CHAP. XIII. 
CIRCULATION OF THE SAP. 
403 
branches and the buds begin to swell : the instant this happens 
a certain quantity of sap is attracted out of the circumjacent 
tissue for the supply of those buds : the tissue, which is thus 
emptied of its sap, is filled instantly by that beneath or about 
it : this is in its turn replenished by the next ; and thus the 
whole mass of fluid is set in motion, from the extremities of 
the branches down to the roots. Du Petit Thouars is, there- 
fore, of opinion that the expansion of leaves is not the 
effect of the motion of the sap, but, on the contrary, is the 
cause of it ; and that the sap begins to move at the extremities 
of the branches before it stirs at the roots. That this is really 
the fact, is well known to foresters and all persons accus- 
tomed to the felling or examination of timber in the spring; 
and to gardeners who are occupied with forcing the branches 
of plants in winter, while their trunks are exposed to the 
weather.’ Some good observations upon this were commu- 
nicated to Loudon’s Gardener’s Magazine^ by Mr. Thomson, 
gardener at Welbeck ; who, however, drew a wrong inference 
from them. 
The following observation gives additional weight to the 
opinion of Du Petit Thouars : — 
M. Gaudichaud found, when in Brazil, that, upon cutting 
through one of the creeping Cissi (C. hydrophora), the sections 
only slightly discharged fluid when the upper part was merely 
divided from the under ; but that when a truncheon, of whatever 
length, was separated from the stem, the sap then ran out in 
great quantity from either end, according to which was held 
downwards, and that it only dropped out slowly when held in 
a horizontal position. Upon examining the next day the cut 
end of the lower part of this stem, it was found dry for 5 or 
6 inches below the wound. M. Gaudichaud ascribes the 
latter circumstance to the pressure of the atmosphere upon 
the orifices of the tubes ; and the absence of any considerable 
amount of bleeding in the upper half, to the power of suction 
in the leaves, &c. ; while he attributes the ready discharge of 
fluid from either end of the separated truncheon to atmo- 
spheric pressure, which, he supposes, operates upon the vessels 
of the Cissus, as it would upon inert tubes. {^Ann, Sc., n. s., 
vi. 142.) 
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