or are better developed and more extensive outside the boundaries 

 of the state. 



Adaptations to Aridity 



How do California plants cope with the Mediterranean climate 

 and its prolonged periods of summer drought, as well as with the 

 unreliable winter rains? There are several ways in which plants 

 have responded to this climatic regime. 



Annuals are plants that complete their Hfe cycle within a year; 

 that is, the seeds germinate and the plants grow, flower, and set 

 seed in less than 12 months. Many California annuals have evolved 

 interesting mechanisms that are direct adaptations to growing in 

 areas with a highly seasonal rainfall. Studies by a number of 

 workers, in particular a group of biologists who worked at the 

 Cahfornia Institute of Technology in Pasadena some years ago, 

 have investigated these adaptations. For example, the seeds of 

 several annual species do not germinate unless they have been 

 drenched with more than a half inch (1 .3 cm) of rainfall (or its 

 simulated equivalent in the laboratory). This water must come 

 from above and actually wash over the seeds; placing the seeds 

 in a bed of wet soil will not induce them to germinate. The basis 

 of this behavior is the leaching of chemical inhibitors from the 

 dormant seeds of these annuals, or the leaching of germination- 

 inhibiting salts from the soil. A desert plant which germinates 

 after the first slight rain in the autumn has a very low chance of 

 continuing to survive and grow to maturation, and some desert 

 annuals do not germinate immediately after the first heavy rains 

 but exhibit a delayed germination phenomenon. The adaptive 

 value of this trait is that such a delay, until after one or more 

 heavy rainfalls, increases the chance that the seedlings will be 

 growing during a period of good soil moisture. All the germination 

 patterns that have been studied in desert annuals are explicable 

 in terms of the average pattern of winter rains in desert areas. 

 Obviously, any species that is unable to respond to this average 

 pattern will have a poor chance of survival over a period of many 

 generations. 



Some of the southern California deserts receive summer rains 

 in addition to the winter rains. It is of interest to note that in a 



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