Tissue Systems: Epidermal, Ground and Vascular

Rather than tracking every individual tissue separately, plant anatomists group the tissues of a mature organ into three tissue systems, based on the position each occupies and the broad job it performs. The same three systems -- epidermal, ground and vascular -- recur in the root, the stem and the leaf, although the detailed cell types within each system differ from organ to organ.

The epidermal tissue system forms the outermost protective covering of every plant organ. In its simplest form it is the epidermis: usually a single layer of flattened, tabular, tightly packed cells with almost no intercellular space and, in most cases, no chloroplasts. On aerial organs the outer (exposed) wall of the epidermis secretes a waxy, waterproof layer called the cuticle, which cuts down uncontrolled loss of water. Set into the epidermis of leaves, and to a lesser extent of green stems, are stomata -- small pores, each flanked by a pair of specialised, bean-shaped (in most dicots) or dumbbell-shaped (in grasses and many other monocots) guard cells that are unusual among epidermal cells in carrying chloroplasts. A small number of epidermal cells lying immediately around a stoma are often structurally distinct from ordinary epidermal cells and are called subsidiary cells; the guard cells, the pore between them, and their subsidiary cells together make up what is called the stomatal apparatus, the structure that opens and closes to regulate both gas exchange (carbon dioxide in, oxygen out, and vice versa) and transpiration (water loss as vapour). The epidermis also carries a variety of outgrowths, or appendages: trichomes, hair-like structures on the stem and leaf that may be unicellular or multicellular, branched or unbranched, and that generally help cut down water loss and deter small herbivores; and, on the root, unicellular root hairs, which are the epidermis's absorptive extensions into the soil. In many monocot leaves, the epidermis additionally shows regularly alternating silica cells and cork cells, a feature not found in dicots.

The ground (or fundamental) tissue system is essentially everything that lies between the epidermis and the vascular tissue -- in a stem this includes the cortex, the pericycle, the pith and the medullary rays; in a leaf it is called the mesophyll. It is made chiefly of parenchyma, though a collenchymatous zone often occurs just under the epidermis (the hypodermis of dicot stems) and sclerenchymatous patches occur here and there for extra mechanical support. In a leaf, the ground tissue (mesophyll) is frequently further differentiated -- as studied later in this chapter -- into a photosynthetically specialised palisade layer and a gas-exchange-specialised spongy layer. …