56 



VEGETABLE PHYSIOLOGY 



minent in those which are crystalline in character, though 

 it is not confined to them. Solutions containing different 

 substances in equal degrees of concentration do not, however, 

 possess equal osmotic powers ; each one has its own special 

 ability which is often spoken of as its osmotic equivalent. 

 With any particular osmotic substance, however, the osmotic 

 efficiency varies with the concentration of the solution. 



Though the process of osmosis as illustrated in the 

 experiment just described is far simpler than that which 

 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 young non-cuticularised 



FIG. 51. VEGETABLE CELLS. 



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

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



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 already described, have an affinity 

 for water, or, to use a more technical phrase, have a fairly 

 high osmotic equivalent. Water consequently passes into 

 the cell, at first only in such quantities as to distend it 

 somewhat. As the process goes on, more liquid is taken up 



