— 169 — 



podiaceae. What was said of the Hemicryptophyles is still 

 more the case here: that the early flowering species may 

 be summer-plants; here every one is such, and not one is 

 known to rest during the summer-periods, although some of 

 the species of Astragalus may shed their leaflets. 



The Transcaspian Fanerophytes occur in greatest numbers 

 and attain their highest development in the Sand-desert. As 

 already stated (p. 79), the Sand-desert has a rain-absorbing 

 soil, but its capillary action is slight, hence there must be 

 larger supply of water in the deeper layers than in the Clay- 

 deserts. This is naturally correlated with the occurrence of 

 trees since these have long roots capable of extending down 

 to the water; compare also p. 48. 



Another factor of importance for the ground-water and 

 therefore for the Fanerophytes, is the density of the vegeta- 

 tion. Wherever in dry regions there is a rich vegetation of 

 herbs or small shrubs, these will intercept and absorb the 

 water, so that it is not allowed to sink into the soil. Ko- 

 STYTSCHEFF points out that forests might grow on the South- 

 Russian steppe if the surface were loosened, so that water 

 might penetrate the soil. Why this does not take place is, 

 he states, because the surface of loess is impervious to water, 

 especially when covered by plants. 



Without entering into the question why the Russian 

 steppe is treeless, it is noteworthy that Transcaspia presents 

 better conditions for growth of trees than South-Russia; this 

 is a result of the absorptive capacity of the soil due to its 

 structure and to the open plant - covering, aided also by 

 climatic conditions. Reference may be made in this connec- 

 tion to Schimper's statement (p. 527) that the South-Russian 

 steppe has a dry as well as a cold winter, an element 

 hostile to the growth of trees. For when dry winds blow 

 at the same time as the ground is frozen, the trees with 

 their branches and buds more exposed than smaller plants, 

 will be unable to compensate for loss by transpiration. 



Somewhat similar conditions prevail on the Spanish 

 steppes; the winter is cold, dry and windy ^). May has the 

 greatest amount of precipitation. 



') Climatic tables for Zaragoza and Valladolid in Meteorol. Zeitschrift 

 9. 1874. p. 218. 



