54 VEGETABLE PHYSIOLOGY 
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 mem- 
brane, such as the zoospores already many times men- 
tioned, can more readily absorb water from without, and 
hence they are not vacuolated to the same extent as are 
those which possess a cell-wall; indeed many of them 
have no vacuole. This cavity when present being always 
filled with liquid, the protoplasm of the cell has ready 
access to water, as much so indeed as the protoplast which 
possesses no cell-wall. The vacuole contains a store which 
is always available. 
The quantity of water which a vacuole can contain is 
very small, and as the needs of the protoplasm are some- 
what extensive, a need arises for the continual renewing of 
its supply. This is evident when we consider that the 
protoplasm draws its nutriment eventually from the water, 
and that it must return to it such waste products as it 
gives off. Its oxygen must be drawn from the same 
source, for this gas can only pass into the interior of a cell 
by entering into solution in the liquid which it contains. 
In cells which are deep-seated the need of oxygen can only 
be supplied by a slow passage from cell to cell of the gas 
which has been dissolved by those abutting upon a free 
surface. Similar considerations apply to the elimination 
of the carbon dioxide which accompanies the respiratory 
processes. 
The life of a plant is consequently very intimately con- 
nected with the renewal of the water which the cells contain: 
Fresh liquid must be taken in, and that which is already 
there must be to a certain extent removed; the plant 
demands in fact a kind of circulation of water, and this 
becomes the more imperative as the mass of the plant 
increases, with the possible exception, however, of those 
massive plants whose habitat is marine. 
In examining the way in which this circulation is set 
up and maintained, it is first necessary to inquire into the 
