46 THE VEGETATIVE FUNCTIONS OF PLANTS 
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. 
