62 VEGETABLE PHYSIOLOGY 



phenomena in the plant differs from any that can be used 

 in the laboratory in one important point, which modifies 

 the process in a fundamental way. It is alive, and exercises 

 an active control over the process. It consists of a cell- 

 wall, on each face of which is a thin film or layer of the 

 living substance. The latter is more complex than the 

 ordinary semipermeable membrane, for it can vary its 

 behaviour from time to time and so regulate the entry of 

 substances both liquid and dissolved, determining not only 

 what shall pass through, but to a large extent in what con- 

 centration they shall pass. In this way the absorption 



FIG. 51. VEGETABLE CELLS. 



A, very young ; B, a little older, showing commencing formation of vacuole. 

 p, protoplasm ; n, nucleus ; v, a vacuole. 



of a particular salt by the vegetable cell is not necessarily 

 in the proportion of its osmotic pressure. 



We can apply the osmotic process to explain the original 

 formation of the vacuole. Consider the case of a young 

 non-cuticularised cell of the external layer 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 

 in its interior. Part, at least, of the cell- wall is in contact 

 with water on the outside. The protoplasm is actively 

 living, and in the course of the chemical changes which 

 are incidental to vital action certain substances are produced 

 by it, which, like the syrup in the experiment first described, 



