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The stem of a dicotyledonous plant is covered, like its root, by a single-layer skin - the epidermis. The cells of the epidermis are close together, elongated in the direction of the longitudinal axis and covered by the cuticle. They do not contain chloroplasts and vents are less common in the skin of the stem than in the leaves, while trichomes (hairs) and thorns, scales, etc. are abundant.
In the case of stems, in which the conductive tissues form a compact cylinder, we distinguish a layer of the primary cortex under the epidermis, which has a protective, mechanical, storage and sometimes assimilation function. It consists predominantly of parenchymal tissue, often rich in starch, and can be divided into three parts. The outer part of the primary cortex - the hypodermis (exodermis) is often called collenchyma and has a mechanical function. The middle thickest parenchymal layer of the primary cortex has primarily a photosynthetic function and the inner layer is not significantly morphologically differentiated (unlike the root). Both layers are called the endodermis The inner endodermis does not usually form Caspary strips, it differs mainly in its physiological function and is referred to as the starch sheath.
A continuous or intermittent ring of sclerenchymatic fibers, referred to as a pericycle, is often formed between the primary cortex and the bast of the vascular bundles. The sclerenchymatic pericycle forms a mechanical sheath of the stem, which reinforces it.
The vascular bundles consist of the phloem (bast part) and the xylem (wood part). The vascular bundles of primary stems are usually collateral or lateral (the wooden part is on the inside and the bast on the outside) and open (cambium is established between the phloem and the xylem, which allows their secondary thickening). In plants of some families, the bundles are bicolateral (bilateral) - they have two bast parts and a wooden part between them. In both cases, the vascular bundles are arranged in one circle.
Between the vascular bundles are strips of parenchymal tissue - the pith rays that protrude from the pith in the middle of the stem.
## Keywords
plants leaf stem monocotyledonous dicotyledonous differences
