22 TRANSPIRATION AND ASCENT OF SAP ch. 



water-glands. However, the extreme apex often withers 

 away in the older leaves, as if some substance had been 

 exuded there from the leaf. In case this tip be the seat 

 of a water-gland, it was removed from all the leaves of a 

 branch which was set in the saturated chamber. After a 

 suitable time it was found that the coloured fluid had 

 risen into all the veins of the leaves, and it was seen in 

 the ultimate blind terminations of the vascular bundles. 

 In Cheiranthus cheiri these terminations are surrounded 

 by cells undifferentiated from the other cells of the 

 mesophyll of the leaf. The coloured fluid must have been 

 drawn into the terminal portions of the veins by these 

 cells, and not by any specialised water-glands. We may 

 conclude that the similar cells along the conduits have the 

 same function. 



It was usually found at the end of all the experiments 

 conducted in the saturated chamber that the surfaces of 

 the leaves had a copious deposit of water upon them, and 

 so it seemed probable that water was actually extruded 

 from the cells of the leaf even after water had begun to 

 condense on them from the surroundings. 



The actual presence of free liquid on the surface of the 

 leaves apparently did not markedly diminish the rate of 

 rise of the coloured fluid in the branch, and so, if the branch 

 was immersed in water before commencing the experi- 

 ment, it was found that the eosin mounted notwithstanding 

 into the dripping leaves. 



In these cases, the pumping cells, being surrounded by 

 water, must possess a directed action, which enables them 

 to draw the water in on one side from a liquid supply, and 

 to expel the water on the other into free liquid. 



This directed action may be more strikingly demon- 

 strated by the following experiment : A branch is fixed 

 water-tight into the lower narrow opening of a glass 

 receiver, so that its upper part and leaves project into the 

 interior, while its base extends beyond the cork in the 



