264 ABSORPTION OF WATER [CH. 



through the plant. Further, in the case of submerged aquatics, 

 the transpiration stream is, for two reasons, of even greater 

 importance than in the case of terrestrial plants. Firstly, it has 

 been shown that the water, in which submerged plants live, is 

 generally still poorer in saline matter than that which percolates 

 through the soil 1 , and, secondly, there seems some reason to 

 suppose that submerged plants depend upon their transpiration 

 stream, not only for their salts, but also, possibly, for some part 

 of their carbon dioxide supply. We have noted the possible 

 importance of the substratum as a source of carbon dioxide 2 

 and, since this gas diffuses slowly, it is reasonable to suppose 

 that the water absorbed by the roots from the soil may be richer 

 in carbon dioxide than that in which the leaves are immersed. 

 Hence it is not impossible that the transpiration stream in 

 submerged plants may have its value in connexion with carbon 

 assimilation 3 . 



The existence of a transpiration current throws light upon 

 the otherwise inexplicable fact that many submerged plants have 

 an elaborate system of roots, often bearing well-developed root- 

 hairs. In the case of some Potamogetons, for instance, the root- 

 hairs are said to survive and play their part after the death of the 

 other cells of the piliferous layer 4 . Such a root system could 

 scarcely be needed merely for purposes of anchorage, and, fortu- 

 nately, we now have direct experimental proof that it serves 

 also for absorption. An American observer, Raymond H. 

 Pond 5 , by means of an ingenious piece of apparatus, succeeded 

 in actually measuring the water taken up by an individual root 

 of one of the submerged Water Buttercups. The root in ques- 

 tion, which was 14 cms. long and clothed with root-hairs, was 

 found to absorb 5 cubic cms. of water in 24 hours. 



Pond also carried out a number of indirect experiments on 



1 Sauvageau, C. (iSgi 1 ). 2 See pp. 253, 254. 



3 The work of Brown, W. H. (1913), appears to support this view, 

 though the author does not himself draw these conclusions, but regards 

 the roots as mere organs of anchorage. 



4 Sauvageau, C. (iSgi 1 ). 5 p on ^ R. H. (1905). 



