The tissue systems

Anatomy is the study of a plant's inner structure. To see it, the plant part is cut into thin slices. Think of slicing a cucumber for salad: round slices show a transverse section, long slices a longitudinal section. Same cucumber, different view, so always check which way a diagram was cut.

Inside, the tissues are grouped into three tissue systems, by their structure and where they are:

• The epidermal tissue system: the outer covering.
• The ground (fundamental) tissue system: the filling.
• The vascular (conducting) tissue system: the pipes for water and food.

Epidermal system. The epidermis is usually one layer of closely packed living cells. On stems and leaves it is coated with a waxy cuticle that stops water loss; roots have no cuticle, since they must absorb water. Roots have one-celled root hairs. Shoots often have many-celled hairs (trichomes), which also help reduce water loss.

Stomata are tiny pores, mostly on leaves, that control water loss (transpiration) and the exchange of gases. Each pore is guarded by two guard cells: bean-shaped in most plants, but dumb-bell shaped in grasses. Their outer walls are thin and inner walls (around the pore) thick, and they have chloroplasts. The guard cells, the pore and the special epidermal cells around them (subsidiary cells) together make the stomatal apparatus.

Ground system: everything except the epidermis and the vascular bundles. It is made of parenchyma, collenchyma and sclerenchyma, found in the cortex, pericycle, pith and medullary rays. In leaves, the ground tissue is the mesophyll, full of chloroplasts.

Vascular system: xylem (carries water) and phloem (carries food), grouped into vascular bundles. Bundles with a layer of dividing cells (cambium) between xylem and phloem are open, as in dicot stems, and can make the stem thicker. Bundles without cambium are closed, as in monocots. In roots, xylem and phloem lie on different radii, alternating (radial bundles). In stems and leaves they lie on the same radius, usually phloem outside (conjoint bundles).