From Seed to Seed:
Plant Science for K-8 Educators

Leaves Up Close

Although leaves vary considerably, most share some basic features. Let's look at a typical leaf up close:

The upper and lower epidermis cells form a protective layer for the delicate interior cells-similar to the way that your skin protects your own inner tissues. The upper epidermis is located on the side that normally faces the sun; the lower epidermis is on the underside of the leaf. The cuticle, (La. cutis = skin) made from a waxy substance called cutin (CUE-tin), covers the entire leaf surface to help prevent moisture loss.

Sandwiched between the epidermal cells is the mesophyll (MEZ-oh-fill; Gr. meso = middle, phyll = leaf). The mesophyll contains two layers. The upper layer consists of tightly packed palisade cells. Most photosynthesis takes place in these chloroplast-packed cells. Below the palisade cells is a layer of loosely-packed spongy cells that allow gases and water vapor to move about. Woven throughout the mesophyll are the veins, or vascular tissues, responsible for transporting liquids to and from cells.

*plural stomata or stomates; singular stomate or stoma

In most plants, the stomates open in the light and close in the dark. However, the plant faces a tradeoff. In order for photosynthesis to take place, the leaf must consume carbon dioxide. So, when the sun comes up, the stomates open and carbon dioxide enters the leaf. Whenever the stomates are open, the leaf is also losing water in the form of water vapor (because of transpiration). Therefore, the plant must balance the need for carbon dioxide with the avoidance of dehydration and regulate the stomata accordingly.