Ask Mr. Smarty Plants

Plants do give off moisture in a process called transpiration. The major way the plant does this is through special holes, or pores, on the leaf surface called stomata (singular, stoma). These stomata must open for the plant to take in the carbon dioxide (CO2) that is crucial to its survival and growth. The stomata open and close dependent on the shape of the two guard cells on either side of the stomatal pore. The guard cells respond to water pressure. When there is plenty of water present and the pressure in the guard cells is high, their shape changes so that the pore of the stomata is open and the plant can take in CO2. When it is open the stomata can also transpire, or lose water. How much water it loses depends on the surrounding temperature, humidity, and air movement. When there is little water pressure in the guard cells, they close and the plant ceases to transpire (as well as to take in CO2). Closing the stomata conserves water for the plant.

Some water can also be lost simply through the surface, or cuticle, of the leaf as well as through the stomata. However, because the cuticle has a waxy surface that prevents the water passing through it, this is usually a very small fraction of transpiration. In some plants there is a very heavy waxy deposit on the cuticle that gives extra protection to keep the plant from drying.

The number and placement of the stomata in the leaves also affects how much the plant transpires. Many species of plants have fewer stomata per square centimeter on the upper surface of the leaves than they have on the relatively protected under surface. Other species have stomata only on the under surface of the leaf where there is more protection from heat and air movement that can increase transpiration.

Here are several Internet sites that give you more information on plant transpiration:

1. University of Arizona Cooperative Extension
2. University of Wisconsin Earth Online
3. University of Michigan Reach Out!