624 



Proceedings of the Royal Irish Academy. 



The following table combines Table II. with. Table IV. for tbe 

 sake of comparison : — 



Table Y. 



Medium. 



Specific 

 Transpiration. 



Specific 

 Evaporation. 



Oxygen, 



Air, .... 



CO2, .... 



Ether, .... 



Chloroform, . 



. 



135-8 



100-0 



87-3 



82-3 



66-4 



104 



100 



89 



81 



59 



From this table it would appear that the rate of transpiration is 

 diminished when the leaves are surrounded by CO2, ether vapour, or 

 chloroform, much in the same degree as the rate of evaporation would 

 be diminished by the presence of these gases. In the case of oxygen, 

 however, the rate of transpiration is increased much more than the 

 rate of evaporation would be from a liquid surface. 



It must be understood that these numbers only apply to the first 

 effects of COo, ether, and chloroform ; for when these gases begin to 

 exercise a killing action on the cells, the rate of transpiration is very 

 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 may alter the surface tension, and so modify 

 the rate of loss. 



"With regard to oxygen, the case is different. The increase in the 

 percentage of this gas increases the rate of transpiration much more 

 than that of evaporation. But, unfortunately, this does not decide 

 the problem as to whether transpiration is facilitated by a vital action 

 in the evaporating cells, or whether the upward current is simply due 

 to the inflow of heat at their evaporating sui-faces. Oxygen may 

 increase the rate of transpiration by Kberating heat in the evaporating 

 cells by the oxidation of combustible materials there. The increase in 

 the rate of exhalation of water fromaleaf surrounded with oxygen would 



