200 SOILS. 



mersed in a (probably not even fully) saturated atmosphere 

 is capable of supplying the requirements of normal vegetation ; 

 thus explaining the obvious beneficial effects on vegetation of 

 the summer fogs prevailing in portions of the arid region, 

 e. g., on the coasts of California and Chile. 



Mayer's experiments relied upon to prove the uselessness of hygro- 

 scopic moisture to plant growth, were carried out in flower-pots, in 

 which it was plainly shown that the plants wilted before even the 

 visible liquid (capillary) moisture of the earth was entirely exhausted. 

 But this simply proves that under such artificial conditions, plants can- 

 not withdraw moisture from the soil rapidly enough for their needs. 

 In nature, and notably in the arid regions, the chief supply of water is 

 received through the deep-going main roots, while the bulk of the 

 active feeding roots of the plant may be surrounded by almost air-dry 

 soil; under which conditions, as Henrici (Henneberg's Journ., 1863, p. 

 280) has shown, slow growth and nutrition occurs even in such plants 

 as the raspberry, a native of humid climates. But in the arid region 

 this is the normal condition of the native vegetation through most of 

 the rainless summer. That a higher moisture-coefficient does not 

 necessarily imply that a larger amount of moisture can be withdrawn 

 from the soil by the plants, is undoubtedly true in some, but not in all 

 cases ; for in soils rich in humus, the moisture is more freely shared 

 with the roots than in non-humous, clay lands. 



The higher moisture-absorption is however of the most un- 

 questionable service in the case of the occurrence of the hot, 

 dry winds that so frequently threaten the entire crops of some 

 regions. In this case the soil containing the greater amount 

 of moisture requires a much longer time to be dried, and heated 

 up to the point of injury to the roots, than in the case of sandy 

 soils of low absorptive power, whose store is exhausted in a 

 few hours and then permits the surface to be heated up to the 

 scalding point, searing the stems and root crowns. That such 

 injury occurs much sooner in sandy lands than in well-culti- 

 vated clay soils, is a matter of common note in the arid region. 



Summary. The significance of hygroscopic moisture in 

 connection with plant growth may then be thus summarized : 



i. Soils of high hygroscopic power can withdraw from 

 moist air enough moisture to be of material help in sustaining 



