1 8 TRANSPIRATION AND ASCENT OF SAP ch. 



If, when the radiated energy is cut off, and the con- 

 ditions are such that water tends to condense on the leaves 

 from the surrounding space, the cells of the leaves still 

 continue to draw up water in the capillaries, then the 

 work done must be at the expense of the 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 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 transpiration 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 

 arrangements are made : A small branch about 30 cm. 

 long is cut and set in water in a cool, dark cupboard. 

 From this it is transferred, still standing in water, to 

 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 gas pressure existing in the water conduits has 

 become equalised to that of the atmosphere, and that, 

 consequently, the external pressure exerted at the base 

 of the branch has ceased to move the water upwards. 

 An open beaker, containing water at 100 C, is now intro- 



