Primary Stem Structure
The plant stem is generally cylindrical, or sometimes ridged or quadrangular (Fig. 2.2). Primary vascular tissue typically consists of either a complete cylinder or a system of discrete vascular bundles. The cortex is the region of ground tissue between the vascular tissue and the epidermis; the junction between the cortex and vascular region is termed the pericyclic region, from which endogenous adventitious roots can arise (Fig. 3.4). The pith is the central region of ground tissue, though in many stems it breaks down to form a central hollow cavity. The stem epidermis often bears stomata and trichomes, as in the leaf epidermis.
The stem primary ground tissue is basically parenchymatous but can be modified into various tissue types or interspersed with fibres and sclereids, and parenchyma cells frequently become lignified as the plant ages. Ridged or angled stems often possess strengthening collenchyma at the angles, immediately within the epidermis. Many stems are photosynthetic organs with a chlorenchymatous cortex, particularly in leafless (apophyllous) plants, which normally occur in nutrient-poor habitats.
Some plant stems possess secretory cells or ducts in the ground tissue. For example, many species of Euphorbia possess branched networks of laticifers in the cortex (Fig. 1.6), which extend throughout the ground tissue of the stem and leaves. Plants with succulent stems, such as many Cactaceae, typically possess regions of large thin-walled cells that contain a high proportion of water. Some stems (e.g. corms of Crocus) are specialized as storage or perennating organs; they store food reserves in the form of starch granules, most commonly in the inner cortex. Sometimes the layer of cortical cells immediately adjacent to the vascular tissue is distinct from the rest of the cortex, and may be packed with starch granules; this is termed a starch sheath, or sometimes an endodermoid layer or endodermis, though the component cellsusually lack the Casparian thickenings that are typically found in the root endodermis.
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