What Is the Job of the Leaf?

Photosynthesis

• Photosynthesis is the process that occurs within the leaf tissues, called the mesophyll, by which absorbed sunlight is converted to the chemical energy needed to manufacture sugars, the nutrient of plants. Leaves are normally as flat as possible so that they can absorb maximum sunlight and thereby maximize their sugar production. The manufactured sugar is transported to other areas of the plant through the leaf vascular system, which is composed of branching internal traces and the visible veins.

Protection From Dehydration

• The visible layer on the top and bottom of the leaf blade is called the epidermis. It secretes a wax-like layer over the leaf surface called cutin. This protects the leaf from overexposure to the sun, and hence dehydration. In addition, there are specialized cells in the epidermis, mostly on the underside of the leaf, which guard microscopic pores called stomata. The guard cells close the stomata on hot days so that internal moisture doesn't escape.

Protection From Disease

• The wax-like cutin layer also is a barrier against some plant diseases. It prevents them by repelling water, much like a raincoat, and thereby preventing any harmful organisms carried by it from infiltrating the plant. Airborne organisms also are hindered by the cutin layer. Many pesticides are manufactured with agents that purposely stick to the cutin so that the product can effectively provide additional insect and organism control.

Nutrient Storage

• Certain types of plants store reserves of manufactured sugars and water within their leaves. Plants grown from bulbs or rhizomes, like onions and irises, are examples. If iris leaves are cut open, a watery sap leaks from them. The same is true if an onion is cut open. An onion is a type of food-storing leaf. Succulent plants such as aloe and sedum also are examples of plants that store nutrients in their leaves.