TüRGESCENCE. 213 



and very elastic, and is thus In so far suited to withstand the endosmotic pressure 

 acting from within; but this property of the cellulose wall would not by itself 

 allow turgescence of the cell to be set up, since the cellulose permits filtration to a 

 very great extent— i. e. the cell-sap pressing from within would be again forced 

 out through the cellulose wall, even with very feeble pressure, and consequently 

 no observable turgescence could arise in this way. As a matter of fact, also, it 

 is shown that all cells merely enveloped with a cellulose wall (e.g. wood cells) 

 are incapable of becoming turgescent. Only those cells in which a protoplasmic 

 utricle is deposited all around the inner surface of the cellulose wall are capable 

 of becoming turgescent. The protoplasmic membrane in fact permits the entrance 

 of the water absorbed by endosmose into the sap-cavity, but it is in a high degree 

 resistent against the pressure of filtration which arises from the increase of the 

 volume of the sap\ To this pressure the protoplasmic membrane of the cell 

 is impermeable, and thus completes the necessary properties of the cell-wall, 

 in such a manner that the water which has penetrated by endosmose cannot be 

 again pressed out. Or, in other words, the cellulose wall as well as the proto- 

 plasmic membrane permit the entrance of the water absorbed by endosmose into 

 the sap-cavity of the cell, which in consequence of this tends to enlarge; the 

 protoplasmic membrane prevents the water filtering out again in consequence of 

 this pressure, while the external cellulose membrane forms a solid, elastic support, 

 to which the protoplasmic membrane becomes appressed by the endosmotic pressure. 

 It depends upon the extensibility and elasticity of the cellulose membrane how 

 far the volume of the cell-sap can be increased; for the protoplasmic membrane 

 itself is in a very high degree extensible and but slightly elastic, and if the sap-cavity 

 were surrounded only by it the vesicle would extend without resistance, in con- 

 sequence of the increase in volume by endosmose. The properties of the 

 protoplasmic membrane and the cellulose membrane thus supplement one another 

 in offering resistance to the endosmotic pressure of the cell-sap. That this is 

 actually the case is proved by the behaviour of a turgescent cell when, by 

 evaporation or by exosmose, a portion of the water of its cell-sap is wdthdrawn^ 

 If, for instance, turgescent cells are placed in a highly concentrated but otherwise 

 uninjurious salt solution — e. g. potassium nitrate — a considerable quantity of the 

 water of its cell-sap is withdrawn from the cell by the attraction of the salt ; in 

 consequence of this the protoplasmic membrane becomes very strongly contracted, 

 in proportion to the decrease in volume of the cell-sap. The cellulose membrane 

 on the other hand is only slightly contracted, because it was only slightly extended. 

 Both membranes thus become separated from one another; and the protoplasmic 

 membrane lies as a closed utricle free in the cavity of the slightly contracted 

 cellulose membrane. If such a cell is again placed in pure water, the latter is 

 attracted by the salts of the cell-sap and penetrates both membranes, the proto- 



* With respect to the co-operation of the protoplasm in the turgescence of cells, cp. my ' Lehr- 

 buch der Botanik; IV Aufl. 1874, p. 866. 



- The relations between turgescence, the protoplasm, and the cell-wall, established by Naegeli, 

 Pfeffer, and myself, were first clearly explained by Hugo de Vries in his ' Untersuchtingen über die 

 mechanischen Ursachen der Zeilstreckimg'' (Leipsic, 1877), a publication which may be recommended 

 in the highest terms to those commencing the study of vegetable physiology. 



