ON THE FLUIDS OF PLANTS. 403 



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 physical 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 thi3 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 



