Introduction

As with humans, water plays an important role in the heat balance of plants and animals, and it is also required for other aspects of an organism’s physiology. For plants, water serves four main functions (Goudie and Wilkinson 1980): cellular material functions only in its presence; it dissolves and transports nutrients; it provides raw material for photosynthesis; and, by absorbing heat and losing it through evapotranspiration, water provides a thermoregulatory mechanism so critical in the harsh thermal environments of deserts and semi-arid lands. It is particularly effective in this last role because of its high specific heat and because of the heat utilized or released upon evaporation or condensation (approximately 600 kcal/g). For animals, water is required for cellular function and it also plays a thermoregulatory role. The ultimate stress is cellular dehydration, and dryland species of plants and animals vary greatly with respect to their tolerance for water loss.

Survival in a desert thus requires that plants and animals maintain both body moisture and temperature within established ranges of tolerance. This is achieved through a variety of mechanisms designed to evade periods of stress or reduce stress by reducing heat gain, maximizing heat loss, optimizing water uptake, and reducing water expenditure (Evenari 1985). These mechanisms are strikingly similar in plants and animals and include adaptations involving morphology, physiology, biological rhythms, habitats and, in the case of animals, behavior.