One controlling factor, then, is dominant in the desert: the scarcity of water. This results not only in the unusual forms and adaptations of desert plants, but also in a distinctive type of plant community. In wetter climes, plants compete mostly for sunlight. They can grow close together, as long as they receive adequate illumination. During the regrowth of forests, several distinct sets of plants appear, each succeeding group more tolerant of shade than the last, as the forest canopy closes. In the desert, there is no such succession. Clear a patch of desert vegetation, and the same species will reappear—spaced out, with bare ground between them as before. For here sunlight is abundant (we might say overabundant), but there is not enough water to allow plants to cover the ground. Dr. Forrest Shreve, who was a botanist at the Carnegie Institution’s Desert Laboratory near Tucson and a master student of deserts, defined a desert as “an area in which deficient and uncertain rainfall ... has made a strong impression on the structure, functions, and behavior of living things.” The distinctive characteristics of desert plants and animals have evolved through millions of years, in a trial-and-error process in which only the better-fitted forms have survived. It would be enlightening to know how many of the species and varieties of plants that developed during the past 60 million years or so have failed to adapt to Sonoran Desert conditions. It is fascinating to study the hundreds of forms that have succeeded and to try to determine what structures they have perfected and what methods they have originated that enabled them to maintain themselves in such a harsh environment. Desert plants can be classified as “escapers,” “evaders,” and “resisters,” according to their means of adaptation to water shortage. Escapers, such as the annuals, avoid the problem entirely by waiting out the dry periods as seeds, to sprout and reproduce only when the rains come. Evaders, such as the ocotillo, reduce their water loss during droughts by such methods as dropping their leaves or going into a state of dormancy. Resisters, however, “hang in there” all year, taking the desert’s worst. The cactuses, prime examples of this group, rapidly soak up water from each rain and store it for use during drought; the mesquite’s deep roots tap a more constant source of moisture. Succulents A large group of desert plants conserve water for use in periods of drought by storing it in specialized tissues during the wet season. Some of these “succulents,” principally the yuccas, eschevarias, and agaves, have developed water-storage tissues in their leaves. A few, notably the NIGHTBLOOMING CEREUS (see appendix for scientific names of plants), have slender stems but an enormous, carrot-shaped root in which the moisture-storage tissue is located. The GOURDS also have large, thick, moisture-retaining roots, as does the WILD-CUCUMBER.