628 Proceedings of the Royal Irish Academy. 



Under normal circumstances transpiration is effected under con- 

 ditions 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 transpiring cells. To maintain this 

 temperature, the leaves of the plant are free to receive light and heat 

 radiations, and heat may he conducted into them, as evaporation 

 tends to lower their temperature below that of their surroundings. 

 This inflow of energy from the external world must, under 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. 



If, when the radiated energy is cut off, and the conditions are 

 such that water tends to condense on the leaves from the surrounding 

 space, the cella of the leaves still continue to draw up water in the 

 capillaries, then the work done must be at the expense of thu stored 

 energy ; and, if this work is no longer continued, when the leaves are 

 killed, we may fairly ascribe it to vital actions pumping or drawing 

 up water from the conduits of the plant. 



It may be pointed out here that this energy could only be made 

 available when the store materials can obtain the requisite oxygen from 

 the plant's surroundings, or from its own substance. And so, in common 

 with other vital actions, it would cease when oxygen is not available. 



Supposing, then, we find that the upward motion of the transpira- 

 tion current continues when radiated energy is cut off, and when the 

 leaves are surrounded by a space saturated with water vapour, we are 

 driven to conclude that the traction exerted on the ascending water is 

 exerted by a vital action, and we can no longer assume that simple 

 physical processes, exactly corresponding to the actual inflow of 

 energy, at the moment, can account for the elevation of water 

 in such a case. On the other hand, the converse will be true if no 

 elevation of water occurs in the plant when it is submitted to the 

 conditions described. 



To put this matter to an experimental test, the following arrange- 

 ments arc made : — A small branch about 30 cms. long is cut and 

 set in water in a cool, dark cupboard, From this it is transferred, 

 still standing in water, under a glass receiver. The internal walls 

 of the receiver are kept wet. After remaining one hour under the 

 receiver, and still screened from light, it is assumed that any reduced 



