SAP CIRCULATES. 53 
upwards; for the cut surface next the ground 
streams with the exuding drops: but if we look 
at the upper surface of the cut, we shall find it 
not to be quite dry, but that here and there 
tiny drops of crystalline fluid are seen to flow 
from countless minute tubes. This teaches us a 
second fact about the circulation of the sap, in- 
forming us that there is also a motion from above 
down again toward the earth. If, again, we could 
water the plant with some coloured liquid which 
it would absorb, and were then to cut it quite 
across, we should see that veins of colour ran to 
and from the pith, indicating that the cellular 
tissue (that is, the pith, and the horizontal rays 
which we see radiating from it,) is also penetrated 
by the sap. Thus, then, the sap moves from the 
roots to the branches, from the branches to the 
roots, and from the outside to the heart of the 
stem. 
Another little experiment will shew us through 
what parts of the stem the two different currents 
of sap move. If we only cut into the bark, a very 
small portion of fluid exudes from the inner part 
of the cut; and this comes from above. We have 
already seen that the course of the sap in the wood 
