14 VEGETABLE PHYSIOLOGY 
permeable, yet presents a certain obstacle to the absorption 
of water, and so even those cells which are living in 
streams or ponds usually possess a vacuole. Cells without 
a membrane, such as the zoospores, already many times 
mentioned, can more readily absorb water from without, 
and hence they are not vacuolated to the same extent as 
the former ones ; indeed, many of them have no vacuoles. 
Where the vacuole exists it always contains water, so 
that the protoplasm of the cell has ready access to it, as 
much so indeed as the cell which possesses no wall. The 
vacuole contains a store which is always available. 
The advantages which water supplies to the plants are 
many. In the first place, we have seen there is a very close 
connection between it and the protoplasm, the life of the 
latter being dependent upon its presence. The information 
we have at present does not enable us to explain the nature 
of this dependence. There are other features of the rela- 
tionship, however, into which we can enter more fully. The 
protoplasm derives its food from substances in solution in 
the water; the various waste products which are incident 
to its life are excreted into it and so removed from the 
sphere of its activity. The raw materials from which cer- 
tain cells construct the food which is ultimately assimilated 
are absorbed from the exterior in solution in water. More- 
over, water is the ultimate medium through which gaseous 
constituents necessary for life reach the protoplasm. 
Passing from the consideration of the protoplasm in 
particular, the plant as a whole shows a similar dependence 
on water. Many parts owe their rigidity to the distension 
of their cells by liquid ; growth of the different members is 
dependent upon the same hydrostatic pressure. In many 
cases communication between different parts of a plant is 
brought about through the same instrumentality, and thus 
the response of the plant to various forms of stimulation 
is facilitated or indeed made possible. 
Another primal necessity of the plant is air. Every 
living organism, with the exception of a few of the very 
