Structure of A Leaf

A leaf consists of a thin outer skin, or epidermis, covering cells that make up the mesophyll (middle leaf). An intricate network of veins runs through the mesophyll.

The Epidermis

consists of a single layer of cells. Its outer surface, or cuticle, is coated with a waxy substance called cutin. Cutin helps to prevent evaporation from the mesophyll. In many plants certain cells of the epidermis grow into hairs. A hair consists of single cells arranged end to end in either single or branching threads. The hairs of tobacco and of a number of other plants secrete a sticky substance.

The epidermis is pierced by a vast number of tiny pores called stomata (singular: stoma), from the Greek word for mouths. The stomata occur chiefly on the undersides of leaves. In the olive and many other plants they occur only on the lower surface; and in still other plants, such as the water lily, they are found only on the upper surface.

Each stoma is enclosed by a pair of guard cells shaped somewhat like beans. When they absorb moisture the guard cells swell up and curve away from each other, thus opening the stoma. When they lose moisture they collapse and close the stoma.

The chief function of the guard cells is to regulate the breathing of the leaf by admitting air and letting other gases escape. They open the stomata in daylight, when the leaf needs air with which to make food, and close them at night. When the leaf wilts the guard cells collapse, thus reducing loss of moisture.

The Mesophyll

is made up of thin-walled cells containing chlorophyll, the green substance that begins the food-making in plants. In most plants the mesophyll cells form two types of tissue: palisade tissue and spongy tissue. The palisade tissue is made up of palisade cells, cylindrical cells set at right angles to the surface of the leaf. Palisade tissue is usually found under the upper surface of leaves. In some erect leaves it occurs under both surfaces.

The spongy tissue consists of irregularly shaped cells. It contains many air spaces that lead to the stomata. Air enters the spaces through the stomata, and waste gases escape from the spaces through the stomata.

Veins extend throughout the mesophyll. They provide passage for fluids and help support the leaf. The veins are continuations of tiny tubes that extend from the stem through the petiole. They supply the cells of the mesophyll with minerals dissolved in water, which they need to make food, and carry food back to the stem.