WATER-RELATION BETWEEN PLANT AND SOIL. 25 



Passing back into the plant body, incipient drying of peripheral 

 walls is surely dependent, under continuous external conditions, upon 

 the rate of water movement from deeper lying parts toward the walls 

 concerned. This rate is obviously a function of the internal resistance 

 offered to such water movement. Among many conditions surely 

 influencing this resistance only two may be mentioned here — the general 

 water-conducting power of the plant body and the water-supplying 

 power of the subterranean periphery. Little has been done in a 

 dynamic way toward a knowledge of how different plants may differ 

 in regard to their conducting power. 



The water-absorbing power of subterranean surfaces of the plant is 

 of course largely dependent upon the actual rate of upward conduction 

 of water, which depends, indirectly, upon the actual transpiration rate 

 and upon internal factors. Nevertheless, as we have already empha- 

 sized, the extent, nature, etc., of the exposed cell membranes of roots 

 must frequently furnish the limiting condition of absorbing power. It 

 is obvious that the rate of upward conduction of water may some- 

 times be determined by this absorbing power; indeed, transpiring power, 

 conducting power, and absorbing power are probably very closely 

 connected in many cases. A knowledge of the water-absorbing process, 

 in its relation to the general well-being of the plant, is thus seen to be 

 deeply involved with an appreciation of the dynamics of water con- 

 duction and transpiration. 



From the last paragraphs it emerges that what we have called water- 

 absorbing power of the root system and designated by AI really refers 

 to much more than the specific properties or characteristics of the 



T . 



roots themselves. It also appears that transpiring power -^ involves 



many features resident only at a distance from the exposed foliar 

 membranes. 



An additional consideration is required here, to prevent our sug- 

 gestions on the measurement of environmental aridity from being 

 misleading. From our deductions and assumptions, as thus far set 

 forth, this aridity seems to become nil when either / or E, as here 

 employed, becomes zero; for D^ = EI. I should become zero when 

 the entire water-absorbing surface of the plant is bathed in free liquid 

 water, as in the case of water cultures. Similarly, E should vanish 

 when there is no evaporation, as when the entire transpiring surface 

 is covered with a water film thick enough so that variations in the 

 evaporation rate from the exposed film surface have no influence upon 

 the water conditions of the plant tissues thus covered. A plant com- 

 pletely submerged in pure water should be subject to no external 

 resistance to absorption and to no external acceleration of water loss. 

 (We ignore the usually negligible phenomena of guttation and external 

 secretion of liquid water.) In all three of these cases D appears to 

 become nil, according to our equation. 



