THE OSMOTIC CHARACTERS OF THE CELL 



13 



in water, the latter at once begins to enter through the wall, and the diluted 

 sugar solution begins to rise in the tube {R). If the U-tube be now filled with 

 mercury {Qu) it will be found that the water enters with a force sufficient to 

 raise and support a column of mercury of considerable height. This pressure is 

 usually explained by postulating an attraction between the molecules of sugar 

 in the porous pot and the molecules of water outside. The attraction, however, 

 affects the water only, for the sugar does not pass out on account of the pre- 

 cipitation membrane being impermeable to that substance. The amount of the 

 attractive force may be estimated, starting from the position of rest, by 

 measuring the height of the mercury column which is supported by the inflow- 

 ing water. 



Pfeffer's osmotic cell may be legitimately compared with a vegetable 

 cell, especially if the precipitation membrane be laid down on the inside of the 

 porous pot, as shown in Fig. 3, as is possible for many experiments without 

 detriment to the rigidity of the membrane. The cell-sap (which itself 

 may contain cane sugar) would correspond to the contents of the pot, the 

 protoplasm to the copper-ferrocyanide membrane and the cell-wall to the 

 wall of the pot. If the cell of an alga (Fig. i) be placed in water, the water 

 streams through the cell-wall and protoplasm into the vacuole, and if it were 

 possible to attach a graduated manometer to this cell, the pressure exerted 

 on the inside of the cell-wall might be estimated. That such a pressure actually 

 exists may be easily demonstrated otherwise. The pressure of the inflowing 

 water induces tension in the elastic cell-wall, and if 

 this tension be relieved, e.g. by piercing the wall, a dis- 

 tinct contraction of the cell may, in many cases, be 

 observed. This internal pressure is termed osmotic or 

 turgor pressure, and by it the protoplasm is pressed 

 firmly against the membrane ; without such a resis- 

 tant layer the protoplasm would have been as little 

 able to resist this pressure as the copper-ferrocyanide 

 membrane would without the supporting clay cell. 



Since protoplasm, however, differs widely in con- 

 sistence from a precipitation membrane made of 

 gelatine-tannate or copper-ferrocyanide, and may be 

 likened rather to a fluid than to a solid, it is im- 

 portant to note that genuine fluids also show the 

 phenomenon of semipermeability. Water, for example, 

 is permeable to ether, but not to benzol, and if a 

 membrane be saturated with water and abuts on one 

 side on pure ether, and on the other on benzol, all the 

 conditions are fulfilled for the establishment of an excessive osmotic pressure 

 on the benzol side (Nernst, 1890). 



If we next ask ourselves what substances the protoplasm is permeable to 

 and what not, two methods are open to us, either to determine those capable 

 of diffusing outwards from the cell-sap to the exterior (exosmosis), or inwards 

 from the environment into the vacuole (endosmosis). Our knowledge of the 

 contents of the vacuole is, at least in some cases, sufficiently extensive to 

 enable us to determine that in the long run exosmosis will take place from it. 

 We know, for example, that the cells of sugar-beet are unusually rich in cane 

 sugar, a substance which may, by chemical methods, be detected even in 

 minute quantities. When thick slices of sugar-beet, thoroughly washed to 

 remove all traces of free sugar released from the cut cells, were laid in water, 

 De Vries (1877) found that even after fourteen days no sugar had passed from 

 the uninjured cells into the water. Again, if this experiment be repeated, 

 using red beet in place of white, we find that the protoplasm is as impermeable 



Fig. 3. Pfeffer's osmotic 

 cell. Z", porous pot ; N, pre- 

 cipitation membrane; /?, mano- 

 metercontaining mercury, Qu ; 

 Z, sugar solution. 



