161 



with RAUNKLER'S statement that the Therophytes are the 

 growth-form best adapted to a dry hot climate, but that they 

 decrease in numbers where the temperature is low. 



As already stated the most important characteristic of 

 the biological spectrum of the Transcaspian lowlands is the 

 great number of Therophytes or annual plants (41 per ct.). 

 This is also characteristic of countries to the south and 

 south-west, and in the desert areas of western North-America, 

 (see table 3 p. 159). According to MAC DOUGAL (1909) and 

 CANNON (1909), the Sonora desert has two precipitation-maxima 

 with spells of dry weather between (comp. p. 59); during the 

 two wet periods there is a great abundance of mesophytic 

 annual plants (ephemerals), the species of the two periods 

 being different although some of them appear twice a year 

 both in summer and winter. (See also THORNBER 1. c.) 



Most of the annual plants of Transcaspia belong to 

 what VOLKENS (1. c. p. 20) terms ephemerals, that is plants 

 which complete their life-history from germination to the 

 ripening of seeds during the short and comparatively moist 

 spring-time (see above p. 59). In countries where the winter 

 is warm, germination sometimes takes place during autumn 

 or winter (VOLKENS p. 19), but in the cold winter of Trans- 

 caspia this is not likely to be the case. 



The existence of the ephemeral plants is dependent on 

 a short comparatively moist favourable period, sufficiently 

 warm and long enough to permit them to complete their 

 development. The adaptations of the plants consist in the 

 capacity for rapid development and in the resistance of the 

 seeds to prolonged dessication; their vegetative parts on the 

 other hand show only slight adaptation against dessication, 

 so that they must be termed mesophytic in structure. 



An important condition for the welfare of the ephemeral 

 plants, which also holds good as regards the longer lived 

 annual plants, is that sufficient open ground is available and 

 that the perennials or the ligneous species do not grow so 

 close as to hamper the germination and growth of those 

 plants which must start afresh from seeds every year. This 

 condition is fulfilled in the Transcaspian lowlands where a 

 dense vegetation is only found in the valleys of the rivers and 



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