Competition among plants.

Competition is a word of various meanings. In biology, it originally was introduced to account for the low survival rate of the potential offspring of all creatures. The number of seeds formed by a pea plant may be a dozen; most annual plants produce hundreds or thousands of seeds; cottonwood trees and orchids seeds run to the millions; and, in the case of fern and mushroom spores, there are billions formed by a single individual. Since in a state of equilibrium each plant can be replaced by only a single other one, processes were considered that eliminated the excess offspring (such as the activity of predators). With Darwin's evolution theory, competition took on additional meaning in relation to survival of the fittest. Competition was not anymore a struggle between equals, but a mechanism to award superiority. Competition became a contest, and considerations of combat, struggle, territorial exclusion, and even war entered in the wake of Darwin's ideas. As Warming (1) states, competition is "a consideration of the means by which plants oust each other from habitats." But, it is hard to conceive of any mechanisms by which stationary plants can combat each other to result in an ousting. In an important experimental investigation, Clements, Weaver, and Hanson (2) studied competition. They concluded that "Competition is purely a physical process. With few exceptions, such as the crowding up of tuberous plants when grown too closely, an actual struggle between competing plants never occurs.. In the exact sense, two plants, no matter how close, do not compete with each other as long as the water-content, the nutrient material, the light and heat are inl excess of the needs of both. When the immediate supply of a single necessary factor falls below the combined demands of the plants, competition begins." When growing sunflower, wheat, and other plants at different distances of each other, Clements et al. (2) found that the closer the plants were spaced to one another, the more they inhibited each other. But, it appeared from their data (see Table 1) that: (i) all plants in a competition plot were equally reduced in growth, and (ii) with increasing density of the planting, the production of the plants per unit area tended to reach a maximum value, which was not changed with further decreases in spacing. This is a common experience in all agricultural spacing tests, a result that shows that this form of competition does not provide a mechanism for selection, since all individuals are equally inhibited. The same experience was gained from observations in the field. In the center of Death Valley near the headquarters of the National Monument, with an average yearly rainfall of about 40 mm, the vegetation is exceedingly poor. Only very few shrubs-such as Larrea, Atriplex hymenelytra, and Tidestroemia-grow in that area, and the number of seedlings of annual plants appearing after rain is small (see Table 2). With the