OSMOSIS 



411 



Fig. 1180. 



interior. As this must be absorbed from without, it becomes 



necessary to inquire into the way in which it effects its entry. 



This is based ui)on a purely physical process which is known as 



osmosis. If two fluids of different densities, for example water 



and syrup, be separated fi'om each other by a homogeneous 



permeable membrane, they will tend to pass through the latter 



till there is a mixture of the two of equal density on each side 



of it. We shall thus have a stream of water passing throuo-h 



the membrane to the syrup, and a stream of syrup similarly 



passing to the water. The rate of flow of 



the two streams will not be the same, 



however, and the first result will be a 



considerable increase in quantity of the 



fluid upon the side of the membrane in 



contact with the syrup, owing to the 



greater amount of water that will have 



passed through. 



A convenient form of apparatus to 



exhibit this process of osmosis is shown in 

 Jig. 1180. It consists of a bladder fastened 



to the end of a narrow tube, which is 

 immersed, as shown, in a vessel of water. 



The bladder and part of the tube are filled 



with syrup, and the height at which the 

 latter stands in the tube is noted. After 



some time the contents of the tube will 

 be increased, and the liquid will stand in 

 it at a higher level, in consequence of the 

 osmotic action that has taken place. If the 

 positions of the water and the syrup were 

 reversed, the liquid would fall in the tube, 

 showing that under these conditions also 

 there is a greater stream of water towards 

 the syrup than of the latter in the opposite 

 direction. 



Though the process thus stated is far simpler than what we 

 have reason to believe takes place in the vegetable cell, we can 

 apply it to explain the original formation of the vacuole. 

 Consider the case of a cell of the dermatogen of a plant which is 

 immersed in water. It is full of protoplasm, and limited or 

 clothed by a cell-membrane, which is permeable more or less 

 readily by water. The protoplasm is saturated with water, but 

 there is no separate accumulation of the latter. Part at least of 



Fiy. 1180. Apparatus to 

 show osmotic action. It 

 consists of a bladder 

 fiUed with syrup to the 

 open eud of which a 

 tube is attached, and 

 the whole placed in a 

 vessel containing water. 



