ON THE FLUIDS OF PLANTS. 
408 
The descent of the proper juice of the plant is also another motion of these 
fluids which is constantly going on, and by which the peculiar secretions of 
plants are deposited in their various organs. 
These, then, are the motions of the fluids of plants that so many theories have 
been furnished to explain; and looking at their variety we can hardly suppose 
that the operation of mere physical causes is capable of explaining them. Yet 
many physical theories have been adopted by eminent botanists. 
Capillary attraction was one of the earliest opinions entertained as the cause 
of these motions. There are, however, many objections in the way of this 
theory. First, plants do not absorb moisture after vitality is extinct, which 
ought to be the case if its absorption depended on molecular affinities. Secondly, 
trees would be filled with the greatest quantity of sap during the winter, when 
no evaporation takes place, which is not the case. Thirdly, heat does not pro¬ 
mote the flow of sap, as may be seen by dipping a branch of Spurge flowing 
with sap into hot water. The flow of sap in the spring of the year, arises from 
the expansion of the leaf-buds, and not from the accession of heat. Fourthly, 
the cause of the descent of the sap would be left quite unexplained by the theory 
of capillary attraction. 
Amongst other ph 3 rsical theories, the following may be mentioned :— Malpighi 
supposed that it depended on the contraction and dilatation of the air enclosed in 
the air-vessels. Borelli attributed it to the condensation and rarefaction of the 
air and fluids of the plant. Du Hamel, who was supported by Linnaeus, 
accounted for it by the agency of heat. Hales demonstrated the insufficiency 
of this principle in accounting for the phenomena of the progression of the sap, 
but did not adopt a theory less objectionable, having embraced the same views 
as Malpighi. Du Petit Thouars attributes the ascent of the sap to the ex¬ 
pansion of the leaves in spring, but this is not referring the phenomenon to a 
distinct principle. If the motion given to the sap by the commencement of the 
function of the leaf be really the origin of this motion, it would appear to depend 
upon atmospheric pressure, fresh particles of sap rising to fill the place of the 
particles consumed by the requirements of the leaf. 
Other physiologists have explained the motions of the sap by referring them to 
vital actions. Brugmanns, Coulon, Saussure, and De Candolle adopted the 
theory of vegetable irritability. The vessels in which the sap was contained 
were supposed to be susceptible of the action of stimuli, and the contact of the 
sap with the tissues of the plant was considered to act as a stimulus, and to pro^- 
duce a contraction and dilatation of the vessels by which the sap was propelled. 
Knight adopted the theory of contraction and dilatation, but he supposed this 
phenomenon took place not in the sap-vessels but in the silver grain or medullary 
