338 APPENDIX 0, 



of the root, and connected the remaining stump with a glass tube 

 filled with water and confined by mercury. This glass tube rep- 

 resents the root lengthened. 



By the perspiration from the surface of the tree, the root im- 

 bibed the water in the tube with such vigor that in six minutes 

 the mercury had risen in the tube as high as 8 inches, which cor- 

 responds to a column of water 9 feet in height. 



This force is very nearly equal to that with which the blood 

 moves in the great crural artery of a horse. ' I found,' says Hales, 

 in his thirty-sixth experiment, ' the force of the blood of several 

 animals, by tying them down alive upon their backs, then laying 

 open the great crural artery where it first enters the thigh, and 

 fixing to it, by means of two brass pipes running one into the 

 other, a glass tube above ten feet long and one-eighth of an inch 

 in diameter. In this tube the blood of one horse rose eight feet 

 three inches, and the blood of another horse eight feet nine inches ; 

 the blood of a little dog, six feet and a half.' 



Hales proved by special experiments^ that the force of suction 

 shown by him to be possessed by the roots of plants, is exercised 

 equally by every individual branch, shoot, leaf, and fruit, in short, 

 by every portion of the surface ; that the motion of the sap from 

 the root to the branches and leaves continues even when the 

 trunk is, in any part, completely stripped of the outer and inner 

 bark, and that this force of suction acts not only from the roots 

 towards the top, but also from the latter towards the roots. 



He concludes, from the results of his experiments, that every 

 part of the plant is endowed with a powerful force of attraction. 



"We know now that it was not this force of attraction in itself 

 that made the mercury and the water rise in Hales' tubes ; and 

 his experiments clearly show, that the imbibing force of plants, 

 and of every leaf and root fibre, arising from surface exhalation, is 

 aided by a powerful force from without, which is simply atmo- 

 spheric pressure. 



By the evaporation of the water from the surface of plants a vacu- 

 um is created therein, and in consequence thereof water and gases 

 soluble in that fluid are readily forced in from without and raised 

 by the pressure of the atmosphere, and it is this pressure from 

 without which, together with capillary attraction, constitutes the 

 principal cause of the motion and diffusion of the sap. 



That the surface of plants possesses the faculty of imbibing 

 gases, is most conclusively demonstrated by Hales. In his twenty- 

 second experiment he says : ' The height to which the mercury 

 rose in the tube did in some measure show the force with which 

 the sap was imbibed, though not nearly the whole force ; for while 

 the water was imbibing, the transverse cut of the branch was cover- 

 ed with innumerable little hemispheres of air, and many air-bubbles 

 issued out of the sap-vessels, which air did in part fill the tube as 

 the water was drawn out of it ; so that the height of the mercury 



