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 upon a purely physical process which is known as 

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

 and syrup, be separated from 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 through 

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

 passing to the water. The rate of flow of 

 the two streams wiU 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 



fig. 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 fiUed 

 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 wfll 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 hmited 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 



Fi^. 1180. Apparatus to 

 show osmotic action. It 

 consists of a bladder 

 fiUed with syrup to the 

 open end of which a 

 tube is attached, and 

 the whole placed in a 

 vessel containing water. 



