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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 Alga, which live in streams, pools, 
&c., 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 epider- 
mal cells, the walls of which in these plants are not cuticu- 
larised, 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. But the great number of 
plants which have a terrestrial habitat, from the nature 
of their environment require a more elaborate mechanism. 
This 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 
