1 6 TRANSPIRATION AND ASCENT OF SAP ch. 



markedly diminished, presumably owing to the reduction 

 of the osmotic pressure in the cells. 



The experiments both on transpiration and evaporation 

 are exposed to two common errors e.g., a certain amount of 

 gas will be dissolved in each case by the liquid present, 

 and this will reduce the loss of weight, and so diminish 

 the rate in both cases. Again, this solution of the gas in 

 the liquid will alter the surface tension, and so modify the 

 rate of loss. 



With regard to oxygen the case is different. The in- 

 crease in the percentage of this gas, or even possibly some 

 impurity carried with it, increases the rate of transpira- 

 tion much more than that of evaporation ; and so furnishes 

 experimental evidence in favour of the view that water is 

 brought forward to the seat of evaporation by secretion. 



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 perceptible effect on the vital actions 

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

 the experiment. 



Thus the problem as to how far secretory actions, taking 

 place in the leaf-cells at the expense of the stored energy 

 of organic compounds, accelerate transpiration is not 

 decided by these experiments ; but I think it will appear 

 that their evidence, although by no means unequivocal, 

 favours the view that such actions have some function in 

 the elimination of water from the transpiring cells. 



Although these experiments cannot be regarded as quantitatively exact, 

 I think they are not without their 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 

 arrest the heat radiations, and thus the leaves of the plant are kept cooler 

 than they otherwise would be. It might, however, 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 



