WATER-RELATION BETWEEN PLANT AND SOIL. 23 



at least dormancy of many vital processes must occur. If the ratio 

 is somewhat less than unity, then at least some growth should be 

 possible, assuming that a surplus of water is necessary for growth 

 processes. It seems possible that the critical value of the ratio here 

 considered, at which growth or other vital activity may be definitely 

 affected, may eventually become a valuable physiological and ecological 

 constant, by which some of the over-discussed ''adaptations" of plants 

 may be quantitatively stated, at least in an approximate way. With- 

 out means for evaluating A (absorption rate) , this statement is mainly 

 valuable in the planning of researches. 



It is to be remarked here that it would be quite as logical, and 

 perhaps more in accordance with usual ways of thinking, to deal with 

 the reciprocals of all the quantities named above, thus deriving the 

 power of the environment to supply water to the plant (environmental 

 hydrating power, H^, perhaps) and the general tendency of the whole 

 system (plant and environment together) to maintain in the tissues 

 moisture adequate to growth, etc. (H, say). The latter concept coin- 

 cides with our idea of the water efficiency of the plant in any given 

 environment and may eventuallj^ give rise to a practical measure of 

 the tendency toward success or failure of any plant when growing 

 under any set of conditions. Here it is merely to be emphasized that 

 it is quite immaterial at present whether we deal with D, D^, and Z)j 



or with H, H„ and Hi, since H^j:, etc. 



It is already clear that the water-relations of a given plant, and their 

 march through its life, can not be understood until there becomes 

 available some method by which the rate of water absorption by roots 

 or the absorbing power of these organs may be approximated, at least 

 in relative terms. Thus, the only value to be attached to the latter 

 portion of our discussion lies in the manner in which it emphasizes this 

 physiological characteristic of the organism. We have assuredly 

 reasoned in a circle and "come out where in we went," but our peregrin- 

 ations seem to show clearly just where constructive research is most 

 needed, as far as plant water-relations are concerned. 



It is but trite to remark that no real knowledge of the dynamics of 

 plant water-relations can be attained without measurements of both 

 internal and external conditions, but the study of these relations has 

 thus far contented itself so thoroughly with statics that insistance 

 upon this axiom may be pardonable at the present time. If our sug- 

 gested means for the study of the soil-moisture conditions proves of 

 value, the environment may be said to be tentatively comparatively 

 well cared for, for the desiccating power of the aerial environment may 

 be rather readily determined. Of the internal conditions we can 

 approximate only the transpiring power, which is easy, because absolute 

 transpiration rates may be directly measured. Of the other internal 



