no THE MECHANISM OF ABSORPTION AND TRANSLOCATION 



Although the exact chemical constitution of the plasmatic membrane 

 is not known, there can be no doubt that it is largely composed of proteid 

 substances. This is indicated by the rigor caused by dilute acids, 

 mercuric chloride, iodine, &c., and so far as any determination is possible, 

 the membrane, when fixed by such treatment, gives the reactions 

 characteristic of certain proteid substances. As the result of the action 

 of very dilute solutions of metallic salts, &c., the plasmatic membrane 

 becomes more and more permeable, without any interruption of its 

 continuity ensuing 1 , i.e., it undergoes changes, which might be explained 

 as being due to the coagulation or fixation of the proteid substances it 

 contains. It is, of course, possible that other substances may be present 

 as well, but from the characteristics already given it is evident that the 

 diosmotic membrane cannot be a film of oil covering the protoplast as 

 Quincke 2 was led to suppose by experiments made upon other objects. 

 Such a view is further contradicted by a variety of different physiological 

 experiences, as, for example, by the fact that diosmosis is not dependent 

 upon the solubility of the diosmosing substance in oil. 



Historical. Nageli was the first to call attention to the importance of the 

 peculiar osmotic properties which are possessed by the cell, and more especially the 

 protoplast. Then Traube's discovery of semi-permeable precipitation-membranes* 

 formed the basis for a correct interpretation of osmosis in general 4 , and hence also 

 of the osmotic properties of the cell. The relationships between the various pro- 

 cesses involved in the absorption of substances by the plant, including the con- 

 ditions under which accumulation and translocation take place, and the influences 

 exercised thereon by osmotic pressure, have been investigated by Pfeffer, as previously 

 described \ The absorption of certain aniline dyes by living cells enables many 

 points to be more clearly demonstrated and accurately determined 6 , and also 

 renders clear the fact that a functional division of labour may exist in the protoplast 

 in regard to the processes involved in diosmosis. Formerly this possibility was 

 overlooked, the protoplast being regarded as behaving uniformly at all points, 

 but neither the existence of such physiological division of labour, nor the presence 



1 Pfeffer, Osmot. Unters., 1877, p. 141 ; Plasmahaut u. Vacuolen, 1890, p. 241 ; de Vries, Jahrb. 

 f. wiss. Bot., 1885, Bd. xvi, pp. 508, 529. 



8 Quincke, Ann. d. Phys. n. Chemie, 1888, N. F., Bd. xxxv, p. 630; seq. u. 1894, N. F., 

 Bd. Lin, p. 625. By this later work, the matter has not been altered in any way (see Pfeffer, I.e., 

 p. 246), and that a slight solubility in oil should be shown by a single diosmosing substance has no 

 general importance. [E. Overton (Ueber die allgemeinen osmotischen Eiigenschaften d. Zelle, 

 Vierteljahrsschrift d. Naturf.-Ges. in Zurich, XLIV, 1899, p. no) concludes that cholesterin or 

 a cholesterin ether impregnates the plasmatic membrane and mainly determines its diosmotic pro- 

 perties and its permeability.] 



3 [A semi-permeable membrane is one which is penetrated by water or any other fluids which 

 it can imbibe, but not by all substances which may be dissolved in the latter.] 



4 Nageli, Pflanzenphysiol. Unters., 1855, I, pp. 1-35. See also Pfeffer, Plasmahaut, &c., pp. 

 242 and 316. 



5 Pfeffer, Landw. Jahrb., 1876, Bd. V, p. 87; Osmot. Unters., 1877. 



6 Pfeffer, Unters. a. d. Bot. Inst. z. Tubingen, 1886, Bd. II, p. 179. A summary, and additional 

 observations, in Plasmahaut u. Vacnolen, 1890. . 



