772 



TEXTBOOK OF BOTANY 



rain at all. At the same time it has the highest evaporation rate. Winter 

 temperatures are mild, and freezing temperatures are of short duration. 

 Here is the most bizarre assemblage of plants on the continent. The 

 largest and greatest variet\' of the columnar cacti are found in this 

 region. Some are much-branched and up to 40 feet in height. They all 

 have shallow wide-spreading root systems. All have exceedingly low 

 transpiration rates, because of impervious outer layers of cortical tissue. 



Fig. 399. The native "desert" palm of southern Cahfornia (Washingtonia) in 

 Palm Canyon, where undergroimd water is always available. Photo by U. S. Forest 

 Service. 



Some have closed stomates during the daytime. A year's supply of water 

 has accumulated in many of these plants, and this may amount to as 

 much as 25 to 30 tons in a single plant. 



In Arizona there are two rainy periods, winter and summer. The rains 

 are typical downpours of short duration. During the winter rains the 

 desert "blossoms" with various flowering plants, some of which are com- 

 mon farther north. During the summer rains another assemblage of 

 annuals and perennials with more tropical affinities becomes conspicu- 

 ous. Between the rainy periods the perennial scattered woody plants and 

 succulents give character to the landscape. Mesquite, creosote bush. 



