; 



=B 



Z^ 



Fk;. 3. 



i THE NATURE OF TRANSPIRATION 23 



neck, and is supplied with a solution of eosin {see Fig. 3). 

 If the receiver be filled with water, so that the leaves of 

 the branch are completely 

 submerged, it will be found 

 that, notwithstanding the 

 presence of the water in 

 contact with the leaves, 

 and the hydrostatic pres- 

 sure due to its depth, the 

 eosin will mount rapidly 

 into the branch. 



In some of my experi- 

 ments the pressure of the 

 water was sufficient to 

 drive liquid back into the 

 intercellular spaces of the 

 leaves of the branch. So 

 that it appears that the pumping action can raise water 

 against a considerable external hydrostatic pressure. 



In carrying out this experiment, of course, care must 

 be taken that the gas-pressure in the branch has become 

 equalised with that of the atmosphere. With this pre- 

 caution, however, the result seems conclusive, i.e., that 

 secretory actions, and not evaporation, cause the rise of 

 the eosin into the branch. 



It will be found that, if the water in the receiver is warm 

 (25-30 C), and if the apparatus is placed in a strong 

 light, the ascent of the eosin will be rapid ; if, on the other 

 hand, the water is cold (below 12 C.) and the light is not 

 strong, the eosin will rise but slowly in the branch. If 

 the apparatus is placed in darkness, the eosin will rise but 

 little or not at all. 



It seems probable that the increased rate is, in part, due 

 to the quickening of the vital processes caused by the 

 rise in temperature when the water surrounding the 

 leaves is warm. 



