12 THE RELATION OF DESERT PLANTS TO 



above. As has been noted, however, conditions other than depth 

 seem to play a part in determining the moisture content on this rocky 

 hill, great variations being manifested in a number of samples taken 

 from different places at the same depth. 



The surprisingly large amount of water contained in this soil rela- 

 tively quite near its surface is probably very largely due, paradoxical 

 as it does indeed seem, to the excessively high rate of surface evapora- 

 tion. After a number of heavy showers, when the soil is quite moist 

 to an indefinite depth, the first few centimeters lose water much more 

 rapidly by evaporation into the air than it can be supplied by the much 

 slower process of diffusion upward through the soil films from lower- 

 lying layers. The result is that there is soon formed a very perfect 

 mulch of air-dry soil, similar to the "dust mulch" of the agriculturists. 

 In this condition the rate of water loss from the true evaporating sur- 

 face, which now lies at some depth within the soil, is governed, not by 

 the power of the free air to vaporize water, but by the rate of diffusion 

 of water vapor through the nearly air-dry layers which lie above. This 

 subject will be again considered in the chapter devoted to atmosphere 

 studies. 



In the chapter devoted to plant studies the question of how much 

 water is needed in the soil in order that seeds of desert plants may 

 germinate and develop into seedlings will be considered, and experi- 

 mental evidence will be brought forward pointing clearly to the con- 

 viction already noted by Spalding (loc. cit. ) that sufficient moisture 

 is probably at all times present in the deeper layers of these soils for 

 the needs of transpiration and even growth of the desert plants which 

 root deeply enough to reach those layers. That there is considerable 

 variation in the water content of the deeper soil layers of Tumamoc 

 Hill is shown by the fact already stated, that at the end of the spring 

 dry season Fouquieria plants which had not lost their leaves were 

 numerous, while many others were leafless, the latter apparently indi- 

 cating a paucity of water in the soil within reach of their roots. It is 

 possible that the latter plants were so situated that their roots did 

 not reach moist soil on account of solid rock or large rock fragments. 



PERMEABILITY OF THE SOIL TO WATER AND RATE OF DOWNWARD 



MOVEMENT. 



As has been stated, the season of summer rains began on July 15. 

 From this date until September 1 heavy showers were frequent, some- 

 times several on the same day, sometimes at intervals of several days, 

 and with each shower the surface of Tumamoc Hill was thoroughly 

 flooded with water. The precipitation flowed off from the general sur- 



