ON GUM AND SUGAR TREES 
will not flow in abundance or it is of such quality 
as to have no value. | 
The manner of production of the sap may 
be more or less accurately inferred from what we 
have already learned of plant physiology. We 
know that the leaves of the tree metamorphose 
water and carbon into sugary substances which in 
turn are transferred to various parts of the plant 
to be stored, usually in the form of starch. In the 
case of the maple, we may assume that the carbo- 
hydrates, as they are manufactured in the leaf- 
laboratories, are transferred in the current of sap 
that flows downward from the leaves through 
branches and trunk as a countercurrent in the 
cambium until it finally finds its way to the roots 
of the tree and is there stored for the winter. 
When spring comes and it is time for the new 
leaf buds to put forth, the supplies of nourishment 
are retransformed into soluble sugars, dissolved in 
the water that is taken in by the rootlets, and trans- 
ferred from cell to cell and along the little canals 
in the wood under the cambium layer of the bark, 
until they reach the twigs where the leaf buds they 
are to nourish are located. 
It is doubtless the so-called “root pressure” 
(which we have been led to interpret as due to 
osmosis) forcing the sap upward that causes it to 
flow from the wound in the tree made by the 
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