THE OSMOTIC CHARACTERS OF THE CELL 



23 



and xylol (Overton, 1899), and also of substances insoluble in water, while it 

 would present no obstacle to the passage of water. It is worth noting in this 

 connexion that lanolin, a derivative of cholesterin, is able to absorb more 

 than double its weight of water. 



Although Overton's hypothesis appears in many respects inviting, still 

 it has not been proved either in principle or in detail. In the first place, there 

 are specific differences in permeability ; Penicillium, for instance, will not 

 permit the entry of copper salts, although these are readily absorbed by the 

 majority of plants. Similarly Beggiatoa is able to take up sulphuretted 

 hydrogen, although Algae closely allied do not do so. Such differences 

 may be explained by assuming specific chemical constitutions for the plasmatic 

 membranes of the species concerned. Moreover, the plasmatic membrane of 

 one and the same individual varies according to external conditions. These 

 variations may depend on changes taking place from time to time in the 

 cholesterin-lecithin mixture, although it is possible that, under certain con- 

 ditions, other substances may play a part in the constitution of the plasmatic 

 membrane. Further, we cannot escape from the criticism that the plasmatic 

 membrane may often be in our experiments not quite in a natural condition. 

 For instance, many plasmolysing substances are instrumental in forming pre- 

 cipitation membranes on the surface of the plasma, and thus it is possible that 

 we might be studying the characters, not of the plasmatic membrane in its 

 natural state, but those of a precipitation membrane artificially produced 

 (Berthold, 1896, p. 152). 



[Wachter's researches (1905, Jahrb. f. wiss. Bot. 41, 165) have shown 

 that exosmosis of sugar from the cells of the onion is prevented by salt-solutions, 

 but that quantities of sugar diffuse from cells when these are surrounded by 

 water. In the case of beet also he has observed exosmosis of sugar to take 

 place, in opposition to the results which De Vries obtained (p. 13). Wachter's 

 work does not, however, provide us with an explanation of the complicated 

 osmotic phenomena seen in beet, although his studies demonstrate very 

 clearly that the osmotic peculiarities of the plasmatic layer are exceedingly 

 variable (compare Lecture XIV).] 



As a matter of fact, a renovation of the plasmatic membrane has been 

 experimentally established. It occurs, for example, on the surface of the 

 protoplasm which exudes from a wound in Vaucheria, and may be observed 

 also after injuries inflicted on plasmodia of Myxomycetes. In the latter 

 case it may be easily shown that new plasmatic membranes are formed on 

 isolated parts of the general cell plasma (Pfeffer, 1890' 



Bibliography to Lecture II. 



Berthold, E. 1886. Studien uber Protoplasmamechanik, Leipzig. 



HoFMEiSTER, Fr. 1901. Die chemische Organisation der Zelle. Braunschweig. 



Nageli. 1855. Pflanzenphys. Unters. i, 21. 



Nathansohn. 1902, Jahrb. f. wiss. Bot. 38, 241. 



Nernst. 1890. Zeitschr. f. physik. Chemie, 6, $;^. 



Overton. 1895. Vierteljahrsschr. d. Naturf.-Gesell. Zurich 



Overton. 1899. Ibid. 



Overton. 1900. Jahrb. f. wiss. Bot. 34, 669-701. 



Pfeffer, W. 1877. Osmotische Untersuchungen. Leipzig. 



Pfeffer, W. 1886. Unters. aus d. bot. Instit. Tubingen, 2, 179. . ^ 



Pfeffer, W. 1890. Plasmahaut und Vakuolen (Abh. math.-phys. Kl. Sachs. 



Gesell. 16, 187). 

 Tamann. 1892. Zeitschr. f. physik. Chemie, 10, 255. 

 De Vries, H. 1877. Die mechan. Ursachen d. Zellstreckung. Leipzig. 

 De Vries, H. 1884. Methode zur Analyse d. Turgorkraft. Jahrb. f. wiss, Bot. 



14, 427- 



