PLANT NUTRITION. 43 
squeezing process, is augmented by the swaying of the 
branches or the movements of the leaves. Even more 
powerful must be the effect of the atmospheric pressure 
urging up the liquid to fill the place of that evaporated 
from the leaf surface. This upward current is naturally 
most active at the period of growth, and the channels 
through which it flows are necessarily those where the 
conditions for osmosis are most propitious. In propor- 
tion, therefore, as the cells become filled with woody or 
earthy material does the current become less, As the 
straw ripens or the timber hardens by the formation of 
wood in its cells, so does the flow of liquid diminish, the 
leaves in their turn and degree become obstructed and 
fall, and the current, deprived of their stimulus, becomes 
feeble. 
But while in thus alluding to some of the duties of 
the stem, we have had to note the existence during the 
period of growth of a current of liquid whose general 
direction is upward, it is necessary to point out that the 
direction is not exclusively upward, but that it is mani- 
fested in whatever direction the resistance is least and 
where growth may be going on most actively at the time. 
Again, it is necessary to guard against the still prevalent 
fallacy attaching to the use of the word “‘sap.” That 
term was first employed when it was imagined that a 
regular circulation of fluid took place in plants from 
root to leaf, and from leaf back to root—just as in ani- 
mals the blood courses from the heart through the arteries 
to the capillaries, and back from the capillaries to the 
heart by the veins. In the case of the higher animals 
there is a continuous series of tubes to convey the fluid, 
and that fluid is uniformily arterial or venous. It is 
quite otherwise with. plants ; there is no continuous tube 
or set of tubes, and there is no fluid of uniformily the- 
same composition throughout. Near the root the juice 
pf the plant has one composition, near the leaf another, 
