126 PERMEABILITY 



posure to light results in an increase in the permeability of the cytoplasmic 

 membranes to many and perhaps to all solutes. It is uncertain, however, just 

 how much of this effect can be ascribed to a direct photo-chemical influence 

 and how much is due to secondary effects such as heating. 



6. "Stimulation." — This ignorance-concealing term is applied to a num- 

 ber of phenomena which result in an increase in permeability, the exact nature 

 of which are so incompletely understood that they cannot be described in any 

 more fundamental terms. For example, it has been found that the perme- 

 ability of the egg cells of certain species of invertebrate animals increases when 

 fertilization occurs. Likewise injury or wounding of cells or tissues, whether 

 due to mechanical or other causes, also usually results in an increase in 

 permeability. 



The Permeability of the Cell Wall Membranes. — The permeability of 

 the cell wall membranes is greatly influenced by their chemical composition 

 and physical organization. Cell walls composed of cellulose and pectic com- 

 pounds, such as those in most parenchymatous tissues, are usually quite per- 

 meable to water, and, if wet, to many dissolved substances. Water and 

 solutes probably pass through such walls largely if not entirely through the 

 hydrophilic intermicellar material. The plasmodesms also undoubtedly facili- 

 tate the movement of water and solutes through the walls of many living plant 

 cells. 



That the differentially permeable membrane of such plant cells lies in the 

 cytoplasm is visually shown when the protoplasm of any cell which contains 

 anthocyanins dissolved in the cell sap is destroyed by heating, freezing, or 

 toxic agents. As long as such a cell is alive and uninjured the red pigment 

 does not diffuse out of the vacuole, even if the cell is immersed in water. Im- 

 mediately upon destruction of the protoplasm, however, outward diffusion of 

 anthocyanins and other solutes occurs through the intact cell wall, indicating 

 that the latter does not appreciably impede their egress from the cells. 



Lignified cell walls are also quite freely permeable to water and, when wet, 

 to solutes as well. Lignification apparently results in little if any decrease in 

 the permeability of the intermicellar material. 



Cell walls in which cutin or suberin are present in appreciable quantities, 

 on the other hand, are relatively impeiTneable both to w^ater and solutes. 



While it appears to be generally true that cell wall membranes which are 

 readily permeable to water are also readily permeable to solutes dissolved in 

 water, a number of differentially permeable cell wall membranes have been 

 discovered and it is probable that differential permeability is more often a 

 property of the cell walls than is generally realized. The coats of a number 

 of different kinds of seeds and fruits have been found to act as differentially 



