Anatomy of the Dicotyledonous and Monocotyledonous Stem

A stem's transverse section, like a root's, is organised outward-in, but its layers differ from a root's in composition and, in a dicot stem, in how the vascular bundles are arranged.

In a dicot stem, the outermost layer is a single-layered epidermis, covered by a cuticle (thicker than a root's, since a stem must guard against water loss rather than aid absorption) and bearing occasional stomata and multicellular trichomes. Just beneath it lies the hypodermis, typically a few layers of collenchyma -- living, unevenly thickened cells that give the young, still-elongating stem mechanical support without sacrificing flexibility, and that can also photosynthesise where they retain chloroplasts. Below the hypodermis lies the general cortex, parenchymatous and used for storage, and its innermost layer is often visibly distinct as a starch-storing layer sometimes called the starch sheath -- functionally similar to, though less sharply defined than, a root's endodermis. Just inside this lies the pericycle, which in a dicot stem is typically not a single continuous layer of thin cells (as in the root) but a discontinuous ring of sclerenchyma patches, called bundle caps, sitting directly above each vascular bundle and adding mechanical protection to it.

The vascular bundles of a dicot stem are conjoint, collateral and open (a strip of cambium runs between the xylem, toward the centre, and the phloem, toward the periphery, of each bundle, as introduced in the tissue-systems section), and they are arranged in a single ring around a large, central, parenchymatous pith. Because the bundles sit in a ring rather than filling the whole cross-section, narrow parenchymatous strips called medullary rays run radially between adjacent bundles, connecting the pith to the cortex and allowing food and water to move laterally across the stem. It is this cambium strip in each bundle -- together with a second cambium that later forms in the medullary rays between bundles, fusing into a complete ring -- that makes the secondary growth described in a later section of this chapter possible. …