46 THE VEGETATIVE FUNCTIONS OF PLANTS 



ink. Under these conditions only secretion can operate 

 to withdraw water from the fibro-vascular bundles, and 

 yet the eosin will rise in the branch and into the leaves. 

 From this and other experiments (not described here) 

 it is evident that the withdrawal of water from the fibro- 

 vascular bundles in the stem by the combined action of 

 secretion, osmosis, and transpiration may account for 

 the ascent of sap, even to the tops of very tall trees and 

 vines. These processes are able to raise the water column 

 on account of its great tensile strength, by which it does 

 not separate, although gravity pulls down on its lower 

 end, and the physiological processes in the leaves result 

 in a pull at its upper end. 



