314 



LECTURE XIII. 



plasmic utricle becomes extended, applies itself in the first place cl6se to the 

 cellulose wall, and as the endosmose proceeds further the latter also becomes 

 distended again to a certain extent, until its elasticity opposes further extension, 

 and the cell is now again turgescent. The cellulose wall may in this sense be 

 compared with a very firm but large meshed wire net, the protoplasmic membrane 

 on the contrary with a very extensible but exceedingly fine, and therefore scarcely 

 permeable, net. 



The turgescence of a vegetable cell may be imitated in its most general 

 features by an artificial apparatus. If a pig's bladder is fastened on to one end 

 of a short wide glass tube, and the tube is then completely filled with a solution 

 of salt or sugar, and the second opening also closed with bladder, a sort of 



FIG. 189.— 1. Young, half-grown cell from the cortical parenchyma of the peduncle of 

 Cephalaria leitcantha. s. The same cell in a 4^/0 solution of potassium nitrate. 3, The 

 same cell in a 6°/o solution. 4- The same cell in a loo/o solution, i and 4 after nature; 

 z and 3 diagrammatic. All in optical longitudinal section. A cell-wall ; / protoplasmic 

 lining of the wall ; Jb celt nucleus ; c chlorophyll grains ; s cell-sap ; e penetrated salt solution 

 (De Vries). 



artificial cell results. If this is placed in a quantity of pure water, the latter 

 penetrates by endosmose into the cell, and the increase in volume causes the two 

 distended membranes to project externally like hemispheres, and at the same time 

 strongly resists. pressure with the finger, like a solid body. If a prick is made with a 

 fine needle in one of the two membranes, the liquid spurts up to a considerable height, 

 the membranes at the same time collapsing elastically. Here also it is evidently 

 the attraction of the dissolved substance for the water of imbibition of the membrane, 

 which supplies the force by which the two membranes are so forcibly distended, 

 and the turgescent condition produced. The diflfererice consists only in that here, 

 there are not two different membranes which oppose the pressure of the penetrated 

 water, as in the vegetable cell, but that one and the same membrane — i. e. the 

 -pig's bladder — allows the entrance of the endosmotic current on the one hand, 

 and, on the other, is atthe same time elastic and resistent to filtration; while in 

 the living vegetable cell the two latter properties are distributed between the cellulose 

 membrane and the protoplasmic sac. The so-called precipitation membranes also 

 behave similarly, though their resemblance to living vegetable cells has been to 



