WATER UTILIZATION BY TREES 73 



wilting point and transport to this dry soil the moisture obtained in a 

 moister soil horizon. It is even possible for a deep-rooted plant to 

 absorb moisture from the subsoil, transport it to a drier soil, and there 

 give it up, where it may be absorbed by a shallow-rooted plant. The 

 part this process plays in desert associations and its practical impor- 

 tance remain for future research to determine. 



ABSORBING OF WATER BY AERIAL ORGANS 



Still another method of providing for water is absorption by aerial 

 organs. Actual absorption of dew by leaves is very slight, although 

 Spalding (199, 200) reported that leaves of Celtis, Parkinsonia, Pro- 

 sopis, and others are able to absorb water vapor from very humid 

 atmospheres. The same may be said of the absorption of rain by 

 leaves and twigs. Although these organs are able to absorb water to 

 a slight extent, they certainly are not able to absorb water as easily 

 as they can lose it. The chief value of rain probably lies in the check- 

 ing of transpiration rather than in the absolute amounts of rain water 

 absorbed by the aerial organs of the plant. 



In this connection one might mention the interesting modification 

 possessed by some shrubs and trees of semiarid regions, such as Tama- 

 risk spp., which, according to Volkens (223), absorb water from the 

 atmosphere with the aid of hygroscopic salts that they secrete and 

 then pass this absorbed water on to the tissues beneath. But it must 

 be admitted that an osmotic system which would permit of this sort 

 of transfer is rather difficult to understand. 



TESTING ABILITY OF TREES TO WITHSTAND DROUGHT 



It is, of course, advantageous to know as much as possible concern- 

 ing the drought resistance of trees at various stages, especially when 

 plantings in regions where the water supply is limited (i. e., semiarid 

 regions) are contemplated. Much information has been obtained em- 

 pirically in the past (173) simply by noting whether or not trees will 

 survive in a given climate, but such methods leave much to be desired. 

 For this reason recent workers have been attempting to obtain expe- 

 rimental data by subjecting plants to a lack of moisture and noting 

 how well they survive. Among the recent students of drought resist- 

 ance, Russian investigators have been notably prominent and the 

 reader is referred especially to the works of Kondo (128) and of Tuma- 

 nov (217), who have been chiefly interested in wilting as an index to 

 drought resistance. 



This entire problem, of course, is connected with the question of 

 relation of species or variety to site and is of great practical impor- 

 tance in the matter of land utilization in general and of the best use 

 of forest properties in particular. Although this matter has been dealt 

 with in various places above, it is of such a fundamental nature that 

 it cannot be stressed too much. Not only is there a difference in water 

 requirement and drought resistence between species, but there may 

 even be a difference between races of the same species. Thus Putten- 

 dorfer (172) attempted to find differences between three races of Pinus 

 sylvestris from the Union of Soviet Socialist Republics, Germany, and 

 France by studying (1) wilting, (2) water content of needles, and (3) 

 osmotic pressure of needle press juice. Differences were greater in 



