308 CIRCULATION OF NUTRITIVE FLUID. 



escape of the sap from the wounded part. These conditions may be 

 attained in many Plants ; most conveniently, perhaps, in the stipules of 

 the Ficus elastica, one of the trees which affords the largest supplies of 

 Caoutchouc ; and it is then found that the movement takes place in 

 the following manner. Distinct currents are seen, passing along the 

 straightest and most continuous vessels, and crossing by the lateral 

 connecting branches of the network. These currents follow no deter- 

 minate direction ; some proceeding up, and others down ; some to the 

 left, and others to the right ; not unfrequently a complete stoppage is 

 seen in one or more of the channels, without any obvious obstruction ; 

 and the movement then recommences, perhaps in the opposite direction. 

 The influence of a force, developed by the act of circulation, which 

 determines the direction of the movement, appears from this : that if a 

 tube be cut off, so as to give its contents an equally free exit at both 

 ends, the sap only flows out at one extremity. The movement is 

 retarded by lowering the temperature of the surrounding air, and it is 

 completely checked by extreme cold ; it is capable of being renewed 

 by moderate warmth ; and a further addition of heat increases its 

 rapidity. By a strong electric shock, the force by which the liquid is 

 propelled seems to be altogether destroyed; for the movement then 

 ceases entirely. 



545. Now it is quite certain that this circulation cannot be due to 

 any vis a tergo ; both because it is not constant in its direction in par- 

 ticular vessels ; and because there is no organ in which any propelling 

 force, that could extend itself through such a complex system of vessels, 

 may be developed. Nor can it be in any way due to the force of gravity ; 

 for although this may assist the descent of the fluids through the stem, 

 it is totally opposed to its ascent from the ends of its branches towards 

 their origin, when, as often happens, the latter are at the higher level. 

 Moreover, it may be noticed that this circulation takes place most 

 readily, in parts that are undergoing a rapid development ; 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 

 f route, 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 subser- 

 vient ; 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. 



546. It is capable of being shown, by experiments on organic bodies, 

 that, if two liquids communicate with each other through a capillary 

 tube, for the walls of which they both have an affinity, but 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 energeti- 

 cally into the tube, and 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 ; for if this porous struc- 



