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The need for water is perhaps the dominating consideration in analyzing the physical environment of plants. In conjunction with their growth processes, leaf-bearing plants give off large quantities of water into the atmosphere, a process termed transpiration. A relatively small amount of water is also required by green plants in the process of photosynthesis in which the energy of light is used together with carbon dioxide and water to produce carbohydrates. The dominant sources of water needed for transpiration and photosynthesis is from the soil, where it is taken up into the plant roots.

The rate of transpiration varies greatly according to the type of plant and the prevailing atmospheric conditions. High temperatures, low humidities and winds favor, particularly of the leaf, largely determine the rate of water loss. Plants with large total foliage surfaces composed of broad, thin leaves have higher rates of losses than plants bearing needle leaves, spines, or small thick leaves (sclerophylls). Under conditions of critical water supply but high rates of evaporation, only those plants can survive that minimize transpiration losses by their special leaf structures and by their small size.

The adaptation of plant structures to water budgets with large water deficiencies is of particular interest to the plant geographer. Transpiration occurs largely from specialized leaf pores, called stomata, which are openings in the epidermis (outer cell layer) and cuticle (an outermost protective layer) through which water vapour and other gases can pass into and out of the leaf. Surrounding the openings of the stomata are guard cells which ~an open and close the openings and thus to some extent regulate the flow of water vapour and other gases. Although most of the transpiration occurs through the stomata, some may pass through the cuticle. This latter form of loss is reduced in some plants by thickening of the outer layers of cells or by the deposition of wax or waxlike material on or near the leaf surface. Thus many desert plants have thickened cuticle or wax-coated leaves, stems, or branches.

Another means of reducing transpiration is the development of stomata deeply sunken into the leaf surface that retard outward diffusion of water vapour into dry air, and the restriction in location of stomata to the shaded undersurface of leaves. A plant may also adapt to a desert environment by greatly reducing the leaf area, or by bearing no leaves at all. Thus needle-leaves and spines representing leaves greatly reduce loss from transpiration. In cacti the foliage leaf is not present and transpiration is limited to fleshy stems.

In addition to developing leaf structures that reduce water loss by transpiration, plants in a water scarce environment improve their means of obtaining water and of storing it. Roots become greatly extended to reach soil moisture at increased depth. In cases where the roots reach to the ground water table, a steady supply of water is assured. Plants drawing from such a sources are termed phreatophytes and may be found along dry channels (draws) and alluvial valley floors in desert regions. Other desert plants produce a widespread but shallow root system enabling them to absorb the maximum quantity of water from sporadic desert downpours, which saturate only the uppermost soil layer. Stems of desert plants are commonly greatly thickened by a spongy tissue in which much water can be stored.

A quite different adaptation to extreme aridity is seen in many species of small desert plants that complete a very short cycle of germination, leafing, flowering, fruiting, and seed dispersal immediately following a desert downpour.