THE SOIL AND WATERCOURSES 41 



some extent consolidated the dune, a number of 

 other plants are able to take root. These all tend 

 to hold together the sand around their roots and 

 to protect the surface of the dune from the wind 

 and from evaporation. There is therefore a ten- 

 dency for the dune to become more and more 

 solidified and immobile, and more and more covered 

 by vegetation : each plant that grows and then 

 dies increases by a little the organic constituents 

 of the dune, and by that much decreases one of the 

 factors which render colonization of its surface so 

 difficult. 



The relationships of sand to rain falling on it are 

 remarkable. Water which falls on its surface sinks 

 in with extreme rapidity, and to a considerable depth: 

 and the tendency for the water to rise by capillarity 

 is slight, because the sand-particles are relatively 

 large. Therefore the rain which falls on sand is 

 of no use to short-rooted plants. On the other 

 hand, as it sinks rapidly and as the surface of the 

 water remains at a considerable depth, loss by 

 evaporation is less than in any other type of desert. 

 In fact, the sand dune is a sponge, and most of 

 the water which it receives it holds. The con- 

 ditions favour long-rooted perennials rather than 

 other types of plant. The sparse fl.ora of a sand 

 dune in the Great Palsearctic Desert often consists 

 solely of perennial grasses and deeply rooted switch 

 bushes, but in parts of Transcaspia, where sand 

 overlies impervious loess at no great depth, quite 

 luxuriant vegetation grows even in summer 

 (Paulsen). 



