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.

28).



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.




GIRDLING]



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.





=EXERCISE=



Let the teacher see that these and all other experiments are

performed by the pupils. Do not allow them to guess, but make them

see.



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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