Osmotic Pressi'Re in Plants. 161 



caped the notice of our author. Whatever may be the nature 

 of the forces which determine whether or not water will migrate 

 into root cells from an adjacent layer of nearly dry soil, the 

 Duiintenance of sucli mo\ement must depend, not merely on 

 the initial gradient of the forces involved, but upon the 

 relation between the rate of removal of moisture from the 

 absording cell membranes into the plant interior and the possi- 

 ble rate of water movement through the soil. Thus, if the con- 

 ditions were such (as they must often be in arid regions) that 

 moisture were more rapidly absorbed from the soil laver im- 

 mediatelv adjacent to the absorbing cells than the soil condi- 

 tions would allow it to be replaced from more remote layers, 

 it is obvious that the absorbing surfaces of the plant must soon 

 become surrounded by a layer of soil too dr>' to give up any more 

 moisture. Thus absorption would be checked, although there 

 might be large amounts of water in the soil as a whole. The 

 possible rate of moisture movement through the soil and the 

 relations of this rate to the physical properties of the latter 

 must ever be an important factor in the control of plant absorp- 

 tion from nearly-dry soils. 



At several points in the paper Fitting expresses the idea 

 though it is nowhere strongly emphasized, that a high osmotic 

 concentraticn of the foliar cell sap must act directly to retard 

 transpiration. This must be undoubtedly true, on physico- 

 chemical grounds, but the amount of such retardation cannot 

 be large enough to exercise a primary, or even a great, intiu- 

 ence upon the success or failure of plant forms. I have else- 

 where * discussed certain aspects of this question, and shall 

 reserve its fuller consideration for a future paper, but may state 

 here that a solution of the strength of the highest concentra- 

 tions found by Fitting in the foliar cell sap of desert plants 

 (something over 100 atsmospheres of osmotic pressure) pos- 

 sesses a vapor tension less than 10 '-/c below that of pure 

 water, and should therefore evaporate more than 90 % 

 as rapidly. It is thiis clear that no great ecological importance 

 is to be attributed directly to the checking of water loss by low 

 vapor tension of the foliar solutions. f What may be the indi- 



♦Livingston, B. E., "Relation of the Daily March of Transpiration to Variations in 

 the Water Content of Foliage Leaves." Bot. Gaz. In press. 



tThis has been clearly showa by Drabble and Drabble, for a humid region. See Drab- 

 ble, E. and Drabble, H., The relation between the osmotic strength of cell sap in plants 

 and their physical environment. Bio-chem. Jour. 2: 117-132. 1907. 



