416 MANUAL OF BOTANY 



CHAPTER II. 



THE TRANSPORT OF WATER IN THE PLANT. 



We have seen that it is necessary for the hfe of the 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 

 var}'. Those which are so constituted that water finds free 

 access to all the cells, such as the unicellular or filamentous algse, 

 which live in streaixis, pools, &c., present no difficulty, as osmosis 

 can go on freely in each cell. Sturdier plants of aquatic habit 

 are almost equally easily supplied ; the water enters by osmosis 

 into the epidermal cells, which we have already seen are pro- 

 vided with membranes that are not cuticularised, and it can 

 pass readily from cell to cell all over the plant body. Plants of 

 terrestrial habit, from the nature of their environment, require 

 a more elaborate mechanism, which is found in the large de- 

 velopment of woody tissue they exhibit. Throughout all such 

 plants a stream of water passes, entering in at the roots, passing 

 along the woody axis of the root, 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 saj). In addition to this 

 comparatively rapid stream, slow currents of diffusion from 

 cell to cell are also maintained as in the plants of humbler 

 development. 



Except in some special cases, the water which passes through 

 the body of an ordinary land plant is obtained from the soil in 

 which its roots are embedded. The soil itself is composed of 

 minute particles of inorganic matter, derived originally from 

 the breaking down of rocks, and of decaying animal or vegetable 

 matter mixed with the inorganic constituents. This organic 

 matter is known as humus and is of very varied composition. 

 The soil thus consists of a loose matrix of granular character, 

 the interstices of which are normally filled with air, but which 

 can contain varying quantities of water. When these spaces are 

 filled with water, the plants growing in the soil are very un- 

 favourably placed for absorbing it. When air is present in them, 

 each particle of soil is surrounded by a delicate film of water, 

 which adheres closely to it. This water, often spoken of as hygro- 

 scopic water, is the source of the plant's supply. It adheres so 

 firmly to the soil particles that it escapes ordinary observation ; 

 if, however, soil which has been allowed to drj^ at any ordinary 



