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 
B 
Fic. 51.—VEGETABLE CELLS. 
A, very young; B, a little older, showing commencing formation of vacuole. 
P, protoplasm ; , 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 
