52' MACDOUGAL^ACTION OF LIPOIDS IN GROWTH. 



and hydroxide of mixtures of agar and gelatine nearly evenly bal- 

 anced. ^^ Although the lipoids present would be liquefied in the 

 stronger solutions, the effect of their presence is not discernible in 

 the action of the dried sections in which the permeability of the wall 

 and other layers has reached the maximum. 



The numerous corrections and amendments to the plasmolytic 

 method of estimating permeability and tonicity of cell-contents are 

 suggestive of the complexity of the factors which enter into the 

 exchange between the cell and the medium. An average of sixty 

 per cent, of the osmotic pressure of the cell sap is due to electrolytes. 

 These with the non-electrolytes affect or determine the degree of 

 hydration of the constituents of the plasma and the wall, upon which 

 permeability depends directly. Such action is with but little refer- 

 ence to the isotonic values of the substances concerned. A knowl- 

 edge of the principal features of the hydration reactions of the 

 plasmatic constituents, and of the cell-wall under the influence of 

 cell-contents and medium is therefore fundamental to any compre- 

 hension of the passage of material through the membranes of the 

 plant. 



According to the recent work of Bartell and Sims, swelling or 

 increase by hydration may be the result of the action of several 

 forces. Whether their conclusion that a solution tending to exercise 

 negative osmose increases hydration of the membrane, while condi- 

 tions which shrink the membrane act positively is in agreement in 

 all of its implications with those of Kahho as to penetration and 

 hydration, is not yet clear. ^* Some of the newly disclosed possibili- 

 ties of the intervention of the lipoids are to be taken into account in 

 any consideration of the passage of material into or out of the cell 

 with resultant changes in volume which constitute the essential fea- 

 tures of growth. 



13 MacDougal and Spoehr, "The Components and Colloidal Behavior of 

 Plant Protoplasm," Proc. Amer. Phil. Soc, 39. 150-170, 1920. See pages 156, 



157- 



!•* Bartell, F. E., and Sims, L. B., " The Relation of Anomalous Osmose to 

 the SweUing of Colloidal Material," Jour. Am. Chem. Soc, 44, 289-299, 1922. 



The Desert Laboratory, 

 Carnegie Institution, 



Tucson, Arizona, April, 1922. 



