42 PLANT 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, 
Root action by itself would not benefit the plant, even if 
it were practicable. Leaf action, apart from the root, 
would soon come to anend. 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 shatl 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 
