— 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 (Yekaterinoslaw) 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 28,1° 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,° 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 VOoLkeEns (l. 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. 
