94 HIGHWAYS AND BYWAYS. 
Some of these questions may be answered by study 
and experiment, while others the most scientific research 
has failed to solve. The chemist can make the analysis, 
but the synthesis which the tree has made surpasses his 
ability to perform. 
The most interesting problem in connection with this 
study, and one perhaps the most diversely explained, is 
jhe manner in which the fiuids circulate or move from 
one part of the tree to another; but whether by expan- 
sion, contraction, capillary attraction, endosmose, osmose 
or permeation, experiment and observation teach that 
most of the crude sap taken up from the earth by the 
roots, after depositing some of its earthy matter in the 
cells to thicken their walls, and taking in return gran- 
ules of chlorophyl, is carried by the pleurenchyma, 
parenchyma and duct cells to the extremities of the stem 
or branches, there to nourish buds, leaves, howers and 
fruit, while the food, principally carbon, taken from the 
air by the leaves, together with portions of vitalized sap, 
is carried downward and deposited in the cambium 
layer to form the new outer rim of wood and the inner 
rim of bark; thus showing, if there is not a complete 
circulation of fiuid, there is at least an upward flow 
through one set of vessels, and a downward flow through ° 
other channels. In connection with the respiration and 
absorption of carbon by the leaves, how far they act as 
capillaries in changing the fluids from one set of cells to 
another is yet unknown, but there is little doubt that 
the principal work of the foliage is to build up the wood, 
while the root sap nourishes the foliage. 
