Anatomy of leaves

A dorsiventral leaf (typical of dicots) has a distinct upper and lower side. In section it has three parts: the epidermis, the mesophyll and the vascular system.

Both the upper (adaxial) and lower (abaxial) epidermis have a cuticle. The lower surface usually has more stomata; the upper surface may have none. This reduces water loss, since the upper side faces the sun.

The mesophyll does the photosynthesis. Just under the upper epidermis is the palisade parenchyma: long cells standing upright, side by side, packed with chloroplasts to catch the light. Below it is the spongy parenchyma: rounded or oval cells, loosely arranged, with large spaces and air cavities between them for the exchange of gases.

The vascular bundles run through the midrib and veins. Their size varies with the size of the vein, because dicot leaves have reticulate (net) venation. Each is surrounded by thick-walled bundle sheath cells, and the xylem faces the upper side.

An isobilateral leaf (typical of monocots like grasses) is alike on both sides. It has stomata on both surfaces, and its mesophyll is not divided into palisade and spongy layers. Its bundles are of nearly equal size, because of parallel venation.

Grasses have large, empty-looking bulliform cells along the veins on the upper surface. When they are full of water (turgid), the leaf stays open. When water is short, they lose water (flaccid) and the leaf curls inwards, cutting water loss.