AS INDICATED BY OSMOMETERS. 53 



osmotic pressure, as though the soil were an aqueous solution of solutes 

 to which the root periphery were impermeable. Similarly, the internal 

 condition controlling water entrance (absorbing power of the roots) 

 might also be stated in terms of osmotic pressure, the imbibing power 

 of the exposed walls being a force comparable in many respects to the 

 water-attracting force of an aqueous solution, which is one aspect of 

 osmotic pressure. 



A preliminary study of some of the relations between transpiration 

 and the resistance offered by the soil to water absorption by roots has 

 been made by Livingston and Hawkins (see the first paper of the 

 present pubUcation), in which these authors present simultaneous 

 graphs of these two features — or at least of the first of them and of 

 what they take to be a measure of the other. While the discussion 

 embodied in that paper is closely related to that in which we are about 

 to engage, it is with a different aspect of the general problem that the 

 present paper has to deal. 



Dynamically, the upper limit of the rate at which water may pass 

 from son to root may be said to be a function of the maximum possible 

 rate of absorption (supposedly to be attained if the roots were to be 

 temporarily placed in pure water) and the maximum possible rate of 

 supply by the soil. These maximum possible rates should be propor- 

 tional to the corresponding static forces just considered. Of course 

 only very short time intervals may here be thought of, for the effect of 

 any rate, or of a force of any given magnitude, must be to bring about 

 a new set of equilibrium relations between soil and plant, with accom- 

 panying alterations in the magnitudes of the forces concerned. How- 

 ever, if the actual rate at which roots absorb water does not exceed 

 the maximum possible rate at which the soil may supply water to the 

 active root surfaces, then the magnitude of the rate of absorption 

 should depend upon internal conditions alone and external resistance 

 to absorption should not arise as a limiting condition. 



If the above considerations are reliable, it should foUow that, as far 

 as water is concerned, the single soil condition which determines 

 whether a plant may grow or maintain tm-gor or wilt or die is the 

 maximum possible rate of delivery of water to its root surfaces. As 

 has been emphasized by Livingston,^ by Free,^ and by Livingston and 

 Hawkins (in the first paper of the present publication) , this rate appears 

 to be the one simple soil property by which different soils can be defi- 

 nitely characterized in regard to water-relations. It is, however, a 

 feature which has been accorded practically no serious attention by 



'Livingston, B. E., Present problems of physiological plant ecology. Amer. Nat. 43: 369-77. 



1909. 

 , Present problems in soil physics as related to plant activities. Amer. Nat. 46 ; 294-301 . 



1912. 

 ^Free, E. E., Studies in soil physics. Plant World 14: 29-39, 59-66, 110-19, 164-76, 186-90. 



1911. Also separately reprinted, repaged. Tucson. 1912. 



