Structure of a Leaf

In figure 34-5, a stomate appears on both the upper and lower surfaces of the leaf. A thin, waxy cuticle is also on the upper surface. This cuticle tends to retard water loss through epidermal cells. The figure also shows an upper epidermis and a lower epidermis. The lower epidermis has a hair called a trichome. The only epidermal cells bearing chloroplasts are the guard cells. Between the two epidermises are mesophyll cells abundant in chloroplasts. The mesophyll cells are of two kinds. In the upper portion of the leaf, the mesophyll cells are elongated and vertically arranged, and are collectively called the palisade parenchyma; and in the lower portion of the leaf, the mesophyll cells are characterized by conspicuous intercellular spaces, and are collectively called the spongy parenchyma, or spongy mesophyll. The intercellular spaces communicate with the substomatal spaces to allow gas interchange with the atmosphere. Carbon dioxide from the atmosphere enters the mesophyll cells through the intercellular spaces, while oxygen, a product of photosynthesis, is conveyed out of the leaf through the stomates.