Grafts et al. — 80 — Water In Plants 



provides a highly hydrated matrix along which ions may migrate much as they do in 

 the protoplasm itself. 



Thus, it seems that the protoplasm is highly impervious to the outward migration 

 of solutes and that the inward movement is usually efifected by an absorptive process 

 that is active and hence not subject to the ordinary laws of diffusion. When the proto- 

 plast is subject to adverse conditions such as lack of oxygen, excessive temperature, 

 excessive concentration of hydrogen or hydroxyl ions or toxic substances, or to lack 

 of organic nutrients, its metabolic activity may be so reduced that loss of nutrients 

 occurs. Response to such conditions may be reversible up to a certain point; beyond 

 this, permanent injury occurs and complete loss of control over solute movement in- 

 dicates death of the cell. 



In view of these properties of the living protoplasm, it seems that the term per- 

 meability, in the classical sense of a passive control or selective action upon passage of 

 ions or molecules, has little to do with the functioning of the living cell. Most cell 

 walls may be considered permeable to water and salts in the classical sense. And loss 

 of solutes from injured or senescent cells takes place by diffusion. But uptake by the 

 active living cell, and possibly migration via the symplast, are active processes in- 

 volving the use of metabolic energy. 



Permeability to Water: — Water passes through the plasma mem- 

 branes with relative ease, although its rate may change due to various in- 

 fluences. For example, the protoplasm of young cells appears to be more 

 permeable to water and salts than that of older cells ; protoplasm of sene- 

 scent tissue is again more permeable (Maximov and Mozhaeva, 1944). 

 Cells in a frost-resistant condition seem to possess increased permeability, 

 and this is proposed as one explanation of hardiness (Levitt and Scarth, 

 1936). 



The permeability of cells to water may be determined in several ways: 

 1) rate of plasmolysis or of deplasmolysis, 2) by change in volume of 

 protoplast (plasmometric method), ^) by change in volume or weight of 

 bulky tissues, 4) by means of conductivity and osmotic pressure measure- 

 ments of expressed sap. 5) Special methods have been devised for studies 

 on large algal filaments by de Zeeuw (1939). He determined the water 

 exosmosis from Chaetomorpha linum by sensitive refractometric and dilato- 

 metric procedures. The first is based on extent of dilution of the external 

 medium, the second on change of cell volume. 



The rate of water movement through isolated protoplasts of onion 

 bulb scale was found by Levitt, Scarth, and Gibbs (1936) to be 0.3 

 cubic microns per minute, passing through one square micron of cell surface 

 under a DPD difference of one atmosphere. Such values are commonly 

 termed diffusion constants. Other reported values of diffusion constants 

 are for Fiicus eggs 0.16 (Resuhr, 1935) ; leaf cells of Salvinia auriculata 

 0.55 (Huber and Hofler, 1930) ; internodal cells of Tolypellopsis stelli- 

 gera 1.08 (Palva, 1939). The above value of 0.3 represents a linear rate 

 of 20 microns per hour for water movement through the cell membrane. 



Brauner and Brauner (1940) found that exposure to light increased 

 the water permeability of plant tissue and postulated an electro-osmotic 

 mechanism as an explanation. Others {e.g., Weber, 1929a) have found 

 opposite results, and still others report no apparent effect of light on per- 

 meability. 



Permeability of cells toward water may vary from cell to cell and from 

 tissue to tissue. Huber (1933) found for Vallisneria leaf a rate 30 to 40 

 times less in the mesophyll than in the epidermis. L. and M. Brauner 

 (1943&) have made a significant contribution toward an understanding of 

 permeability of plant cells to water. They state that passage of water 

 through membranes is determined by two factors, each operating in an op- 



