CHIEF ENVIRONMENTAL CONDITIONS. 125 



receive some attention. The water requirement, in this special sense, 

 denotes the ratio of the total water-loss by transpiration to the yield 

 of plant material for any given period of time, usually for the entire 

 growing-season. This conception has recently received renewed 

 attention and very thorough study at the hands of Briggs and Shantz,^ 

 who have employed it as a physiological criterion by which to compare 

 the relative drought resistance of agricultural plants. 



If several different plant-forms be grown under the same set of 

 climatic conditions, it is found that the different forms differ in their 

 water requirements; the amount of water required to produce unit 

 weight of crop is greater in one case than in another. In such a case 

 the plant with the lower water requirement is the one giving the 

 larger crop with the smaller amount of water, and it is obvious that 

 this criterion must be very valuable in the study of agricultural con- 

 ditions in arid and semiarid regions. It is also clear that such com- 

 parisons between plant-forms are always made with reference to some 

 given set of climatic conditions; if one form has a lower water re- 

 quirement than another for one climate it does not follow that the 

 same relation must hold for the same two forms in another climate. 

 Thus, relative water requirements need always to be stated with 

 reference to a particular environmental complex, which must be de- 

 fined as precisely as is possible. Of course the water requirement of 

 a given plant-form may also be employed as a criterion by which dif- 

 ferent sets of environmental conditions may be compared, the physio- 

 logical properties of this plant-form being the standard of measure- 

 ment in such a case. This whole matter promises much for both 

 agricultural science and the ecology of uncultivated plants, and it 

 may be predicted that water requirement will assume greater impor- 

 tance in discussions of plant water-relations, as this field becomes 

 more thoroughly investigated. 



The foregoing general and incomplete treatment of the subject of 

 the influence of water conditions upon plants may suffice for the 

 present. It is to be hoped that the future may furnish well-collected 

 and well-related data on these questions and that some of the experi- 

 mentation to be carried out in the future may be more adequate to 

 the purposes of ecology and agriculture than is much of the hap- 

 hazard experimentation so far predominating in the literature. 



^Briggs, L. J., and H. L. Shantz, The water requirement of plants, II, A review of the 

 literature, U. S. Dept. Agric, Bur. Plant Ind. Bull. 285, 1913. This is a very complete and 

 valuable annotated bibliography of the subject and includes discussion of and references to the 

 writers' own work. In this connection see also Shive, J. W., A study of physiological balance 

 in nutrient media, Physiol. Res., 1: 327-397, 1915. P. 379. 



