular System of the stem is more complex than that of the ROOT. The Water/123.html">Primary Anatomical Structure of the stem originates in the growth cone at the level of leaf primordia, where procambium is laid down. After the procambium is replaced by the cambium, the Secondary anatomical structure is formed. In mature stems, the structure varies, yet remains specific to particular taxonomic groups (monocots, dicots, gymnosperms, ferns) and life forms (herbaceous, woody), as well as being distinctive in certain ecological groups (xerophytes, hydrophytes, and hygrophytes).

STEMS OF MONOCOTYLEDONOUS PLANTS

The structure of the stems in herbaceous monocots is primary and bundle-type. It is characterized by the absence of lateral Meristems — procambium, cambium, and phellogen — and the presence of numerous leaf traces (closed collateral bundles) extending from the leaves into the stem (Fig. 1.31). These are distributed across the entire cross-section of the central cylinder; specifically, at its periphery, the bundles are small and numerous, whereas in the central region they are larger and fewer in number. The pith is either morphologically indistinct or hollow.

Class="center">Fig. 1.31. Diagrams of the path and arrangement of vascular bundles in the stem of a monocotyledonous plant: A — longitudinal section; B — cross section; 1 — individual leaf traces; 2 — fused leaf traces

The primary cortex is most often poorly developed or may be entirely absent, consisting of several rows or patches of parenchyma, containing a ring or separate strands of sclerenchyma, and in aquatic plants, aerenchyma (Fig. 1.32). The arrangement of Tissues in cereals is illustrated using the solid stem of maize (Fig. 1.32, A) and hollow straw-type stems (rye, reed), where the vascular bundles are arranged in 2–3 circles (Fig. 1.32, B, C). The structure of solid stems of the lily family type, featuring a developed, homogeneous cortex, is demonstrated using the asparagus stem (Fig. 1.33).

Fig. 1.32. Anatomical structure of stems in Representatives of the Poaceae family: A — maize; B — rye; C — reed; 1 — epidermis; 2 — chlorenchyma; 3 — sclerenchyma; 4 — ground parenchyma; 5 — vascular bundles; 6 — phloem; 7 — xylem vessels; 8 — air passages; 9 — cavity

Fig. 1.33. Asparagus stem: 1 — epidermis; 2 — chlorenchyma of the primary cortex; 3 — pericyclic sclerenchyma; 4 — ground parenchyma of the central cylinder; 5 — closed collateral bundles: a — xylem; b — phloem

RHIZOMES OF MONOCOTS

The rhizome is an underground Modification of the shoot adapted for nutrient storage and vegetative renewal of the plant. The predominant tissue of rhizomes is storage parenchyma (Fig. 1.34). The anatomical structure of monocot and dicot rhizomes differs slightly.

Fig. 1.34. Rhizomes of monocotyledonous plants: A — lily of the valley; B — couch grass; 1 — epidermis; 2 — storage parenchyma of the cortex; 3 — endodermis; 4 — sclerenchyma; 5 — closed collateral bundle; 6 — centrophasic (centrophloem) bundle; 7 — storage parenchyma of the central cylinder; a — xylem; b — phloem

Characteristic Anatomical Features of monocot rhizomes include: a protective tissue consisting of a long-lived, sometimes lignified epidermis lacking Stomata, trichomes, and cuticle; a well-represented primary cortex consisting of a multi-layered storage parenchyma and a 1-, 2-, or rarely multi-layered endodermis with U-shaped thickenings of The Cell walls; and a central cylinder containing collateral closed or centrophloem bundles distributed throughout the cylinder. The pith is more or less distinct, indistinct, or hollow.

STEMS OF HERBACEOUS DICOTYLEDONOUS PLANTS

In dicotyledonous plants, the Introduction/19.html">Primary Structure of stems is rapidly replaced by a Secondary structure resulting from the emergence and activity of the cambium. Depending on the pattern of initiation and subsequent Differentiation of the cambium in herbaceous plants, the central cylinder can exhibit bundle, non-bundle, or transitional types of structure (Figs. 1.35, 1.36). The primary cortex is typically well-developed, comprising chlorophyll-bearing and storage cortical parenchyma, and often collenchyma. The endodermis is frequently distinct, separating the cortex from the central cylinder. Endodermal Cells may contain reserve starch, in which case it is referred to as a starch sheath, or calcium oxalate crystals, referred to as a crystal sheath. The stem pith can be solid and homogeneous or heterogeneous, and may also be hollow (Apiaceae, Caryophyllaceae, Lamiaceae).

The bundle type of stem structure (Figs. 1.35, 1.36, B, C; 1.37, A; 1.38) is characterized by the presence of permanent collateral or bicollateral bundles formed initially by procambium strands and subsequently by interfascicular/fascicular cambium. The bundles are separated by living or lignified parenchyma or sclerenchyma. In the internodes, the bundles are arranged in an orderly circular pattern, as leaf traces in the nodes unite into synthetic bundles and merge with the stem bundles (Fig. 1.35). The bundles are open and continually replenished with tissues.

Fig. 1.35. Diagrams of the path and arrangement of vascular bundles in the stem of a herbaceous dicotyledonous plant: A — cross section; B — longitudinal sections (three-dimensional and planar-unfolded); 1 — primary cortex; 2 — central cylinder; 3 — leaf traces; 4 — stem bundles

In stems of the transitional type (Figs. 1.36, D, E; 1.37, B), interfascicular cambium arises from the ray parenchyma, producing additional vascular bundles. These are located between the main bundles and are consistently smaller than the latter. As they grow, the main and additional bundles may fuse, forming an unevenly thick ring of xylem. The phloem either coalesces into a thinner, irregular ring or is situated in patches separated by ray parenchyma, sclerenchyma, or sclereids. Such a transition from a bundle stem structure through a transitional type to a non-bundle type is typical of representatives of Asteraceae, Fabaceae, Lamiaceae, Brassicaceae, and others.

Initially, the non-fascicular, or bundleless, type (Fig. 1.36, E, 1.37, B, 1.38, B) is characterized by the absence of vascular bundles, since the procambium and subsequently the cambium are laid down as a continuous ring. The conducting system consists of a wide inner cylinder of xylem and a narrower outer cylinder of phloem. In transverse sections, the xylem and phloem are arranged in a ring (e.g., Caryophyllaceae, Linaceae, Rubiaceae, etc.).

Fig. 1.36. Tissue development in dicotyledonous stems and The formation of various types of anatomical structure (diagrams): A — longitudinal section; B — E — transverse sections of stems of various structural types across three zones — procambium initiation (a), cambium appearance (b), secondary thickening (c); B — primary fascicular structure; C — secondary fascicular structure with parenchyma and sclerenchyma between bundles; D — transitional type; E — bundleless type formed from the transitional type; E — bundleless type formed by a uniform cambial ring; 1 — apical meristem; 2 — protoderm; 3 — ground meristem; 4 — pith; 5 — procambium; 6 — epidermis; 7 — primary cortex; 8 — primary phloem; 9 — primary xylem; 10 — cambium; 11 — pericycle; 12 — secondary phloem; 13 — secondary xylem; 14 — pith ray parenchyma; 15 — sclerenchyma

Various systematic plant groups and species exhibit specific individual features of anatomical structure (Fig. 1.37, 1.38), which are used as diagnostic features in plant Taxonomy as well as in the identification of plants and plant-derived raw Materials.

Fig. 1.37. Examples of stem structure in herbaceous dicotyledons: A — fascicular type (clover); B — transitional type (sunflower); C — bundleless type (flax); 1 — epidermis; 2 — lamellar collenchyma; 3 — cortical parenchyma; 4 — schizogenous canals; 5 — starch sheath (endodermis); 6 — open collateral bundle; 7 — thick-walled phloem; 8 — thin-walled phloem; 9 — fascicular cambium; 10 — interfascicular cambium; 11 — accessory bundle; 12 — xylem; 13 — lignified interfascicular parenchyma; 14 — ray parenchyma; 15 — pith; a — sclerenchymatous fibers; b — bast fibers; c — parenchyma; d — sieve tubes with companion cells; e, f, g, h, i — vessel elements: pitted, reticulate, scalariform, spiral, and annular; k — parenchyma; l — libriform fibers

Fig. 1.38. Stem of pumpkin: A — diagram; B — fragment of a transverse section; 1 — epidermis; 2, 3 — primary cortex: 2 — angular collenchyma in ridges and inter-ridge areas; 3 — chlorenchyma; 4 — 7 — central cylinder: 4 — pericyclic sclerenchyma; 5 — ground parenchyma; 6 — cavity; 7 — bicollateral vascular bundle: a — outer and inner phloem; b — multiseriate cambium; c — xylem

RHIZOMES OF DICOTYLEDONS

The rhizomes of dicotyledons may exhibit fascicular, bundleless, or transitional structures. They are largely similar to aerial stems, but function as underground storage organs and differ from aerial stems in the following ways: they are usually covered by a periderm, lack chlorenchyma in the cortex, have less developed mechanical and conducting tissues, and possess a notably prominent parenchyma throughout all regions (Fig. 1.39). This parenchyma stores not only reserve nutrients but also BIOLOGICALLY ACTIVE SUBSTANCES, which makes the rhizomes of many plants suitable for use as food and medicinal plant materials. Quite often, rhizomes contain specialized endogenous secretory structures, a factor taken into account when diagnosing plants and herbal raw materials.

Fig. 1.39. Rhizome of coltsfoot (diagram of a portion of a transverse section): 1 — periderm; 2 — storage cortical parenchyma; 3 — mucilage cavities; 4 — cortical vascular bundles; 5 — endodermis; 6 — open collateral bundles; 7 — pith with a central cavity

STEMS OF WOODY DICOTYLEDONS

Woody stems are characterized by a secondary bundleless structure. The protective tissue is a periderm with lenticels, or bark (rhytidome). Due to the seasonal activity of the cambium, annual growth rings are formed in the wood; these consist of autumn tracheal elements — tracheids and libriform fibers — and spring tracheal elements — vessels, pitted tracheids, and wood parenchyma (Fig. 1.40).

Fig. 1.40. Fragments of transverse stem sections of linden: A — diagram; B — sector layout; C — fragment of phloem (secondary cortex); D — fragment of xylem; 1 — periderm; 2 — collenchyma; 3 — primary cortex parenchyma; 4 — druses; 5 — bast fibers; 6 — thin-walled phloem elements — sieve tubes with companion cells; 7 — pith rays; 8 — cambium; 9 — wood annual rings; 10 — spring elements of the annual ring; 11 — autumn elements of the annual ring; 12 — pith

Pith rays are uniseriate or multiseriate, primary (connecting the pith with the cortex) and secondary (not reaching the pith). Diagnostic significance is attributed to the thickness of the periderm, the composition and arrangement of histological elements of the wood, and the secondary cortex — hard (thick-walled) and soft (thin-walled) phloem. Wood may be ring-porous, diffuse-porous, or radiate (Fig. 1.41).

Fig. 1.41. Types of wood: 1 — ring-porous; 2 — diffuse-porous

STEMS OF CONIFEROUS PLANTS

The anatomical structure of conifer stems differs from that of angiosperms in the following respects: the periderm usually lacks lenticels but features cracks; bark begins to form early on the trunk; mechanical tissues are absent or poorly developed in the cortex; the phloem includes sieve cells or sieve tubes without companion cells, parenchyma, and occasionally bast fibers or sclereids; annual wood rings consist of spring tracheids with bordered pits, which perform a conducting function, and autumn fiber-like tracheids without bordered pits, which perform a supportive function; all PARTS OF THE stem typically contain schizogenous resin ducts. A typical structure is observed in the stem of Scots pine (Fig. 1.42).

Fig. 1.42. Anatomical STRUCTURE OF THE pine stem: A, B, C — transverse sections; G, D — wood in longitudinal sections — radial and tangential; 1 — periderm; 2 — cortical parenchyma; 3 — resin ducts; 4 — 11 — central cylinder; 4 — phloem; 5 — cambium; 6 — wood annual rings; 7 — spring tracheids; 8 — autumn tracheids; 9 — pith rays; 10 — pith