402 
PHYSIOLOGY. 
BOOK II. 
young uniformly falls off; while, on the contrary, if not 
allowed to set until the stem is well formed, and much sap 
accumulated for its support, it swells rapidly, and ripens 
without experiencing any deficiency of food in the course of 
its growth. In like manner, if a fruit tree is by any circum- 
stance prevented bearing its crop one year, the sap that would 
have been expended accumulates, and powerfully contributes 
to the abundance and perfection of the fruit of the succeeding 
year. 
The cause of the motion of the sap is a subject which has 
greatly excited curiosity, and given rise to numberless con- 
jectures. It \yas for a long time believed that there was a 
sort of circulation of the sap of plants, to and from a common 
point, analogous to that of the blood of animals ; but this was 
rendered improbable by the well-known fact that a plant is 
more analogous to a polype than to a simple animal ; that it 
is a congeries of vital systems, acting indeed in concert, but to 
a certain degree independent of each other, and that con- 
sequently it has myriads of seats of life. It was, moreover, 
experimentally disproved by Hales. This excellent observer, 
whose Statics are an eternal monument of his industry and 
skill, thought that the motion of the sap, the rapidity of 
which he had found to be greatly influenced by weather, de- 
pended upon the contraction and expansion of the air, which 
exists in great quantities in the interior of plants. Others 
have ascribed the motion to capillary attraction. Knight was 
once of opinion that it depended upon a hygrometrical pro- 
perty of the plates of silver grain (medullary rays), which 
traverse the stem in all directions. The same physiologist 
considered that the mechanical agitation of stems and branches 
by wind was favourable to the motion : he confined the stem 
of a tree, so that it could vibrate only in one plane ; and, 
at the end of some years, he observed that its section was an 
ellipse, whose greater axis lay in this plane. Other theorists 
have called to their aid a supposed irritability of the vessels; 
but no contraction of the vessels has ever yet been noticed, 
except under the influence of frost, as shown by Biot. Du 
Petit Thouars suggests that it arises thus : — In the spring, 
as soon as vegetation commences, the extremities of the 
