THE PROCESSES OF ABSORPTION AND EXCRETION 103 



are incapable of diosmosing find no point of entry or exit, either during or 

 after the passage of the oil drop or solid particle. There can be no doubt 

 that the molecules, or molecular complexes, of dissolved substances penetrate 

 between the component elements (micellae) of the plasmatic membranes, 

 and diosmose only when the imbibitory force with which the substances in 

 question are attracted is sufficiently powerful to overcome those molecular 

 forces which may be operating antagonistically (Sect. 16). 



The relationships just indicated are of perfectly general application. 

 The quality of the separating membrane, whether fluid or solid in nature, 

 and the size of the molecules of the diosmosing substance, are of importance 

 in determining the possibility of diosmosis as well as its character 1 . 

 Nevertheless, as the living organism shows, the relative size of the 

 diosmosing molecules in relation to that of the micellar interstices through 

 which they pass is by no means all-important 2 . Thus, the same protoplast, 

 which is impermeable to NaCl or KNO 3 , may allow the larger molecules 

 of methyl-blue, of albuminous substances, and perhaps also of other 

 colloids to penetrate freely. This selective peculiarity, which is of the 

 utmost importance to the plant, may be connected in some way with 

 the fact, indicated by the ready penetration of solid particles, that the 

 passage through the plasmatic membranes involves only a trifling expendi- 

 ture of energy. 



The conditions which regulate absorption and diosmosis have already 

 been indicated when dealing with the phenomena of imbibition and swelling 

 (Sect. 12). It is possible that the molecular forces, which are here at work, 

 may induce molecular decompositions or other chemical changes, and this 

 may indeed frequently form an essential part of the process of diosmosis. 

 Thus it is possible, for example, that KNO 3 is only enabled to penetrate 

 into the cell by being split up into its component ions, or that cane sugar is 

 absorbed by the plasmatic membrane as a monosaccharide, and is re- 

 condensed into saccharose on excretion into the cell-sap. 



When chemical union takes place a substance may very readily be 

 transmitted through a solid non-porous membrane. This would be the 

 case with nitric acid, for example, supposing that on one side of a dry 

 cellulose membrane nitrification took place, while on the other side nitro- 



1 The problem has already been investigated from this point of view in Osmot. Unters., 1877, 

 and also in Unters. a. d. Bot. Inst. z. Tubingen, 1886, Bd. n, p. 301. Direct proofs by experiments 

 with artificial precipitation membranes have also been brought forward. More recently other 

 authors have come to the same conclusion: Tamman, Zeitschr. f. physik. Chemie, 1892, Bd X, 

 p. 255 ; Walden, ibid., p. 699 ; Fiinfstiick, Ber. d. Bot. Ges., 1893, Generalvers. , p. 80; Overton, Ul 

 osmot. Eigensch., 1895 (Sep.-abdr. a. Vierteljahrsschr. d. Naturf.-Ges. in Zurich, Jahrb. 40). On th 

 influence of the character of the membrane : Raoul, Zeitschr. f. physik. Chemie, 1895, Bd. xvn, p. 737. 



2 [That is, absorption does not necessarily run parallel with the rapidity of diffusion, or with 

 the osmotic power, of a given substance, both of which are dependent upon the size of t 



of the dissolved substance.] 



