lowland cismontane non-desert station such as Berkeley, for 

 example, the average monthly rainfall for the month of August 

 is only 0.05 inch (0.1 cm); in Indio, in the desert, the August 

 rainfall averages 0.38 inches (1 cm). Summer rains generally are 

 less reliable than winter rains, and the amount of rain that falls 

 during the summer in these desert areas usually is much lower 

 than that which falls during the winter. As a result, some areas 

 of the Colorado Desert (particularly that portion located in Ari- 

 zona) support two somewhat different sets of annual plant 

 species. Winter annuals germinate and grow during the winter and 

 flower in the spring; these species provide the spectacular displays 

 in such areas as the Anza-Borrego Desert in southern CaHfornia. 

 Less well known is the smaller number of summer annuals which 

 germinate after summer rains and flower during summer months. 



The winter annuals that have been studied germinate only 

 when the temperatures are relatively low, thus being prevented 

 from germinating during the summer rains. In addition, such 

 plants will not flower until the days reach a critical length in the 

 spring, after the cool wet winter season. These winter annuals 

 are "informed" that spring has arrived by day -length rather than 

 by temperature or moisture conditions, perhaps because over the 

 long run day -length is a more reHable indicator of season than are 

 other environmental conditions. In contrast, summer annuals 

 germinate only at a warm temperature and thus appear during 

 the summer but not the winter. These plants have no photoperiod 

 requirement for flowering; they will flower when the plants have 

 reached a suitable size for the production of flowers. Because 

 these plants carry out their growth during the relatively benign 

 temperature regimes that exist in the summer, they do not require 

 a further mechanism to delay their flowering until a specific 

 season has been reached. 



Another class of plants that live in arid regions are called 

 phreatophytes. Phreatophytes are perennial plants that have ex- 

 tensive and deep root systems that enable them to tap under- 

 ground sources of water. Greasewood (Sarcobatus vermiculatus, 

 Chenopodiaceae) is an example of a phreatophyte that occurs in 

 desert regions. The young seedUngs of most phreatophytes 

 produce extensive root systems very rapidly during the winter 



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