382 R. G. Leavitt — Cause of Boot-Pressure. 



Osmose being defined as the diffusion of a solution through 

 a membrane, and water being in no sense a solution, " endos- 

 mose of water " is mistaken phraseology. Some other appro- 

 priate term should be employed, such as infiltration. " Osmotic 

 pressure " is unmistakable, but it is well to keep in mind 

 that pressure arises through failure of the solution to accom- 

 plish osmose. 



Coming now to the plant-body as an osmotic machine, we 

 may distinguish two categories of osmotic pressures with their 

 consequent filtrations of water, according as (1) the solutions 

 are confined within single (living) cells, or (2) the osmotic sub- 

 stances permeate the whole cellular system from root-tip to 

 leaf. The first may be termed localized or intracellular 

 osmotic pressure, the second general internal, or sap-pressure. 



The difficulties involved in ascribing root-pressure to local- 

 ized osmotic action of the root parenchyma are well under- 

 stood, and need not be dwelt upon. Some of them seem to be 

 obviated by attributing the phenomenon in question to general 

 internal osmotic pressure. The root immersed in the wet 

 soil may be compared to the thistle-tube of the common 

 experiment. The root-surface or surface tissue will then 

 answer to the membrane stretched over the mouth of the tube. 

 Tracheae, tracheids, intercellular spaces, and all parts dead or 

 alive through which sap-solutions percolate, may answer as a 

 whole to the interior of the thistle-tube. Soil water may cor- 

 respond to the water in the outer dish and sap with its con- 

 tents, to water with its salt or sugar inside the thistle tube. 

 The water in the stem of the latter, rising by reason of 

 upward osmotic pressure on its free surface, overflows in an 

 osmotically driven stream. The outflow from the root, when 

 the stem is cut off, may have a like cause. In the artificial 

 apparatus the solution washes out with the stream, and the 

 end of the process ensues. In the case of the living apparatus 

 stores of insoluble organic matters (as starch) are present, from 

 which the reduced fund of osmotic substances (as sugars) may 

 be replenished from time to time by action ot enzymes, causing 

 periodic overflows of the kind described by Sachs. The flows 

 would naturally decrease in volume as the stock of reserve 

 materials became gradually exhausted. 



The observations of Sachs on the effect of heat in increasing 

 absorption, and of Clark on pressure in tree-trunks, accord well 

 with the supposition that both kinds of phenomena are due to 

 general osmotic sap-pressure. 



The agency of osmotic sap-pressure as more or less import- 

 ant among several possible factors in maintaining or restoring 

 the continuity of free water between root and leaf during 

 transpiration, is here suggested. So-called negative pressure 

 may probably be shown to be compatible with a simultaneous 

 osmotic sap-pressure of considerable intensity, when the supply 

 of water at the root falls below the demand. 



Harvard University, April 14, 1899. 



