SOIL MOISTURE AND TO EVAPORATION. 73 



and Tribulus exhibit no special adaptation to the arid climate of the 

 desert, as far as germination is concerned. 



As soon as germination occurs, in Cereus, Fouquieria, Covillea, 

 Tribulus, and Boerhavia, a very rapid elongation of the primary root 

 sends the tip of this organ far into the soil. While this is taking place 

 the aerial parts grow but slowly. In the case of Fouquieria and Cereus 

 the cotyledons are the only leaves for many days and even weeks. In 

 Fouquieria a curious transverse thickening of the hypocotyl accom- 

 panies the rapid root growth, so that after two or three weeks the stem 

 of the seedling is exceedingly thick and woody and is covered with a 

 corky layer, while the root may be still unbranched and may have 

 extended many decimeters into the soil. 



This habit of growth is well adapted to desert conditions. During 

 the rainy season the soil is often sufficiently moist for germination, and 

 by the end of the summer the perpetually moist soil of the deeper 

 layers is continuous upward to within a few centimeters of the surface, 

 so that seedlings which exhibit the phenomenon of growth just de- 

 scribed should find themselves well rooted in perpetually moist soil long 

 before the drying out of the upper layers could result in their death. 

 It seems that moisture conditions alone can not account for the notice- 

 able lack of seedlings and young plants in the desert, but that the rav- 

 ages of animal life must play an important part in restricting vegetation. 



The clay soil of Tumamoc Hill has a high moisture-retaining 

 power, being able to hold water to an amount about equal to 41 per 

 cent of its wet volume. While this prevents rapid percolation of pre- 

 cipitation water from the surface layers to those more deeply seated, 

 thus keeping much of the water of the first rains of the summer 

 near the surface and thus poorly protected from evaporation, this phe- 

 nomenon favors water absorption by those storage plants which take 

 moisture mainly from the surface layers of the soil. Mrs. Spalding has 

 noted that when the ground about a Cereus plant is moistened, either 

 artificially or by rain, absorption begins almost immediately, long before 

 the water could have reached the deeper soil layers. This must mean 

 that these plants, and probably also the other cacti of the region, absorb 

 water very rapidly from the wet surface soil directly after the rains. 

 Thus the high retaining power of the clay gives to such plants practi- 

 cally all of the water which falls in their vicinity, excepting what is lost 

 by evaporation before they have time to absorb it. 



The saps of Cereus, Echinocactus, and Opuntia exhibit osmotic 

 pressures no higher than those commonly found in plants of the humid 

 regions. Therefore, for these cacti at least, adaptation to desert con- 

 ditions is not manifest in increased concentration of the cell sap. 



