ABSTRACT
The evolution of terrestrial plants has been associated with the development of complex plant responses to atmospheric humidity. In contrast to life in the aqueous milieu when the earth began, the survival of terrestrial plants requires either the maintenance of a suitable hydration level within active cells or the ability to survive periods of desiccation. Maintenance of sufficient water within cells requires sophisticated regulation of water loss especially when atmospheric humidities are low. The mechanisms by which vascular plants control the loss of water vapor also influence the exchange of other gases, such as carbon dioxide and oxygen, which are essential for plant function. Leaf structures which reduce the outward flow of water vapor also reduce photosynthesis so that it may be anticipated that gas exchange of plants is regulated at intermediate levels which vary depending upon environmental conditions and plant characteristics.
