Components of the Ground Tissue System

Working from the epidermis inward, the ground tissue system is built of a recognisable sequence of named layers. The hypodermis is one or two continuous or discontinuous protective layers just below the epidermis - collenchymatous in a dicot stem (adding mechanical strength while the stem is still elongating) but sclerenchymatous in a monocot stem (since monocot tissue is laid down all at once with no later secondary thickening to rely on). The general cortex, between epidermis and pericycle, is a few to many layers of mainly parenchymatous cells, sometimes with intercellular spaces, capable of storing starch grains, oil, tannins or crystals, and sometimes turning chlorenchymatous (chloroplast-bearing) near the surface of a young stem. The endodermis is the cortex's innermost layer - a single ring of compactly-arranged, barrel-shaped cells with no intercellular spaces - and here the root and stem diverge sharply: a true root endodermis develops casparian strips (bands of lignin, suberin and other carbohydrates on the radial and inner tangential walls) that force water to cross the plasma membrane rather than leak between cells, whereas a stem's endodermis-like layer usually just accumulates starch instead and is called a starch sheath. Wherever casparian strips are present, the endodermal cells sitting directly opposite the protoxylem stay thin-walled and strip-free - these passage cells are the one remaining route by which water and dissolved salts can move radially from cortex to xylem, since the casparian strips block that path everywhere else and, just as importantly, stop water re-entering the cortex once it is already inside the xylem. Just inside the endodermis sits the pericycle (a sing …