G8 
COURSE OF THE SAP. 
[Part II. 
Third office of 
leaves, the 
changing of the 
sap from th e 
upward con¬ 
duit, the 
wood, to the 
downward con¬ 
duit, the bark. 
The growth in 
girthing is from 
the downward 
sap, or depend¬ 
ent on it; proof 
by experiment: 
general rule. But vast quantities of leaves fail 
to mature buds, vast quantities of buds fail to 
produce shoots, and vast quantities of shoots 
fail to grow. Wind, or an insect, or drought, 
or any thing which injures or destroys a leaf, 
will injure or destroy its bud, and consequently 
the next year’s growth of that bud. This is one 
chief cause of the baneful effect of wind on trees. 
Besides this, the circulation of the sap is depen¬ 
dent on the leaf throughout the summer, as it is 
dependent on the bud in the winter. The leaf is 
the turn-table which shifts the sap from the up 
to the down-line in summer, as the bud is this 
turn-table in the winter. Summer and winter 
this traffic is constant; but as it is less in winter, 
so the plant and staff required to wort it is less. 
Keep your turn-tables in order, or lose your traffic. 
That the growth in the girthing, or diameter 
of trees, is a downward growth, that is, from the 
descending sap, or at least that the descending 
sap is necessary for the growth in girth, seems 
clear from this : If a ring of bark is taken off 
round a branch of a tree, so as permanently to 
lay bare the wood, and to intercept the return 
of the sap through the bark, as long as the 
branch lives it will continue to increase in 
girthing above the ring, but not below it; and 
