WATER-RELATION BETWEEN PLANT AND SOIL. 21 



Although not experimentally dealt with here, we may, for the purpose 

 of the general theory, let the symbol dr denote the reciprocal of root 

 absorbing power (^7) ; that is, the power effective at root surfaces to 



retard water entrance. Thus dr = ^-jf-, with the notation given above. 



The resistance offered by the soil to water absorption was considered 

 proportional to the water-absorbing power of the soil, as shown by its rate 

 of removal of water from the unchanging surface of the auto-irrigator. 

 This may be denoted by ds, the effective power in the soil, tending to 

 cause water loss from the roots or retardation of water entrance, and 

 may be termed the water-attracting power of the soil. 



The whole environment, subterranean and aerial, may be considered 

 as a machine always tending to remove water from the plant, and the 

 plant may be similarly regarded as a machine tending to give off water 

 to the environment. We may express this water-removing tendency 

 of the total environment as its desiccating power, De, which should be 

 proportional to the product of the desiccating power of the aerial sur- 

 roundings (rfa) and the water-attracting power of the soil {ds). Thus, 

 De = daX ds, which means that if da is m times as great at hour 2 as at 

 hour 1, and if ds is similarly n times as great, then Dg is mn times as 

 great for the second period as for the first. We may also say that De, 

 for any hour, is a measure of the aridity of the environment as a whole, 

 expressed in terms of its aridity for some other hour taken as unity. 

 It appears that we may possibly have here what at least may approach 

 being a single-term measure of all that ecologists connote by environ- 

 mental conditions, as far as water is concerned, and for our particular 

 cultures. The application of this method of reasoning to plants under 

 other conditions than the ones here employed, as in the open ground, 

 will of course involve new difficulties; this is a matter for future 

 dynamic studies on plant water-relations. A beginning is perhaps well 

 made by carrying out an investigation of these features for a few indi- 

 vidual potted plants, and this, in a crude and preliminary way, is all 

 that is here to be attempted. 



It has been remarked that the water efficiency of a plant {i. e., its 

 power to maintain water in its tissues sufficient for the usual physio- 

 logical processes) is not dependent alone upon the surroundings, but is 

 also a function of internal conditions, of foliar transpiring power and 

 of root absorbing power. These two powers are operative in opposite 

 directions; the former tends to accelerate desiccation, the latter to 

 retard it. If the root absorbing power were brought into our argu- 

 ment as it now stands, we should have to use its reciprocal, as has been 

 noted. 



Attempting to state in a single term the effectiveness of the internal 

 conditions tending to promote desiccation, as we have just done for 

 the external ones, we obtain an equation like the following : Di = diX dr. 



