Botany - B.E. Yakubenko 2017

Part One. Plant Anatomy and Morphology
Chapter III. Vegetative Plant Organs
3.7. Anatomical Structure of the Stem

The Water/140.html">Anatomical Structure of the stem is generally distinguished into Primary and secondary structures. Within this complex Organization, several distinct groups of typical Tissues can be identified, which determine their structural features.

Primary stem structure. The Primary Structure arises As a result of the activity and Differentiation of the Meristems located in the apical meristem (growing point) of the SHOOT. According to Schüepp's and Schmidt's theories, the shoot apex contains two zones of initial Cells that divide to form two histogens: the tunica and the corpus. The epidermis—and occasionally several layers of the primary cortex—is formed from the outer meristematic layer, known as the tunica. The inner Cells of the apex, or the corpus, give rise to the inner layers of the primary cortex and the vascular cylinder. Consequently, the primary stem structure comprises the epidermis, the primary cortex, and the central cylinder.

The epidermis typically consists of a single layer of living parenchymatous cells with sinuous Cell walls, which imparts increased mechanical resilience to the protective tissues. This allows them to withstand the pressure generated by the growth and formation of new Cells and Tissues. Stomata are present within the epidermis, and various appendages may develop on its surface. Lying deeper is the primary cortex. In dicotyledonous plants, the outer layers of the primary cortex contain collenchyma, the cells of which possess METABOLISM/14.html">Chloroplasts and perform an assimilatory function. The presence of collenchyma helps the stem resist mechanical stresses such as wind and rain. In addition to collenchyma, many plants contain strands of sclerenchyma. The middle layers of the primary cortex consist of parenchyma cells and serve a storage function.

The innermost layer of the primary cortex forms the endodermis, whose cells fit tightly together; in some plants, The Cell walls may become lignified or suberized over time. In most herbaceous plants, the endodermis is absent and is instead replaced by a starch sheath (starch-storing starch-sheath), the cells of which are packed with starch grains.

In the outer region of the central cylinder, one or two layers of parenchymatous and prosenchymatous pericycle cells can be distinguished. The parenchymatous cells give rise to medullary rays, sclerenchyma, and adventitious buds, while the prosenchymatous cells of the pericycle form primary bast fibers (for example, in hemp).

The greater part of the stem is occupied by the pith, into which the vascular bundles are embedded. Its cells are parenchymatous. Extending between the vascular bundles, it forms the pith (medullary) rays. In the center of the stem, the pith often disintegrates, causing the stem to become hollow.

Vascular bundles originate from the procambium of the shoot apex. During the division of procambial cells, the primary elements of the phloem and xylem are formed; the xylem consists of protoxylem and metaxylem, while the phloem comprises protophloem and metaphloem. The first elements to appear are annular and spiral vessels, followed later by scalariform and reticulate vessels, as well as sieve tubes with companion cells.

The procambium may differentiate either as discrete strands or as a continuous cylinder (ring). If the procambium develops as separate strands, a collateral (bundle) type of structure is formed; otherwise, a non-bundle (stele-like) type is established.

In some cases, the procambium is entirely consumed in The formation of phloem and xylem, resulting in the primary type of structure characteristic of monocotyledonous plants. In contrast, in dicotyledons and gymnosperms, the procambium gives rise to the cambium, which subsequently generates new xylem and phloem elements.

Several specific features are notable in the anatomical STRUCTURE OF THE stem. Monocotyledonous plants encompass woody and herbaceous species, as well as annuals and perennials, which exhibit diverse structural types.

Plants with stems filled with parenchyma (such as members of Liliaceae and Iridaceae) exhibit the following structure: an epidermis, a primary cortex consisting of chlorenchyma (chlorophyll-bearing parenchyma), storage parenchyma, and an endodermis. Lying deeper is a pericyclic sclerenchyma. The bulk of the stem is composed of the pith, within which closed collateral bundles are scattered.

The stems of grasses (Poaceae) also possess unique characteristics. The primary cortex is typically absent. From the periphery to the center, the stem structure comprises: a single-layered epidermis with or without stomata, the cells of which die off fairly quickly; and a ring of sclerenchyma. In this arrangement, the chlorenchyma containing air cavities and stomata is located between the ridges of the sclerenchyma. Enclosed by the sclerenchymatous sheath is the ground parenchyma, which fills the entire stem. Embedded within it are closed collateral vascular bundles, which are larger toward the center and smaller near the periphery. Above each vascular bundle lies a sclerenchyma cap that provides additional mechanical strength to the stem. The central part of the pith breaks down in most grasses, forming a central cavity. Such hollow stems are referred to as culms.

Woody monocotyledons, such as *Dracaena*, *Yucca*, and *Agave*, are characterized by secondary thickening of the stem, which is driven not by cambial activity, but by the proliferation of the parenchyma.

Secondary stem structure. In the stem anatomy of dicotyledonous plants, depending on whether the procambium develops as discrete strands or as a continuous cylinder, a bundle or non-bundle type of structure develops, respectively. In transverse sections of the bundle type, the following tissue regions are distinguishable: the epidermis, the primary cortex, the central cylinder, and the pith. The primary cortex includes collenchyma, cortical parenchyma, and the endodermis, although in some plants it is represented solely by parenchyma; it frequently stores not only nutrient reserves but also other metabolic products. The central cylinder begins with the pericycle, from which sclerenchyma later forms (either as a continuous ring or separate strands), housing open collateral vascular bundles arranged in a circle. These vascular bundles are formed by fascicular (bundle) cambium. At the level of the fascicular cambium, interfascicular cambium differentiates from the parenchymatous cells; this interfascicular cambium produces the parenchyma of the medullary rays rather than conductive elements. The central region is occupied by the pith, consisting of living parenchymatous cells. This type of structure is characteristic of most herbaceous dicots.

The non-bundle type of stem structure is characterized by the absence of discrete vascular bundles, with the xylem and phloem arranged in continuous cylinders. This type of structure is typical of most shrubs and woody trees, and is only rarely found in herbaceous forms, such as cultivated flax (*Linum usitatissimum*). In this case, the stem consists of the following tissue systems: the epidermis and the primary cortex, which comprises parenchyma and a starch sheath (collenchyma is absent, and the chlorenchyma lies directly beneath the epidermis, although collenchyma is typically present in trees and shrubs). Beyond the primary cortex lies the central cylinder, starting with a pericyclic sclerenchyma, followed by the primary and secondary phloem, which is represented by bast fibers and sieve tubes with companion cells. The ring of secondary phloem results from The activity of the cambial ring, which simultaneously produces a robust layer of secondary xylem toward the center. This secondary xylem consists of regular rows of vessels, tracheids, and libriform fibers, interspersed with narrow strips of medullary ray parenchyma. The center of the stem contains the pith, surrounded by the annular and spiral-annular Vessels of the primary xylem.

With age, a portion of the pith disintegrates, resulting in the formation of a central cavity.

Herbaceous dicots such as cultivated flax and hemp possess a non-bundle type of structure, the detailed features of which are examined in laboratory practicals.

Certain plants exhibit a transitional type of stem structure, shifting from a bundle to a non-bundle type. In the sunflower (*Helianthus*), for instance, a bundle type of structure is formed during the early Selection/3.html">Stages of development. Subsequently, interfascicular cambium arises between adjacent fascicular cambia. Joining with the fascicular cambium, it forms a complete cambial ring that lays down continuous layers of xylem and phloem, thereby establishing a non-bundle (secondary) stem structure.

Anatomical structure of woody stems. Woody plants possess a non-bundle type of structure. In woody dicots, xylem and phloem are cut off in cylindrical layers through the activity of the cambial ring. In the peripheral region, cork cambium (phellogen) develops, producing cork (phellem) outwardly and phelloderm inwardly. Thus, the Introduction/11.html">Secondary structure of stems and young branches in woody plants is formed by secondary lateral meristems: the cambium and the cork cambium.

The cork cambium produces the periderm, which is eventually replaced by bark (rhytidome) as the plant ages.

Located beneath the periderm is the primary cortex. Its outer layers typically consist of lamellar collenchyma filled with chloroplasts, while its bulk is made up of large parenchymatous cells. The parenchyma stores nutrients, Gums, and calcium oxalate druses. The innermost layer of the primary cortex is a single-layered starch sheath. The primary cortex is a temporary structure that Functions for 5 to 10 years, gradually transforming into bark as phellogen activity progresses.

In the secondary phloem (inner bark), primary medullary ray regions consisting of living parenchymatous cells can be distinguished, alongside the secondary phloem itself, which is represented by hard bast (bast fibers) and soft bast (sieve tubes, companion cells, and phloem parenchyma). Bast fibers are exceptionally strong and capable of withstanding heavy bending loads; in some plants, such as the linden (*Tilia*), they are used to manufacture various woven products. Sieve tubes perform the conductive function, while the phloem parenchyma serves for nutrient storage.

The cambium consists of living, thin-walled parenchymatous cells neatly stacked in radial rows. Cambial cells undergo tangential divisions, with the mother cell producing daughter cells, one of which remains initial (meristematic), while the other specializes into an element of secondary phloem or secondary xylem. Notably, a significantly greater volume of xylem is produced compared to phloem. Lying just inside the cambium is the prominent secondary wood (xylem). Due to the periodicity of cambial activity, distinct growth rings (annual rings) are formed here, consisting of large-pored early wood (spring wood) and small-pored, thick-walled late wood (summer/autumn wood). In transverse sections of a woody stem, early wood appears porous and lighter in color, whereas late wood is dense and darker. In spring, cambial cells produce thin-walled, pitted, and scalariform vessels that ensure an ample supply of water and nutrients to the developing leaves. During the second half of the growing season, cambial activity slows down, and it cuts off thick-walled tracheids and libriform fibers. The xylem laid down over the course of the vegetation period comprises both early and late wood, which together constitute a single annual ring. Cambial activity ceases for the winter and resumes the following spring. Because last year's late wood and this year's early wood are sharply demarcated, the age of the plant can be accurately determined by counting the growth rings. In the secondary xylem, secondary medullary rays are visible as narrow parenchymatous strips that facilitate the horizontal Transport of substances.

The pith occupies the central region of the stem and contains both living small cells and larger, colorless dead cells. It serves as a storage site for nutrients. The pith is surrounded by the primary xylem, which persists only partially in the form of annular and spiral vessels and xylem parenchyma. Primary medullary rays extend from the pith toward the secondary cortex, remaining narrow in the xylem region and widening within the cortex.

As a tree trunk ages, specific developmental changes occur. In the peripheral region, the primary cortex and part of the secondary phloem die off, transforming into bark (rhytidome). As the trunk expands in girth, the bark cracks and fissures. The color of the bark and the pattern of fissures are species-specific. Changes also take place within the xylem. The older zones adjacent to the pith gradually lose their water-conducting function as their vessels become plugged with tyloses; in some plants, such as oak and elm, this inner region of the xylem acquires a dark coloration and is termed heartwood (*duramen*). The peripheral, younger region of the xylem—the sapwood (*alburnum*)—retains both conductive and mechanical functions.

The wood of gymnosperms has certain distinct characteristics. Its xylem lacks libriform fibers, xylem parenchyma, and vessels. Tracheids form The basis of the xylem and perform both conducting and mechanical functions. Typically, the conducting function is served by wide-lumen spring tracheids, whereas autumn tracheids, which feature significantly thickened walls, are better adapted for mechanical support. Tracheids are pierced by numerous bordered pits. In addition, both the bark and the wood contain vertical and horizontal resin ducts that accumulate oleoresin. This resin is harvested by tapping and is utilized in various industrial sectors. The radial rays in the gymnosperm stem have a complex structure, comprising both living starch-storing cells and dead tracheid-like cells, and they also accommodate horizontal resin ducts.