66 VEGETABLE PHYSIOLOGY 
CHAPTER V 
THE TRANSPORT OF WATER IN THE PLANT 
We have seen that it is necessary for the life of a plant 
that all its living cells shall be freely supplied with water. 
According to the habit of life of plants the mode of supply 
must necessarily vary. Those which are so constituted 
that water finds free access to all the cells, such as the 
unicellular or filamentous Alg@, which live in streams, 
pools, &¢., present no difficulty, as osmosis can go on freely 
in each cell, water entering its vacuole from the exterior. 
Sturdier plants of aquatic habit are almost equally easily 
supplied ; the water enters by osmosis into the vacuoles of 
the epidermal cells, the walls of which in these plants 
are not cuticularised, and from them it can pass from 
cell to cell all over the plant-body. No force in addition 
to osmosis is necessary in these undifferentiated plants. 
Others, which have a terrestrial habitat, from the nature of 
their environment require a more elaborate mechanism, 
which is found, as we have already pointed out, in the 
well-differentiated system of conducting tissue, composed 
largely of lignified vessels, fibres, and cells. Throughout 
all such plants a stream of water passes, entering at the 
roots, passing along the woody axis, and so rising up 
the stem into the leaves, where a very large part of it is 
evaporated. This stream of water is often known as the 
ascending sap. In addition to this comparatively rapid 
stream, slow currents of diffusion from cell to cell are also 
maintained, as in the plants of humbler type. These 
diffusion currents, depending mainly on osmosis between 
contiguous cells, have not the definite direction of the 
