CHAP. XXVIII.] THE CAPILLARY FORCE. 371 



capillary system, whilst its existence in greater power at one point 

 than at another, would cause a greater afflux of blood in the former 

 than in the latter direction. 



The best illustration of the exercise of this force, which, for the 

 sake of brevity, may be designated the capillary force,* is found 

 in the circulation of the sap in plants. It is exercised in two 

 situations at the roots and in the leaves, constituting in the one 

 & vis a tergo, and in the other a vis a f route. At the roots, a 

 rapid imbibition of fluid takes place with such energy, that it 

 pushes before it the fluid above ; thus if the stem of a vine, in 

 which the sap is rising, be cut across, a bladder tied over it, will, 

 after a short time, be burst by the fluid accumulated beneath it ; 

 or if a bent tube, containing a column of mercury, be affixed to it, 

 the mercury will be raised to the height of forty inches or more. 

 And that a force of attraction is exercised at the leaves, may be 

 shewn by placing the lower end of the upper division of the cut 

 vine in water, when an active absorption and circulation of water 

 will take place as long as the vital changes in the leaves go on ; 

 but if the vine be taken into a dark room, so as to check these 

 vital changes, the absorption and circulation will likewise cease. 

 So also the elaborated sap or latex, which, from its containing 

 the elements for the nutrition and for the various secretions of 

 the plant, may be likened to the arterial blood of animals, circu- 

 lates through a complex system of anastomosing vessels (like 

 the capillaries of animals), in the under surface of the leaves and 

 in the bark, and will ascend towards the stem, even against 

 gravity, in a dependent branch. The circulation of these fluids 

 takes place with the greatest activity in growing parts, in which 

 nutrient and chemical changes are going on most actively. 



Professor Draper, of New York,f has given a definite expression 

 to the nature of the forces which operate in the production of the 

 circulation of the sap in plants, and in that of the blood in 

 animals. The laws of endosmose and exosmose resolve themselves 

 into the following dogma: " That if two liquids communicate 

 with one another in a capillary tube, or in a porous or parenchy- 

 matous structure, and have for that tube or structure different 

 chemical affinities, movement will ensue ; that liquid, which has 



* The name Capillary force, which was given by Dr. Carpenter, must be 

 taken as merely denoting that the force is exerted at the Capillaries, whether 

 it be exercised by their walls or by a mutual action between the blood within 

 and the tissues outside them. 



t On the Forces which produce the Organization of Plants. 1845. 



