CIRCULATION. 111 
stem, and finally to its last ramification, namely, the leaves? What 
is the route which the sap follows in ascending ? Does it traverse 
the pith, the bark, or the wood? or, finally, does it permeate through 
all three at the same time ? 
When a tree is cut down in spring it is easily seen that the sap flow- 
ing in it is then in the wood. If a plant is made to absorb coloured 
liquid, or if the branches are plunged into the 
same liquid, it is easily seen that it does not 
rise first either in the bark or pith. It is the 
wood or ligneous body through which it mani- 
festly takes its passage. This passage is effectu- 
ated through all the ligneous elements, cells, 
fibres, and vessels. The anatomical construc- 
tion of these vessels, their large number, their 
strength in the prostrate filiform and slender 
stems, which often attain a very considerable 
length, and which require to be traversed by a 
_ large quantity of sap in order to supply what 
is necessary for evaporation by the leaves,—all 
these general facts leave no doubt as to the 
part which the wood vessels play in the circu- _ 
lation of the sap. There is besides nothing 
easier than to ascertain directly the presence of 
sap in the interior of the wood. Such, then, 
we may conclude, is the true path followed by 
the ascending sap. 
Dr. Hales, an English physiologist, to whom 
science is indebted for numerous experiments 
throwing light upon the movement of the 
nourishing juices in plants,. was anxious to dis- 
cover the force with which the sap rose in the 
stem. In order to ascertain this, he fastened a EES 
bent tube on the top of an ascending branch of ¥s- shacoasing this boos a 
@vine-stem in the spring; having carefully fixed ate. > 
the tube upon the transverse section of the vine- stock, the lower bend 
ing filled with mercury. The flowing sap accumulated in the in- 
terior branches began little by little to move the mercury, till it rose 
to about forty inches in the tube (Fig. 136). The flow of sap then 
