Spalding: Absorption of atmospheric moisture 375 



twelve species of desert perennials subjected to experiment all 

 exhibit some slight capacity for direct absorption of water vapor 

 from the atmosphere, but in general the amount absorbed is incon- 

 siderable in comparison with that given off in corresponding periods 

 in dry air. As yet there is no evidence that these almost infinites- 

 imal quantities of water are utilized in the body of the plant. 

 The promptness with which the water thus absorbed is returned 

 to the atmosphere suggests rather a superficial process, purely 

 physical in its nature, and of no immediate physiological signifi- 

 cance. In any case the present experiments, taken in connection 

 with earlier ones already referred to, show that the net result to 

 the plants found to be capable of absorbing either liquid water or 

 water vapor is plainly much less in the latter than in the former 

 case. Nevertheless, as shown most clearly in the experiments with 

 Fauguieria. and Covillea, a high degree of saturation of the atmos- 

 phere, by directly or indirectly preventing excessive transpiration, 

 may be far more advantageous to the plant than a succession of 

 light rains which are insufficient to moisten the soil ; the advantage, 

 in this case, being manifested in most striking fashion to the eye in 

 the living green with which these otherwise sombre-hued shrubs 



of the desert are clothed. 



Possibly the most important conclusion to be drawn from this 

 study and a preceding one on leaf-absorption is the patent fact 

 that the roots of desert plants constitute their only reliably active 

 agent in providing a normal water supply. This conclusion, 

 indeed, was to be expected ; but it is none the less important to 

 establish, in the face of evidence that might be interpreted other- 

 wise, the capital fact that upon the roots of these desert plants 

 rests practically the entire burden of securing and transmitting the 

 water upon which they depend, and, however far some of them 

 have departed from the habits of present-day mesophytes, they 

 remain in this particular fundamentally unchanged. 



Desert Botanical Laboratory, 



Tucson, Arizona. 



