Stem of Dicotyledonous Plants - Detail

The stem of a dicotyledonous plant is covered, like its root, by a single-layered epidermis. The cells of the epidermis are tightly packed, elongated along the longitudinal axis, and coated with a cuticle. These cells lack chloroplasts, and stomata are less common on the stem's epidermis than on leaves. However, the stem's surface often has abundant trichomes (hairs), thorns, and scales. In stems where the conductive tissues form a compact cylinder, there is a primary cortex layer just beneath the epidermis that serves various functions, including protection, mechanical support, storage and sometimes photosynthesis. The primary cortex is predominantly composed of parenchymal tissue, often rich in starch, and can be divided into three parts. The outermost layer, called the hypodermis (or exodermis), is often referred to as collenchyma and provides mechanical support. The middle, thickest parenchymal layer of the primary cortex mainly serves a photosynthetic function, and the innermost layer, unlike in roots, is not morphologically distinct. These layers are referred to as the endodermis. This 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, develops between the primary cortex and the phloem of the vascular bundles. The sclerenchymatic pericycle acts as a mechanical sheath, strengthening the stem. The vascular bundles consist of the phloem (bast) and the xylem (wood). In primary stems, the vascular bundles are generally collateral or lateral (with the xylem on the inside and phloem on the outside) and open (with cambium between the phloem and xylem, allowing secondary growth). In some plant families, the bundles are bicolateral, featuring two phloem sections with xylem in between. In both cases, the vascular bundles are arranged in a single circle. Strips of parenchymal tissue, known as pith rays, lie between the vascular bundles and extend outward from the pith at the stem's center. ## Keywords plants leaf stem monocotyledonous dicotyledonous differences