BOTANY WITH BASICS OF HYDROBOTANY (AQUATIC PLANTS OF UKRAINE) - B.Ye. Yakubenko - 2011

VIII. BASICS OF PLANT MORPHOLOGY AND ANATOMY

Morphology and Anatomy of the Stem and Shoot

Anatomical Structure of the Stem

The stem exhibits an extraordinary diversity in its internal Structure, reflecting the adaptation of plants to various environmental conditions. However, three main parts can always be distinguished within it: the epidermis, the primary cortex, and the stele (the central axial cylinder). The Water/140.html">Anatomical Structure of the stem changes over time. As the embryonic SHOOT grows, a Primary Structure is formed, which in dicotyledons eventually transforms into a Secondary structure As a result of The activity of secondary Meristems.

The Introduction/19.html">Primary structure of the stem is characteristic of monocotyledonous plants and persists throughout their entire life span. It is formed due to the activity of the primary apical meristem of the shoot apex. The protoderm, which gives rise to the epidermis, is formed from the tunica Cells, while the ground meristem, which subsequently develops into the primary cortex, originates from the inner Cells of the tunica and the outer cells of the corpus. The procambium, which gives rise to the Tissues of the central axial cylinder, is formed from the corpus.

The outer surface of the stem is covered by the epidermis, which consists of a single layer of cells with cutinized walls. Its primary function is to protect the stem from environmental influences. As a rule, the epidermis is covered with a cuticula that prevents excessive moisture loss and often bears appendages in the form of various trichomes. The epidermis contains Stomata that facilitate gas exchange with the external environment, although their number per unit area is significantly lower than that in leaves.

Directly beneath the epidermis lies the primary cortex. It is composed of collenchyma, primary cortical parenchyma, and endodermis. Collenchyma is a living mechanical tissue that provides strength to the stem. Parenchyma is a living tissue with thin walls. Sometimes it contains METABOLISM/14.html">Chloroplasts and is capable of Photosynthesis; this type of parenchyma is called chlorenchyma. In the stems of aquatic plants, it is distinguished by the presence of large intercellular air spaces where air accumulates. This variety of parenchyma is known as aerenchyma. The parenchyma of the primary cortex also houses epithelial cells of the secretory tissue (secretory cavities and mucilage canals) and stone cells, and it stores calcium oxalate crystals. The innermost layer of the primary cortex is the endodermis, which surrounds the central cylinder. As a rule, it consists of a single layer of cells with Casparian strips (suberified/lignified radial walls). This tissue is most optimally developed in cereals and aquatic plants. In many angiosperms, it is replaced by a starch sheath (starch-storing layer). Its cells contain starch grains that are not utilized in metabolic processes but serve as a gravity-sensing organ (gravireceptor) for the stem.

The center of the stem is occupied by the vascular cylinder (stele), which includes the pericycle, vascular bundles, and pith. The pericycle, located directly beneath the endodermis, generally consists of a single layer of thin-walled cells and acts as a primary lateral meristem. Parenchymal cells give rise to the medullary rays and form sclerenchyma (bast fibers) as a continuous ring or separate strands opposite the vascular bundles, ensuring their rigidity. Adventitious roots, buds, and secondary meristems originate within the pericycle. In many monocotyledons, the pericycle does not form.

Vascular bundles are formed from the procambium. Primary phloem arises immediately beneath the primary cortex and consists of narrow sieve tube elements, companion cells, and phloem parenchyma. Primary xylem develops in a centripetal direction and consists of vessels.

The central part of the stem is occupied by the pith (medulla). It is represented by large, thin-walled cells that store nutrient reserves. In some plants (such as Poaceae and Apiaceae), the pith cells quickly die off, leaving the stem hollow. In cereals, such a stem is called a culm (straw). The pith is connected to the parenchyma of the primary cortex via medullary rays, which consist of living parenchymal cells and ensure the horizontal Transport of mineral and organic substances throughout the stem.

In monocotyledonous plants, vascular bundles are incapable of secondary thickening. In dicotyledons and gymnosperms, a cambium forms within the vascular bundle, the activity of which subsequently leads to The Development of the Secondary anatomical structure of the stem.

The central part of the stem contains the pith, formed by large parenchymal cells that sometimes store nutrient reserves. The pith connects to the primary cortex through strands of medullary rays, which facilitate the horizontal Transport of substances across the stem. In many cereals, the pith breaks down, forming a central cavity.

Within the pith, there are discrete vascular bundles or continuous cylinders of phloem and xylem separated by a cambium derived from the procambium. Depending on the pattern of procambium formation—as discrete strands or a continuous ring—a bundle-type or non-bundle (stele-type) stem structure is formed. The procambium of monocots forms only primary phloem and xylem elements, resulting in The formation of closed vascular bundles. If the procambium gives rise to a cambium In addition to phloem and xylem, open vascular bundles are formed, which are typical of most dicotyledons.