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The Rotation Of Crops
The Sap Current

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The Sap Current

The root-hairs take nourishment from the soil. The leaves manufacture
starch and sugar. These manufactured foods must be carried to all parts
of the plant. There are two currents to carry them. One passes from the
roots through the young wood to the leaves, and one, a downward current,
passes through the bark, carrying needed food to the roots (see Fig.

If you should injure the roots, the water supply to the leaves would be
cut off and the leaves would immediately wither. On the other hand, if
you remove the bark, that is, girdle the tree, you in no way interfere
with the water supply and the leaves do not wither. Girdling does,
however, interfere with the downward food current through the bark.

If the tree be girdled the roots sooner or later suffer from lack of
food supply from the leaves. Owing to this food stoppage the roots will
cease to grow and will soon be unable to take in sufficient water, and
then the leaves will begin to droop. This, however, may not happen until
several months after the girdling. Sometimes a partly girdled branch
grows much in thickness just above the girdle, as is shown in Fig. 29.
This extra growth seems to be due to a stoppage of the rich supply of
food which was on its way to the roots through the bark. It could go no
farther and was therefore used by the tree to make an unnatural growth
at this point. You will now understand how and why trees die when they
are girdled to clear new ground.


It is, then, the general law of sap-movement that the upward current
from the roots passes through the woody portion of the trunk, and that
the current bearing the food made by the leaves passes downward through
the bark.


Let the teacher see that these and all other experiments are
performed by the pupils. Do not allow them to guess, but make them

Girdle valueless trees or saplings of several kinds, cutting the
bark away in a complete circle around the tree. Do not cut into the
wood. How long before the tree shows signs of injury? Girdle a
single small limb on a tree. What happens? Explain.

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