CIRCULATION IN PLANTS AND LOWER ANIMALS. 309 



ture be previously saturated with the fluid, for which it has the less 

 degree^ of attraction, this will be driven out and replaced by that for 

 which it has the greater affinity, when it is permitted to absorb this. 

 Now if, in its passage through the porous solid, the liquid undergo such 

 a change, that its affinity is diminished, it is obvious that, according to 

 the principle just explained, it must be driven out by a fresh supply of 

 the original liquid, and that thus a continual movement in the same 

 direction would be produced. 



547. Now this is precisely that which seems to take place in the 

 organized tissue, permeated by nutritious fluid. The particles of this 

 fluid, and the solid matter through which it is distributed, have a certain 

 affinity for each other ; which is exercised in the nutritive changes, to 

 which the fluid becomes subservient during the course of its circulation. 

 Certain matters are drawn from it, in one part, for the support and 

 increase of the woody tissue ; in another part, the secreting cells demand 

 the materials which are requisite for their growth, as starch, oil, resin, 

 &c. ; and thus in every part that is traversed by the vessels, there are 

 certain affinities between the solids and the fluids, which are continually 

 being developed afresh by acts of growth, as fast as those which pre- 

 viously existed are satisfied or neutralized by the changes that have 

 already occurred. Thus in the circulation of the elaborated sap, there 

 is a constant attraction of its particles towards the walls of the vessels, 

 and a continual series of changes produced in the fluid, as the result of 

 that attraction. The fluid, which has given up to a certain tissue some 

 of its materials, no longer has the same attraction for that tissue ; and 

 it is consequently driven from it by the superior attraction then pos- 

 sessed by the tissue for another portion of the fluid, which is ready to 

 undergo the same changes, to be in its turn rejected for a fresh supply. 

 Thus in a growing part, there is a constantly-renewed attraction for the 

 nutritive fluid, which has not yet traversed it ; whilst, on the other hand, 

 there is a diminished attraction for the fluid, which has yielded up the 

 nutritive materials required by the particular tissues of the part ; and 

 thus the former is continually driving the latter before it. 



548. But the fluid which is thus repelled from one part, may still be 

 attracted towards another ; because that portion of its contents which 

 the latter requires, may not yet have been removed from it. And in 

 this manner, it would seem, the flow of sap is maintained, through the 

 whole capillary network, until it is altogether exhausted of its nutritive 

 matter. The source of the movement is thus entirely to be looked for 

 in the changes which take place in the act of growth ; and the influence 

 of heat, cold, and other agents, upon the movement is exercised through 

 their power of accelerating or retarding those changes. The fluid which 

 thus descends through the stem and roots, seems to be at last almost en- 

 tirely exhausted ; a portion of it appears to find its way into the interior 

 of the stem, and to be mingled with the ascending current ; but all the 

 rest seems to have been entirely appropriated by the different tissues, 

 through which it has circulated. Thus there is no need of any general 

 receptacle, into which it may be collected, and from which it may take 

 a fresh departure ; such as is afforded by the heart of Animals. And 

 as the purpose of this circulation is only to supply the nutritive mate- 



