104 MINERAL SALTS ABSORPTION IN PLANTS 



suitable experimental materials might be: meristematic cells in tissue 

 culture, myxomycetes in the plasmodial condition, and leaves of 

 certain mosses e.g. Mnium punctatum. 



3. Salt Relations of Cell Components 



a. Cell walls. Most plant cells are surrounded by a rigid wall 

 containing cellulose, hemicelluloses, polyuronides (e.g. pectic acid, 

 pectinic acid and pectin), lipids and protein. The water content of 

 cell walls may be as high as 90 per cent or more (Frey-Wyssling, 

 1952) and most of them are freely permeable to salts in aqueous 

 solution. The low resistance offered to diffusion of salts and other 

 dissolved substances is demonstrated by the phenomenon of 

 plasmolysis. Bennet-Clark and Bexon (1946) showed that when 

 plasmolysed onion epidermis cells are transferred from one plasmo- 

 lysing solution to another, the liquid in the intramural space rapidly 

 equilibrates with the new medium. In mature cells, walls often 

 become impregnated with fats and lignin (suberin) and are then 

 much less permeable. 



Cellulose, hemicelluloses, polyuronides and phospholipids are 

 capable of binding salts in readily exchangeable forms, and the wall 

 therefore behaves as a Donnan system containing an appreciable 

 concentration of negative charges. Ions move through this system 

 from the external medium, by diffusion and exchange, to the surface 

 of the cytoplasm, and by virtue of its binding properties the wall 

 exerts some effect on the availabilities of ions at this point. Readily 

 available cations are held at a high concentration near the cyto- 

 plasmic surface, while the concentration of anions tends to be lower 

 than in the external medium. 



b. Cytoplasm. Early cytologists concluded that a distinct 

 membrane exists within the cell wall at the outer surface of the 

 cytoplasm and to this the terms "plasmalemma", "ectoplast" and 

 "plasma membrane" have been applied. (Plate I, facing p. 95). Simil- 

 arly, the cytoplasm is thought to be bounded on its inner surface by 

 the "tonoplast" or "vacuolar membrane". Owing to the imperme- 

 abihty of these membranes, dyes injected into the cytoplasm are 

 retained there. The mechanical stability of both the plasmalemma 

 and tonoplast has, in some cases, been demonstrated by micro- 

 dissection. Both membranes have about the same thickness (60-100 

 A), and are composed principally of lipids and protein which are 



