GENERAL 89 



accurate representation of the nature of cork or cuticle. In practice, 

 paper prepared in this manner is assumed to be impermeable to water, 

 but nevertheless slight quantities of gases may pass through, as well as any 

 other substances that are at all soluble in the impregnating oil. Similarly, 

 a thin film of india-rubber shows that a membrane impermeable to water 

 may nevertheless be penetrated in perceptible amount by carbonic acid 

 and other gases. The impermeability of cork and cuticle is only relative, 

 although it may often be extremely pronounced. This property is of 

 great importance to the aerial organs, which are, as a general rule, covered 

 either by a cuticle or by a corky investment, so that protection is afforded 

 against any excessive and ineffective loss of water, while gaseous inter- 

 change, though rendered more difficult, may still take place with sufficient 

 activity to supply all needs. Cells, however, in which the walls are 

 suberized on all sides, and which are thus only with difficulty permeable 

 to water, are, as a general rule, completely dead, probably because the 

 amount of exchange allowed is insufficient for the requirements of the 

 living protoplast. The specific nature of the cell-wall determines whether 

 dissolved substances will reach the protoplasm, and in what relative amount. 

 Almost without exception, however, all substances that can diosmose through 

 the external limiting plasmatic membrane can also pass through the cell- 

 wall so long as it is saturated with imbibed water, and also through corky 

 and cuticularized cell-walls, though usually with excessive slowness. On 

 the other hand, very many substances are unable to penetrate the living 

 plasma, although they may readily diosmose through the moist cell-wall. 

 Apparently, however, the diosmotic properties of the plasmatic membranes 

 are liable to modification, frequently of a regulatory character in connexion 

 with the temporary needs of the living organism, so that a particular 

 substance can be absorbed at one time, but not at another. The means 

 by which changes of the diosmotic properties of the plasma may be induced 

 are probably of very special character, and it is conceivable that move- 

 ments and changes of position of the component micellae of the peripheral 

 plasmatic membrane may modify its diosmotic character. The suberiza- 

 tion which particular cell-walls may undergo is a similar change, directly 

 due to the activity of the living protoplast, and induced for the benefit of 

 the organism as a whole. 



The character of the cell-wall and of the plasmatic membranes 

 determines whether a given substance will penetrate to the interior of 

 a cell, and any such substance will continue to be absorbed until a con- 

 dition of equilibrium is reached, when all further absorption ceases, 

 however, this condition of equilibrium is continually disturbed, continuity 

 of absorption may result, and in this way, relatively large quantities of 

 a particular substance may be absorbed from an extremely dilute solution 

 or of two given substances, one may be absorbed in large amount, t 



