Dixon — On the Effects of Gases on Transpiration. 625 



in that case be due to a rise in temperature, and to an increased facility 

 to diffusion at the seat of evaporation ; and the more complex processes 

 of vital action taking place in the protoplasm need not be appealed to. 

 These two causes would then be responsible for the fact that the rate 

 of transpiration is about 30 per cent, greater in oxygen than in air 

 But, again, it may be that vital processes giving rise to puniping 

 actions (to which I have already alluded as possibly occurring in the 

 leaf-cells) may be made more vigorous by the greater quantities of 

 oxygen available for respiration — that, in fact, these actions, in com- 

 mon with vital processes generally, are quickened when resj)iration is 

 more vigorous. 



"With the other gases there is practically no difference between the 

 specific transpiration and specific evaporation. The logical conclusion 

 from this seems to be that these gases were without effect on the vita 

 actions of the leaf-cells, so far as transpiration is concerned, during the 

 experiment. 



Thus the problem as to how far pumping actions, taking place m 

 the leaf-cells at the expense of the stored energy of organic compounds, 

 accelerate transpiration is not yet decided ; but I think it will aj)pear 

 that the evidence of the foregoing experiments, although by no means 

 decided, favours the view that such actions largely control the elimi- 

 nation of water from the transpiring cells. Other evidence in favour 

 of this view I have cited in my paper on the Physics of the Transpira- 

 tion Current.^ 



However this question is ultimately decided, I think the subject 

 matter of this research is not without its bearing on plant physiology. 



It is a matter of frequent observation that many plants which are 

 natives of arid regions, secrete a relatively large amount of ethereal 

 oils. It has been urged^ that the vapours of these ethereal oils form 

 a screen which arrests the heat radiations, and thus the leaves of the 

 plant are kept cooler than they otherwise would be. It might, how- 

 ever, be said against this theory that such an absorptive screen in 

 contact with the leaves (and it would evidently be most effective at 

 the surface of the leaves) would rather tend to raise their temperature. 

 Be that as it may, it seems that the property of vapours ia checking 

 evaporation, emphasised by this research, afford a simpler explana- 

 tion of the function of these oily secretions. "When the vapour of 

 the ethereal oils is liberated from the leaf -tissues, it will surround the 



1 " Notes from the Botanical School, T. C. D." No. 2, p. 88, 1897. 

 - "The Origin of Plant Structures," George Henslow, p. 82. 



