tfi^ LECTURE XXXIII. 



prevented by the peculiar properties of the protoplasmic sac lining the cell-wall ; 

 (3) Although the sac of protoplasm itself prevents filtration, it is true, it would be 

 more and more distended by the water forcibly entering by endosmose, were this 

 not prevented by the firm elastic cellulose wall which surrounds it. The protoplasmic 

 vesicle does not allow filtration, but it is exceedingly extensible ; a limit is imposed 

 to its actual extension, however, by the slight extensibility and great elasticity of 

 the cellulose wall, which however on its part permits filtration to a large extent ^. 



It is now important to make out by means of what forces the water is absorbed 

 from without through the cellulose wall and the protoplasmic vesicle. We know so 

 far of no other cause for this than the attraction which the substances dissolved in the 

 water of the cell, and in part contained in the wall itself, exert on the surrounding 

 water: by means of this attraction the molecules of water are drawn through 

 the cellulose wall and the protoplasmic sac into the interior of the cell, and increase 

 the volume of the sap, which therefore presses the walls outwards. Since, how- 

 ever, the cellulose walls in virtue of their elasticity oppose considerable resistance 

 to this pressure, the entrance of the water must take place with still more consider- 

 able force ; or, in other words, the endosmotic attraction must be greater than the 

 elasticity of the cellulose walls. It has been found that this endosmotic attraction 

 may attain considerable magnitude, so much in fact that the absorbed water may exert 

 a pressure of 6-7 atmospheres on the surrounding wall — i. e. the cell-wall under these 

 circumstances must behave as if a column of mercury 6-7 times as high as the 

 barometer column were pressing on it from within ^. It was formerly believed that 

 such powerful endosmotic effects could only arise if the cell contained large quantities 

 of dissolved substances; and it was also assumed that it is chiefly 'substances of the 

 nature of gums, sugars, or proteids which bring about the endosmotic action. But 

 the investigations of Pfeffer have shown that crystallisable salts under the circum- 

 stances here given exhibit far greater endosmotic force than the organic substances 

 mentioned; that, for example, saltpetre acts much more vigorously, and that 

 even very dilute solutions of this and other salts exert vigorous endosmotic action. 

 Subsequently De Vries pointed out good reasons for believing that in the turgescence 

 of the growing parts of plants the vegetable acids or their salts are especially 

 important; in fact, as I had already shown in 1862, all parts of plants which are 

 actively growing in length have an acid reaction ; and Graham had pointed out that 

 the organic acids common in the plant, and their potassium salts, are remarkable for 

 their attraction for water, and according to De Vries sensitive phenomena in motile 

 organs of plants are accompanied by an increase in the acid reaction, when an 

 increase of turgescence is taking place at the same time ^. 



1 The difference in the significance of the protoplasm and the cellulose wall for the processes of 

 diffusion in living cells was first recognised and thoroughly worked out by Naegeli, in his treatise 

 on the ' P7-imordial UtricW (Pflanzen-physiol. unters, von Naegeli und Cramer, H. I, 1S55, p. i). 

 The principal subsequent workers on this subject have been De Vries {'Sit?- la permcabilite du proto- 

 flasma des betleraves rouges,' Arch. Neerlandaises, VI, 1871, p. 117), and Pfeffer {'Osmotische 

 Untersuchungen' 1877). 



^ Very detailed statements on the magnitudes of endosmotic force and the resulting turgescence 

 of the cells are found in Pfeffer's '■Osmot. Unters.,' Leipzig, 1S77. 



3 Hugo de Vries, ' Über die Bedeutung der Pflanzensäuren für den Turgor der Zellen,' Bot. 

 Zeit., 1879, p. 847, 



