MACDOUGAL AND SPOEHR— GROWTH AND IMBIBITION. 291 



ing or shortening would ensue until an adequate supply reached the 

 expanding region. 



The proposal of Rahn^ to explain the relation of growth to tem- 

 perature upon the basis of a direct integration of enzymatic action 

 and enzyme destruction does not seem adequate. It is true that 

 among the reactions upon which the growth of plants depends are 

 syntheses or renewals of thermo-labile material, and upon metab- 

 olism possibly including oxidation of carbohydrates. Each of the 

 separate processes or reactions, enzymatic or otherwise, goes on at 

 a rate determined by the temperature, and by the concentration of 

 its products, and to an extent limited by the amount of material 

 brought into its reactions. The extent to which, for example, the 

 sugars are oxidized determines the degree of acidity or alkalinity of 

 the cell thus affecting its water relations in a very serious manner. 

 Also as will be shown later, the swelling of colloids, and presum- 

 ably the growth capacity of a cell, may be modified by proteins, 

 while its volume or measurable variation in volume is at all times 

 a function of the balance between water-accession and water-loss. 



The cell itself may be considered as a mass of colloidal material 

 variously altered from the globular by pressure and contacts. The 

 outermost layer being of greater density or compactness is usually 

 designated as a membrane, and much has been written during the 

 past few years concerning the permeability and the modifiable semi- 

 permeability of such structures. The meristematic or embryonic cell 

 with the action of which we are chiefly concerned in growth, is in 

 its earlier stages dense and shows none of the cavities or clear 

 spaces which form such a large part of the volume of a mature cell, 

 while the relatively large nucleus shows even greater density. 



The enlargement of this mass consists to an extent as great as 

 98 or 99 per cent, in swelling by the imbibition of water. The rate,. 

 extent and total amount of such swelling will be determined by the 

 character of the colloidal mixture, by salts, acidity or alkalinity 

 of the solutions present, and only to a very slight extent by osmosis; 

 as this process takes place in colloids. Hence turgidity may play a 

 very minor part in the earlier stages. 



1 Rahn, O., " Der Einfluss der Temperatur und der Gifte auf Enzym- 

 wirkung, Garung, und Wachstum," Biochcm. Ztschrft., 27: 351, 1916. 



