34 BOTANY OF THE LIVING PLANT 



Osmotic Phenomena in Plant Cells. 



Most free cell-wall surfaces tend to bulge outwards. This is seen 

 even in embryonic cells (Fig. 9), and is a general feature of mature 

 cells of the epidermis (Figs. 22, 47). A tendency to convexity also 

 appears in the free cell-walls of the Cucumber hair (Fig. 17). Such 

 observations are evidence of a feature common in the living cells 

 of plants, namely, the state of Turgor (or Turgescence), arising 

 from the presence of internal pressure (Turgor Pressure) acting 

 equally upon the whole inner surface, and tending to round off the 

 contours of the cell. This state of turgor is due to the osmotic pro- 

 perties of the cell. 



At this point we may consider an experiment in which the arrange- 

 ment shown in Fig. 17A is employed. A parchment thimble is initially 

 filled with a solution of sugar and securely attached to a rubber 

 stopper fitted with a long glass tube. The thimble is then immersed 

 in water. Very soon liquid commences to rise in the tube, and if the 

 sugar solution is sufficiently concentrated a column several yards 

 high may be obtained in a day or two. Evidently water is passing 

 from the outer vessel into the sugar solution, and this is leading to 

 the development of a considerable hydrostatic pressure within the 

 thimble. This movement of water through a membrane is known as 

 Osmosis and would be produced with a solution of most crystalloidal 

 substances. The motive agent in the movement of the water is what 

 is known as the Osmotic Pressure of the sugar solution : the precise 

 nature of osmotic pressure is uncertain, but its existence is connected 

 with the fact that while the molecules of water pass through the 

 membrane rapidly, those of sugar penetrate only slowly. A measure 

 of the osmotic pressure of a given solution is obtained by noting the 

 maximum hydrostatic pressure that is developed when the solution 

 is enclosed in an apparatus of the type shown in Fig. 17A, using a 

 semi-permeable membrane (see below). Such measurements reveal 

 that by osmosis, hydrostatic pressures of considerable magnitude 

 may be set up. Thus the osmotic pressure of a 10 per cent, solution 

 of cane sugar is equal to approximately 8 atmospheres, or 120 pounds 

 per square inch : that is to say, the solution would set up a hydro- 

 static pressure of this magnitude when enclosed in a suitable apparatus. 

 Osmotic pressure varies with the concentration of a solution, and 

 within limits there is a direct relation between the two values. 

 Osmosis does not occur only from pure water into a solution : if 

 one solution is separated by a membrane from a second solution of 



