— 164 — 



The figures in table 4 show that while in Denmark 

 four-fifths of the annuals flower after the first of July, we 

 have in the South-Russian steppe (Yeliaterinoslaw) about one 

 half, in Greece and Spain about one third, and in the Trans- 

 caspian lowlands about one fifth which are late-flowering. 



In the series: Libyan desert, Transcaspian desert, Spain, 

 South-Russian Steppe, Denmark, the number of summer- 

 annual plants thus steadily increases. And throughout 

 the same series the summer becomes less warm and 

 less dry. In Cairo there is no rain during 4 — 6 summer 

 months and the mean temperature of July is 29** C; in 

 Merw 3 — 4 summer months are rainless, and the mean July 

 temperature is 30,2° C, (Askhabad 29,7° C, Petro Alexan- 

 drowsk 28,7° C; Table 1 p. 17). In Spain no month is 

 perfectly rainless though July and August are very dry, and 

 the mean July temperature is, for instance, Murcia 29° C, 

 Madrid 24,5° C. Yekaterinoslaw has as mean July temperature 

 of 23,0° C. and the precipitation is greatest in July and 

 smallest during winter.^) 



This leads us to the conclusion that the number of 

 summer-annual plants is more especially dependent on the 

 summer rainfall, as might be expected. From north towards 

 the south the conditions of life during summer become more 

 and more unfavourable, so that fewer and fewer annual 

 species are able to endure, namely only those adapted to 

 withstand increasingly unfavourable conditions. 



But the winter temperatures may also play a part in 

 determining the relative number of annual summer-plants, 

 since plants have a better chance of hibernating in coun- 

 tries with warmer winters than where the cold is severe. 

 In this way annual summer-plants might become perennials. 

 The statement by Volkens (1. c. p. 21), that facultative 

 annual and perennial plants are characteristic for the Libyan 

 desert, is perhaps not only correlated with the summer but 

 also with the winter conditions. 



The mean temperature at Cairo for Jan. is 11,9° C, and 

 even if (according to Hann) a few degrees of frost are some- 



') The temperature records are cited from Hann 1897, III. 



