66 VEGETABLE PHYSIOLOGY 



CHAPTEE V 



THE TKANSPOBT 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 Algcz, 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 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 



