PLANTS. 
11 
another, and extending in a horizontal line from 
the outward bark across the other two, and the wood, 
and so to the pith itself: these vessels are generally- 
filled with sap. Beside the fibres that ascend from the 
root, and constitute the wood and bark, there are 
other vessels disposed in the same manner, and 
ranged along the fibres at proper intervals of distance, 
through the trunk and branches of the wood ; these 
form the air vents, and the vessels properly so called. 
They are a set of tubes, composed of fibres revolving 
in a spiral line, and in one part extended in little 
ramifications to the external air ; in the other con- 
tinued and enlarged to the very root. These vessels 
are always empty. 
The Sap. 
The consideration of the vessels which are ap- 
pointed for the conveyance of nourishment to the 
different parts of the plant, naturally leads us to the 
circulation of the sap, about which such contradic- 
tory opinions have been maintained. Some suppose 
that the vegetable sap circulates in the plant in the 
same manner that blood does in animals; while others 
affirm that it only ascends in the day-time, and de- 
scends again in the night : these different opinions 
have both had their advocates; but as the most pow- 
erful reasoning seems to be against the regular cir- 
culation of the sap, we shall confine ourselves to 
that side of the question, and relate some few of the 
arguments which Dr. Hales has made use of to esta- 
blish the point. 
