Thus in every factor there is conspicuous environmental heterogeneity or variability. What are the consequences for living organisms?
First of all, the distribution of the rainfall in two seasons separated by a period of intense heat and dryness, in a region affording sufficient temperature for growth throughout the greater part of the year, results in two distinct vegetative seasons. The first is the period of winter and spring annuals, shrubby or frutescent perennials. The second is that of summer annuals and frutescent and arborescent perennials.
The annuals developing in the winter and spring months and those appearing after the torrential rains during the heat of July and August are not only subjected to widely different conditions of growth, but are specifically distinct and physiologically dissimilar. The life cycle of these winter annuals may be short or long, depending upon the distribution of temperature. They may germinate and begin growth with November rains and mark time in development throughout the colder winter months, and complete vegetation and fruition with the precipitation of February and March and the warmth of March and April. On the other hand, germination and initial growth may be delayed by low temperature and inadequate moisture until well into March, when, if water is scarce and temperature high, the whole life cycle of the plant may be of remarkable brevity. Under these circumstances, many of the plants open their flowers and even nearly or quite mature their fruits with the cotyledons still apparently functional.
If the winter rains be supplemented by heavy spring showers, the winter annuals, which would otherwise be dwarfed, except in the most favored spots, may show long-continued growth and attain a large size.
While the winter and summer annuals pass the periods of greatest extremes of temperature and of dryness in the form of resistant seeds, the woody perennials must remain exposed to the most extreme conditions of the year. In their physiological activities, they show the greatest diversity. Some respond to the winter moisture and spring warmth by foliation and fruition. Others lie dormant throughout the first growing season to burst into leaf and flower after the heavy summer rains. Some are physiologically active in one growing season only, others in both. Fouquieria loses its tender leaves whenever the soil becomes too dry, and clothes itself with green again, whenever temperature and soil moisture are favorable. Mortonia retains its tough leaves for years.
The second consequence of the division of the rainfall into two seasons, instead of one period of precipitation, usually found in desert regions, is a fairly luxuriant growth of tree-like perennials, as well as of small rapidly maturing annuals. Thus, these southwestern deserts have fittingly been called arboreal deserts; the greenest of all deserts.
It is this covering of trees, often mere shrubs, if size be the criterion
