NUTRITION. THE MACHINERY. 39 



for the general benefit of the plant. Each separate leaf, 

 like each separate cell, has a life of its own, and to some 

 extent is independent of every other leaf ; but, if they are 

 to be of any use to the plant as a whole, there must be co- 

 operation. The stem and its branches supply the means 

 for this co-operation. Moreover, there must also be co- 

 operation between the root and the leaves. Root action 

 by itself would not benefit the plant, even if it were prac- 

 ticable. Leaf action, apart from the root, would soon come 

 to an end. The leaves by their copious evaporating sur- 

 faces act as suckers to draw up the water from the soil by 

 the agency of the roots. Thus as the stem is the agent 

 between leaf and leaf, so it is the go-between betwixt the 

 roots and the leaves. Apart, then, from its function of 

 bearing leaves and flowers in such numbers and in such 

 manner as shall secure the greatest benefit to the plant 

 under the circumstances in which it is placed apart also 

 from its office of storing up food for future use we have 

 to consider how it is that the stem acts as the go-between 

 betwixt the root and the leaves, and between the leaves 

 themselves. 



Ascent of Liquids. It is certain that liquids, chiefly 

 water, and gases, mount up from the soil to the leaves. 

 How they enter the root has been explained, and their 

 passage up the stem against the direction of gravity may 

 be accounted for on like principles of diffusion. There are ? 

 however, various circumstances which aid the upward flow 

 of the liquid. The distended condition of the cells and 

 the swollen state of their walls must exercise pressure on 

 the contained fluids, the direction of which is mostly from 

 below upwards. This pressure, or squeezing process, is 



