The Water of Planis and [ts Movements. 59 
or air are drawn into the stem, and thus the pressure is 
maintained. 
The popular idea, that the flow of sap in spring is due to 
a rapid rise of water through the stem at that season, is er- 
roneous. ‘The sap is really rising through the stem much 
faster in midsummer than in early spring. Sap ceases to 
flow on the opening of the buds, because transpiration (75) 
from the foliage quickly relieves the abnormal pressure. 
SQ. The Current of Assimilated Food. The food of 
the protoplasm in the different parts of the plant is assimi- 
lated almost wholly in the leaves (121). But we know that 
growth occurs, not only in the leaves, but in the stem and 
roots as well. It is clear, therefore, that when the stem and 
roots are growing, a movement of food matter must occur 
from the leaves into these organs. This movement may be 
demonstrated by a simple experiment. If a notch is cut 
into the stem of any of our common woody plants during 
spring or summer, deep enough to pass through the bark 
and a little into the wood, a callus, or cushion of new cells 
(73), will soon form on the upper side of the notch, but not 
on the lower, showing that the material from which new 
cells are formed is passing downward. Close examination 
will show that this callus forms just outside the union of the 
bark and wood. In all plants having more than one coty- 
ledon (46), this current is through the inner layers of the 
bark. The assimilated matter is dissolved in the water that 
saturates the cell-walls, and passes from the leaves to other 
parts of the plant by diffusion (64). 
81. Killing Trees by Girdling. To destroy the life 
of a tree that can not be conveniently removed, we “girdle” 
it by cutting a notch about the trunk beneath the lowest 
