SOIL MOISTURE AND TO EVAPORATION. 69 



humid regions are uniformly not quite so resistant to drought as the 

 desert forms studied. The difference is not very marked, however, and 

 in explaining the existence of desert plants emphasis is apparently to be 

 laid, not upon the greater resisting power of such forms to paucity of 

 soil water, but upon the facts that there seems to be always considerable 

 moisture in the soil under discussion, that this moisture is conserved by 

 comparatively slow transpiration, and that most non-storage forms of 

 the desert root very deeply. 



The general conclusions from these studies of the moisture require- 

 ment for the desert forms experimented with are: (1) Those plants 

 which exist throughout the dry season can withstand a somewhat drier 

 soil than those which appear only in the rainy season, and even these 

 latter may often resist wilting in a drier soil than can such non-desert 

 plants as squash, cabbage, etc. (2) There is sufficient moisture in the 

 soil of Tumamoc Hill, and this is near enough to the surface, to 

 supply the transpiration needs of such plants as Euphorbia and seedlings 

 of Fouquieria. The larger plants of Fouquieria, as well as the other 

 shrubs, must be considered as having a root system well enough dis- 

 tributed through the soil to correspond to their comparatively large 

 transpiration surfaces. They probably root very deeply in rock crevices. 

 (3) The roots of seedling Fouquierias elongate directly downward so 

 rapidly as to make it appear possible for them to reach a permanent 

 and adequate water supply before the soil, wet thoroughly by the fre- 

 quent showers of the rainy season, can produce injury through condi- 

 tions of drought. After their roots have reached a depth of 30 cm. 

 the plants are probably safe on the hill in most seasons. 



The open formation of desert vegetation doubtless makes it possible 

 for the plants to draw upon a very large volume of soil for their water 

 supply. The noticeable scarcity of seedling or even young plants of the 

 more typical desert forms, even in the rainy season, would seem to indi- 

 cate that conditions other than those of available moisture are effective 

 to reduce the number of these. It may be that in most years the sur- 

 face layers of the soil do not remain moist long enough after each 

 shower to allow the seedlings to obtain a foothold. It seems more prob- 

 able, however, that the depredations of animal life, especially of insects 

 and the smaller mammals, are the most important factor in preventing 

 the growth of seedlings. As has been stated, when young plants are 

 left exposed in the early part of the rainy season, before the desert has 

 assumed the semimesophytic aspect of this season, they are almost sure 

 to be cut off by animals within a day or two. The importance of animal 

 life in determining the nature of desert vegetation is well substantiated 

 by the patent observation that plants which succeed well in arid regions 

 are generally well protected from animals in one way or another. 



