ROOT-PRESSURE AND ENDOSMOSE. 



375 



with great force? I attempted, in my 'Handbook of Experimental Physiology,' 1865 

 (p. 204), to make this problem clear, and partly to solve it, with the aid of the 

 diagram here reproduced. 



Fig. 212 is intended to represent, diagrammatically and simply,' a piece of 

 a young absorbing root. A, A are so many cortical cells of the root, on which we 

 may suppose protuberances to arise as root-hairs; these immediately surround 

 a vessel, B. By means of the dissolved substances contained in the cells A, the 

 water of the surrounding soil is absorbed by endosmose ; this causes the cells, since 

 they possess a protoplasmic lining to the walls on the inside, to become highly 

 turgescent. Now, as I have already shown, turgescence in general depends upon 

 the absorption of water by endosmose taking place through cell-walls which resist 

 filtration to so great an extent that a strong hydrostatic pressure becomes estab- 

 lished in the cells. Now we may imagine hypothetically that the outer walls a, while 

 favourable to the inflow of the water by endosmose, offer much resistance to its filtra- 

 tion, whereas the portions of wall 6, which bound the vessel, may allow filtration 

 to a greater extent. It is then obvious that 

 when a cell A is highly turgescent, the 

 water taken up through a may be filtered 

 out with force through the wall 5 ; in con- 

 sequence of this, the vessel £ must be- 

 come filled with water, and this will 

 escape above at the transverse section of 

 the root-stock, since the vessel is closed at 

 the root-tip below. Since we know now 

 that the turgescence of a cell may be equal 

 to a pressure of several atmospheres, it is 

 intelligible that the water enters from the 

 cells A into the vessel £ with a pressure 

 which is. able to drive it up to a height 

 of 10-15 metres. To a certain extent, 

 this argument may be demonstrated by an 

 artificial apparatus. In the accompanying fig. 212. 



figure (Fig. 213), z indicates an artificial cell 



composed of a wide glass tube g, g, which is closed at a with a double bladder, and 

 at b with a single one. After filling z with a solution which acts endosmotically, the 

 glass tube r is fastened over b by means of a caoutchouc cap K, K. If this apparatus 

 is now laid in a vessel full of water, the cell z absorbs the water by endosmose 

 through the double membrane a, and when the turgescence has reached a sufficiently 

 high degree the fluid absorbed through a again filters out through the single mem- 

 brane b, which offers a smaller resistance to filtration : it then collects and ascends 

 in r. Incomplete though this apparatus may be, "it at any rate illustrates the funda- 

 mental ideas on which our conception of the essence of root-pressure depends, 

 and in the sixteen years which have elapsed since my publication quoted above, 

 nothing better has been put in its place. I then neglected to mention the various 

 functions which devolve on the one hand upon the cellulose membrane, and on 

 the other hand upon the protoplasmic utricle of the cell, in turgescence and 



