620 Proceedings of the Royal Irish Academy. 



the mean of a number of observations with a branch of Cytisus Labur- 

 num in CO2. the index moved 1 cm. in 38" ; with the same branch in 

 oxygen it moved 1 cm. in 28". 



When the rate of transpiration in air was compared with that in 

 oxygen, it was found that when a branch was surrounded with the 

 latter gas, transpiration was slightly more rapid. The index for the 

 branch in air moved 1cm. in 36"; for the same branch in oxygen it 

 moved 1cm. in 33". 



If, while a current of air was passing through the receiver, a piece 

 of cotton wool soaked in chloroform was introduced, a much more 

 marked difference in the rate of transpiration became apparent in a 

 short time. Thus the index was traversing 10cm. in 50"8" when 

 chloroform was introduced ; at the end of 30 minutes it took 516" to 

 traverse the same distance. The chloroform was then removed, and 

 the air current maintained for 60 minutes. At the end of this time 

 the index moved 10cm. in 120." This result is the mean of a number 

 of experiments made with a small branch of Acer macrophyllum. 



With another branch in air the motion of the index was 10cm. in 

 127". When surrounded with chloroform vapour for 45 minutes the 

 index took 642" to traverse the same distance. 



A similar diminution in the rate of transpiration is observed when 

 the branch is surrounded by ether vapour. Thus, with a branch of 

 Acer macrophyllum in air, the index moved 10cm, in 205"; with the 

 same in ether vapour it moved 10cm. in 265". 



These experiments indicate a large difference in the rate of tran- 

 spiration in the rate of different gases. The figures given here will 

 serve only as examples of the results of such experiments, for, 

 although they were the means of a number of observations, these 

 latter are made so precarious by a number of circumstances, that they 

 can only be taken as indicating a difference, and not as giving a 

 measure of it. The sticking of the index in the capillary tube, and 

 the opening of the receiver to introduce the anaesthetics, bring in 

 errors, which render the method unsuited to exact observation. 



In order to eliminate these sources of inexactness I had recourse 

 to the arrangements shown in the figure. The results obtained by 

 this method are displayed in the following tables. The difficulties of 

 keeping the flow^ of gas exactly constant through the apparatus, and 

 other experimental errors, lead to variations between the individual 

 observations often amounting to 10 per cent. ; but by multiplying these 



1 It is to be observed the rate of flow of the different gases will be diflEerent, 

 even if the pressure be the same. 



