XEROPHYTISM : SUBMERGED PLANTS 201 



§ 33. Promotion of Transpiration 



If we assume that transpiration is not simply a necessary 

 evil, but has a useful role in the plant's economy — e.g. that 

 of promoting the supply of mineral nutrients — we might 

 expect to find in plants growing in situations where evapora- 

 tion is very low, not only an absence of xeroph5^ic characters 

 but even the presence of features which might be interpreted 

 as increasing transpiration. If such features are to be 

 found, and if their effect can be definitely demonstrated, 

 we shall have indirect but valuable evidence in favour of 

 the importance of transpiration. Attempts have not been 

 wanting to discover such features. 



{a) The clearest case is the undoubted existence of a 

 " transpiration current " in submerged plants. Aqueous 

 vapour is not given off, but that there is an ascending stream 

 of water in such plants as Elodea and Potamogeton has been 

 shown by Snell (1908) and Thoday and Sykes (1909). 

 This might be interpreted as a survival of an ancestral 

 function. There is, in land plants, a regular increase in 

 osmotic pressure from the root upwards, which may have 

 something to do with the ascent of water in the stem. Such 

 a gradient in a water plant might cause a flow from root to 

 tip, and Hannig (19 12) has shown that the gradient does 

 exist. We might possibly believe that here we have a 

 condition derived from some ancient land ancestor with no 

 relation to the plant's present condition, like the functionless 

 stomata on the lower sides of some floating leaves. But, 

 as we have seen, it has been shown that these plants thrive 

 better when rooted, and it seems highly probable that the 

 reason is a better supply of salts, due to the transpiration 

 current ; but Brown (191 3) suggests that the increased 

 vigour is due to the anchoring of the plant near a rich source 

 of carbon dioxide — the mud with its decaying organic 

 matter — and Arber thinks that carbon dioxide may be 

 carried up in the ascending current. 



{b) Under conditions of reduced transpiration and 

 favourable absorption — high temperature, well- watered soil, 



