PERMEABILITY AND RATE OF ABSORPTION 127 



permeable membranes. These coats may be 'several cell layers in thickness, 

 but the cells of which they are composed are often dead. Even in those in 

 which the cells are living destruction of the protoplasm by one treatment or 

 another does not destroy the property of differential permeability. Clearly 

 therefore, this property is inherent in the cell walls. Many such membranes 

 are relatively permeable to water but show different degrees of permeability 

 to solutes, ranging all the way from practical impermeability to some to a 

 marked permeability to others. Among the species whose seed or fruit coats 

 have been found to show this property are many of the grains, peach, apple, 

 bean, horse-chestnut, sugar-beet, sunflower and cocklebur. For example, Shull 

 (191 3) found that the seed coat of the cocklebur was im.permeable to ether, 

 acetone, chloroform, ethyl alcohol, gljxerol, sugars, NaCl, CUSO4, HCl, and 

 tartaric acid among others, but was permeable to a greater or lesser degree 

 to KCl, KOH, NaOH, H0SO4, HNO3, AgNOg, NaNOs, acetic, citric, and 

 lactic acids, as well as some others. 



Relation between Permeability and the Rate of Absorption. — There 

 is not necessarily any correlation between the permeability of the cell mem- 

 branes to a given compound and the rate at which that substance will enter 

 the cell from other cells or a liquid medium. The permeability of the cell 

 membranes does not control the rate at which compounds pass either into or 

 out of a cell but is merely one factor which influences their rate of move- 

 ment through the cell wall and the cytoplasm. For example, the membranes 

 of a. cell may be readily permeable to a given substance which nevertheless 

 may enter very slowly because the vacuolar concentration of that substance 

 is already relatively high. Another substance, to which the membranes are 

 relatively impermeable, may move into the cell continuously because it is being 

 utilized in some metabolic process within the cell. 



Recent work on the absorption of ions by plant cells (Chap. XXIV) 

 indicates that the results of many studies upon permeability to electrolytes 

 are somewhat misleading. In actively metabolizing cells ions often accumu- 

 late until their vacuolar concentration is much greater than in the outside 

 medium. The penetration of electrolytes into such cells often occurs at a 

 relatively rapid rate, in spite of much evidence which indicates that the cyto- 

 plasmic membranes are not very permeable to them. Similarly there is evi- 

 dence that sugars sometimes move into at least certain kinds of cells at a 

 relatively rapid rate. A probable explanation of this apparent discrepancy is 

 that most studies of permeability have been made upon cells which do not 

 possess the capacity of absorbing electrolytes or other polar compounds at a 

 rapid rate. This capacity seems to be restricted largely, if not entirely, to 

 meristematic or otherwise rapidly metabolizing plant cells. 



