20 WATER-RELATION BETAVEEN PLANT AND SOIL. 



radio-atmometer^ had been employed, but it was not. We may term 

 this desiccating power da and the atmometric reading E, so that da is 

 taken as equivalent to E for any time period in these experiments. 



The transpiring power of the plants was taken as the relative trans- 

 piration ratio (Livingston, 1906, 1913), which is the absolute transpi- 

 ration rate, for a given period, divided by the evaporation rate for 

 the same period, the latter from a standardized atmometer exposed 

 with the plants. For our present purpose the area of the plant surface 

 need not enter into the discussion; area may be regarded as one of the 

 internal conditions determining general transpiring power. If trans- 

 piring power is represented by di, and if T represents (as above) the 

 actual transpirational water loss for the time period in question, then 



T 



we have di = -^. 



hi 



The absorptive power of the roots was not measured, as we have 

 said, but it is probably safe to suppose that it does not independently 

 vary in magnitude from hour to hour, in any great degree; at least it 

 can exhibit no such marked and regular fluctuations as does foliar 

 transpiring power, for the roots of our plants were never subjected to 

 great changes in their water surroundings, and roots in general have 

 never been shown to exhibit anything parallel to stomatal movement. 



If it were possible to measure actual absorption as absolute transpira- 

 tion may be measured, it should then be easy to determine the absorb- 

 ing power of the root system. The treatment would be quite parallel 

 to that resorted to by Livingston (1906) in deriving the transpiring 

 power. In the latter case, as we have seen, the absolute transpira- 

 tion rate is divided by the evaporation rate from a standard physical 



T 



surface; ^, the relative transpiration ratio, is the measure of transpir- 

 ing power. In the case of subterranean conditions we should divide 

 the absolute absorption rate by the power of the soil to deliver water 

 to the roots, thus obtaining what might be termed the relative absorp- 

 tion rate, or absorbing power. AVe suppose here that the rate of water 

 loss from the irrigator is reciprocally proportional to the water-supply- 

 ing power of the soil; for if a soil has at one time a water-supplying 

 power two (or three, etc.) times as great as at an earlier period, then its 

 resistance to root absorption must be one-half (one-third, etc.) as great 

 as this resistance was before. If, then, the rate of loss from the irri- 

 gator be taken as 7, then -^ is the supplying power sought. Hence, if 



A represents the absolute absorption rate, then the rate of relative 

 absorption (or the absorbing power of the root system) should be the 



A . . . . T 



ratio Y = A /, which corresponds to the relative transpiration ratio ^. 



7 



'Livingston, 1911 (1 and 2). 



