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 go 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 
Fic. 51.—VaaETasne Crrzs. 
A, very young; B, a little older, showing commencing formation of vacuole. 
p, protoplasm; 2, 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, 
