544 OF THE CIRCULATION OF BLOOD. 



stances, with considerable rapidity ; it is accelerated by heat, and retarded by 

 cold; and it is subject to all those minor irregularities (such as the cessation 

 of movement, or change in the direction of the current, in a particular chan- 

 nel), which are so constantly to be noticed by any one who attentively watches 

 the capillary circulation of Animals, and which clearly prove the operation of 

 some causes independent of the heart's action ( 734). The general direction 

 of the elaborated sap, through this capillary system, is downwards ; but that 

 the force of gravity cannot have much to do with the movement, is shown by 

 the fact that, in dependent branches, it has to ascend towards the stem, which 

 it will do without interruption from this cause. Moreover it may be noticed 

 that this circulation takes place most actively, in parts which are undergoing a 

 rapid developement; and that its energy corresponds with the vitality of the 

 part. Further; it may be observed to continue for some time in parts that 

 have been completely detached from the rest; and on which neither vis a 

 tergo, nor vis a fronte, can have any influence. It is evident, then, that the 

 force, whatever be its nature, by which this continued movement is kept 

 up, must be developed by the processes to which that movement is subserv- 

 ient ; in other words, that the changes involved in the acts of nutrition and 

 secretion are the real source of the motor power. The manner in which they 

 become so, is the next object of our inquiry: and on this subject, some new 

 views have recently been put forth by Prof. Draper,* which seem to account 

 well for the phenomena. 



a. It is capable of being shown, by experiments on inorganic bodies, that, if two liquids 

 communicate with each other through a capillary tube, ibr the walls of which they both 

 have an affinity, and if this affinity is stronger in the one liquid than in the other, a 

 movement will ensue; the liquid which has the greatest affinity being absorbed most ener- 

 getically into the tube, ami driving the other before it. The same result occurs when the 

 fluid is drawn, not into a single tube, but into a net-work of tubes, permeating a solid 

 structure; ibr if this porous structure 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 the latter is permitted to outer it. Now if, in its passage through 

 the porous solid, the liquid undergo such a change, that its affinity be diminished, it is ob- 

 vious that, according to the principle just explained, it must be driven out by a fresh supply 

 of the original liquid; and lhat thus a continual movement in the same direction would be 

 produced. 



l>. Now this is precisely that which seems to take place in the organized tissue, per- 

 meated by nutritious fluid. The particles of this fluid, and the solid matter through which 

 it is distributed, have a certain affinity Ibr each other; which is exercised in the nutritive 

 changes, to which the fluid becomes subservient during the course of its circulation. Cer- 

 tain matters are drawn from it. in one part, for the support and increase of the woody tis- 

 sue; in another part, the secreting cells demand the materials which are requisite for their 

 growth, as starch, oil, resin, &c.; and thus in every portion that is traversed by the vessels^ 

 there are certain affinities between the solids and the fluids, which are continually bem- 

 newly developed by acts of growth, as fast as those which previously existed are satisfied 

 or neutrali/ed by the changes that have already occurred. Thus in the circulation nf the 

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

 and a continual series nf changes produced in the fluid as the result of that attraction. The 

 flnni, 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 

 possessed by the li-sue 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 

 coiistantly-rencu-ed attraction for the nutritive fluid, which has not yet traversed it; whilst 

 on the other hand, there is a diminished attraction lor the fluid, which has yielded up the 

 nutritive materials required by the particular tissues of the part ; and thus the former is con- 

 tinually driving the latter beliire it. 



c. But the fluid, which is thus icpclled from one part, may still be attracted towards ano- 

 ther; 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 that the flow of sap is maintained 



On the Forces which produce the Organization of Plants, Chap. in. 



