164 THE LIFE OF THE PLANT 



we can easily see whether we have cut across the path 

 of the ascending current of water or not. Experiment 

 has proved that we can cut through the whole of the 

 rind, and even peel it right off, without making the 

 plant wither, which means that its aerial parts go on 

 receiving water from the soil. We can also cut through 

 the pith, which by the way often dies naturally in old 

 trees, leaving hollows at the heart of them ; in course 

 of time decay spreads also to the inner layers of old 

 wood, and yet the tree does not seem to suffer for a 

 long time. Apparently the ascending current of water 

 must flow through the wood, and moreover through 

 the young wood. This inference is also proved by 

 another experiment which has already been mentioned, 

 by the experiment in which the vascular bundles are 

 stained by coloured solutions. This experiment gives 

 specially clear results with leaves which are variegated, 

 or wholly white. In a short time the whole network 

 of veins stands out in colour on the white ground. 

 Microscopic investigations show that the wood vessels 

 are the first to become stained, which means that wood 

 is to be considered as the course for the ascending 

 current of water in the stem. 



And how can we explain the reason of this ascent of 

 water, sometimes to a height of three hundred feet ? 

 The reason of this movement must lie in the stem, as 

 well as in the root ; in the stem, because stems and 

 branches when cut off a tree continue to absorb water, 

 carrying it to the leaves ; in the root, because if the 

 stem be cut off close to the ground, and even more so if 

 the upper part of the root be cut, water will exude from 

 the cut surface of the part remaining in the soil. Let us 

 first study this phenomenon of the exudation of water 

 from the upper cut end of the root, which is apparently 

 the primary cause of the penetration of water into the 

 stem. It was long ago noticed that sap flowed in abun- 

 dance from some wounded or cut stems ; to this pheno- 



