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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 Algce, 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 



