CHAP. XT. 
CIRCULATION OF THE SAP. 
331 
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 Thenars 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. Thom- 
son, gardener at Welbeck ; who, however, drew a wrong- 
inference from them. 
Dutrochet has formed a theory of all the motions of fluids 
in plants depending upon the agency of galvanism. He found 
that small bladders of animal and vegetable membrane, being 
filled with a fluid of greater density than water, securely 
fastened, and then thrown into water, acquired weight; he 
also remarked, that if the experiment was reversed, by filling 
with water and immersing them in a denser fluid, the con- 
trary took place, and that the bladders lost weight : he took 
a small bladder, and filled it with milk, or gum arable dis- 
solved in water; to the mouth of this bladder he adapted a 
tube, and then plunged the bladder in water : in a short time 
the milk rose in the tube, whence he inferred that water had 
been attracted through the sides of the bladder. This experi- 
ment was also reversed, by filling the bladder with water, and 
plunging it in milk : the fluid then fell in the tube, whence he 
inferred that water had been attracted through the coat of the 
bladder into the milk. From these, and other experiments, 
Dutrochet arrived at the inference, that if two fluids of un- 
equal density are separated by an animal or vegetable mem- 
brane, the denser will attract the less dense through the 
membrane that divides them : and this property he calls 
