72 THE RELATION OF DESERT PLANTS TO 



osmotic pressure, they undoubtedly decrease the rate of evaporation. 

 It has been suggested by Aubert (1892) and others that the low trans- 

 piration rate observed in the case of the cacti is in part due to the 

 presence of large amounts of organic acids, gums, and slimes in the cell 

 sap of such plants. How important the latter substances may be in 

 Cereus, Opuntia, and Echinocactus should be well worth a determination. 



CONCLUSION. 



The most important results of the three lines of investigation already 

 discussed separately will now be brought together. Probably the most 

 interesting fact discovered through these studies is that the deeper 

 lying soil layers of Tumamoc Hill contain at the end of the spring 

 dry season, and therefore probably at all times, a relatively large water 

 content. During the two weeks just preceding the beginning of the 

 summer rains, tests indicated that the soil contained from 12 to 15 per 

 cent of moisture at a depth of not over 40 cm. 



This surprisingly large water content of the lower soil layers is 

 probably largely due to the fact that the evaporation rate from the soil 

 surface far exceeds the rate of movement of soil water, thus causing 

 the true surface of evaporation to lie some distance below the soil surface, 

 the water lost finding its way to the air in the form of vapor, which 

 diffuses upward very slowly through the air-dry layers. In this way 

 the deeper portions of the soil are to a great extent protected from loss 

 of moisture by a layer of dry surface soil resembling a dust mulch. 

 The deeper soil layers are doubtless also protected by the presence of 

 numerous rock fragments and by the hard-pan of caliche, which is very 

 slowly permeable to water. 



Downward penetration of precipitation water, while it takes place 

 slowly through the soil itself, is on the whole comparatively rapid 

 on account of the oblique rock surfaces, along which movement is not 

 markedly checked. 



The amount of soil moisture at a depth of half a meter or less is 

 sufficient to supply the transpiration needs of such typically desert 

 plants as were experimented upon (Euphorbia and Fouquieria) , and is 

 probably also adequate for Tribulus and Allionia, and perhaps even for 

 Boerhavia, the most mesophytic desert form studied. These annuals, 

 however, may not root deeply enough to avail themselves to any great 

 extent of this water. 



Seeds of Fouquieria and Cereus fail to germinate in soils containing 

 less than 15 per cent of moisture by volume, not differing in this respect 

 from Phaseolus and Triticum. It is thus apparent that Fouquieria 



