168 THE LIFE OF THE PLANT 



insufficient to explain the raising of water to the summit 

 of the highest trees ; and we also know that cut stems 

 sunk into water are themselves able to absorb it. But 

 in order to explain the reason of this absorption of water 

 by the stem, we must first of all begin by studying the 

 part played in this process by the leaves. The best 

 way to do this, is to perform the following experiment. 

 We cut off a small branch, say of a birch-tree, covered 

 with leaves and dip its cut end in water. On taking 

 the branch out of water we notice a drop suspended at 

 the cut part, but in half a minute, if not less, the drop 

 will be absorbed ; we dip the branch in again and notice 

 once more the rapid disappearance of the drop, showing 

 the greediness with which our branch absorbs the water 

 provided for it. 



The root drives water into the stem ; the stem 

 greedily absorbs it and drives it further on. What will 

 become of this water when all the parts of the plant are 

 saturated with it ? Apparently, if it continually enters 

 at one end it must pass out at the other. This passing 

 out of water is very clearly observed under certain 

 special circumstances. If on a warm damp evening in 

 May or June we bend to the ground after the sun has 

 set and glance at the surface of a field of oats, we shall 

 observe round drops on the very tips of the upright 

 blades. If we patiently observe one leaf for some time, 

 we shall see the little drop growing larger and larger until 

 it rolls down ; in its place, at the very edge of the leaf, 

 another drop will appear and so on. The same pheno- 

 menon will be observed if oats are sown on a plate 

 covered with a glass bell. Small drops of water will 

 continually appear at the tips of the leaves and dis- 

 appear whenever the glass bell is removed. This 

 phenomenon is seen in some plants much more clearly, 

 and they give off water in considerable quantities. 

 Anatomical investigation has even revealed special 

 apertures in these leaves at the points of exudation. 



