i THE NATURE OF TRANSPIRATION 17 



notion of vapours in checking evaporation, emphasised by this research, 

 affords a simpler explanation of the function of these oily secretions. When 

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

 surround the leaves, and fill the intercellular spaces. In these positions we 

 might expect that it will exert a retarding action on transpiration 

 and evaporation, in accordance with the experiments quoted above. I 

 have only been able to make a few experiments on the matter, but 

 these indicate the surmise given here is correct. I found that the 

 vapour given off from chopped-up leaves of Artemisia absinthium 

 reduced the rate of transpiration very considerably. Thus, if we denote 

 tlie rate of transpiration of a branch of Syringa vulgaris, in a current of 

 dry air, by 100, this rate will be reduced to about 87 if we allow the air- 

 current to pass over chopped leaves of this Artemisia, and so carry 

 some of the vapour given off by these leaves round the transpiring branch. 

 The air is, of course, dried after passing over the leaves. In a similar 

 manner I found that the same vapour reduced the rate of transpiration 

 of a branch of Gystisus laburnum from 100 to 93. In these experiments 

 the temperature lay between 16 and 17 3 C. At higher temperatures, it is 

 possible that the effects would be more marked. 



Information as to the nature of the forces effective in 

 bringing forward the water to the seat of evaporation was 

 also sought by examining the possibility of transpiration 

 into a saturated space. 



Transpiration into saturated spaces. In normal 

 circumstances transpiration is effected under conditions 

 favourable to evaporation. The transpiring surfaces are 

 at such a temperature that the vapour pressure in the 

 surrounding space is less than at the surface of the tran- 

 spiring cells. To maintain this temperature, the leaves 

 of the plant are free to receive light and heat radiations, 

 and heat may be conducted into them, as evaporation 

 tends to lower their temperature below that of their 

 surroundings. This inflow of energy from the external 

 world must, in ordinary circumstances, be taking place 

 during transpiration. In addition to these sources of 

 energy, the cells of the leaves may do work at the expense 

 of the potential energy of the store materials they possess. 

 This stored energy, which is, of course, ultimately derived 

 from the radiant energy entering the plant, is the only 

 remaining source of energy available for the leaves. 



c 



