DROUGHT RESISTANCE 



299 



Hot, dry winds sometimes sweep across grain fields of the western United 

 States, killing or seriously injuring plants, even during periods when the soil 

 water content is still relatively high. 



Most species which grow in semi-arid regions, such as the "deserts" of 

 the southwestern United States, or in locally dry habitats, can be conven- 

 iently classified into three groups: (i) ephemerals, (2) succulents, and (3) 

 drought-enduring species. 



The ephemerals are a prominent feature of the vegetation of all semi- 

 arid regions which are characterized by definite rainy seasons. With the 



Fig. 78. Semi-desert vegetation near Tucson, Arizona. The prominent succulent 

 is the Sahuaro {Carneg'tea gigantea). Photograph courtesy of the Desert Laboratory 

 of the Carnegie Institution of Washington. 



advent of rains the seeds of such species germinate, and the entire life cycle 

 of the plant is completed within a few weeks. The new crop of seeds survives 

 the intervening dry period until the next rainy season. Such plants have 

 been termed "drought-escaping" (Shantz, 1927). They are no more drought 

 resistant than many mesic annual plants. 



Succulents constitute a considerable proportion of the vegetation of most 

 semi-arid regions (Fig. 78), and are frequently found in locally dry habitats 



