42 PLA2STT LIFE ON THE FARM. 



One leaf's work would probably not suffice to build up a 

 grain of wheat, the aggregation of them serves to form a 

 sheaf of plump ears. The stem, in fact, is the agency 

 by which the work of individual leaves is combined and 

 concentrated 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 a 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. 

 Eoot action by itself wOuld not benefit the plant, even if 

 it were practicable. Leaf action, apart from the root, 

 would soon come to an end. The leaves by their copious 

 evaporating surfaces 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 up- 

 ward flow of the liquid. The distended condition of 

 the cells and the swollen state of their walls must exer- 

 cise pressure on the contained fluids, the direction of 

 which is mostly from below upwards. This pressure, or 



