360 APPENDIX C. 



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 atmospheric pressure. 



By the evaporation of the water from the surface of plants 

 a vacuum 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 pres- 

 sure from without which, together with capillary attraction, con- 

 stitutes 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 

 covered 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 could only be proportionable' to the 

 excess of the quantity of water drawn off, above the quantity 

 of air which issued out of the wood. 



' And if the quantity of air, which issued from the wood 

 into the tube, had been equal to the quantity of water imbibed, 

 then the mercury would not have risen at all, because there 

 would have been no room for it in the tube. 



