626 ProceecUnrjii of the Royal Irish Academy. 



leaves, and fill the intercellular spaces. In tfiese positions we might 

 expect that the vapour -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 as yet, 

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

 vapour given off from chopped-up leaves of ^7' i^errt2's«'a^5sm!;/<m»i reduced 

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

 of transpiration of a branch of Syringa vulgaris, in a current of dry air, 

 as 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 transpu'ing 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 

 trauspiration of a branch of Cytisus Lalurnum, from 100 to 93. 

 In these experiments the temperature lay between 16° and 17° C. 

 At higher temperatures, it is possible that the effects would be 

 more marked. 



