Epidermal Tissue System

The epidermal tissue system is the plant's outer skin (Gk. epi = upon, derma = skin) - its single point of direct contact with the external environment, built from protoderm, and interrupted here and there by stomata. Its detailed structure differs by organ. In the root it is called the piliferous layer or epiblema: a single compact layer of parenchyma cells, packed tightly with no intercellular spaces, carrying no cuticle and no pores (since the root's job is absorption, not water-proofing), and bearing single-celled root hairs that both absorb water/minerals and physically protect the root surface. In the stem it is a single layer of compactly-arranged, rectangular, living parenchymatous cells whose outer wall carries a cutin-based cuticle (sometimes with added epicuticular wax) to limit transpiration; chloroplasts are normally absent here except in the guard cells of any stomata present, and many stems also bear numerous epidermal hairs. In the leaf it forms an upper and a lower epidermis, usually more thickly cuticled on top, interrupted by stomata (a pore flanked by a pair of guard cells, generally bean-shaped in most dicots/monocots but dumb-bell shaped in grasses and sedges, sometimes further ringed by distinct subsidiary cells) which are typically more numerous on the lower surface; some xerophytes (Cycas, Nerium) sink their stomata into protective stomatal crypts to cut transpiration further, grasses additionally carry large bulliform/motor cells (for leaf rolling) and silica-filled cells, and some plants (Rose) bear vascular-free, sharp …