34 GENESIS AND DEVELOPMENT 



siderable areas, and it seems often almost inexplicable that no rippling 

 or dune formation takes place. The explanation of their non-formation 

 is to be sought in the fact that the sand-sized particles are too small 

 in proportion to the mass of material, and further, the deposition of 

 dust takes place so rapidly that the wind is not able to carry it away, 

 leaving the coarser particles to accumulate. 



CONDITIONS FOR PLANT LIFE. 



There is a great variation in the conditions for plant life on different 

 sand formations. The climate has something to do with this result as 

 well as the quality of the soil. ' Sea air and saline constituents of the 

 soil destructive to some plants may be beneficial to others. The mobility 

 of a drifting sand dune on the coast may be a condition of life to one 

 plant, while dry atmosphere and the stability of an inland sand field may 

 be essential to the growth of another. Even Pinus maritima, which has 

 produced such wonderful results on the Landes of Gascony, does not 

 grow everywhere even on sand formations in France. It is therefore 

 necessary to study in every case the natural conditions of the locality be- 

 fore the problems of ecological relationship can be solved. 



Some of the conditions of sand formations are, however, everywhere 

 the same and these will here be briefly considered. One of the most 

 important points in this connection is the relation to moisture. The 

 rain-water sinks easily into the sand, the better the coarser the grains are. 

 Generally speaking, the power of retention of water is very small and of 

 all soils sand ranges lowest in this respect. The sandy soil has also a 

 very low power of absorption, and is able to condense only a small por- 

 tion of the atmospheric moisture. This is especially the case with quartz 

 sand. 



Further, sandy soil dries easily, and it is therefore heated quickly by 

 the sun ; but it also cools very soon at night. The difference between day 

 and night temperature can be as high as 40-50 C. In consequence of 

 this, sand is subjected to a considerable bedewing at night, a factor which 

 is important for its capability of carrying a vegetation cover. The great 

 variation of temperatures of the soil is disadvantageous to the plants in 

 one respect, they being more liable to injury by frost, than if growing on 

 any other soil. Sand floras, on the other hand, are always developed 

 earlier, because of the greater heat capacity of the soil. 



