MOTION OF THE SAP IN PLANTS. 81 



HALES shewed by special experiments, that the 

 absorbent force which he pointed out in the root is 

 found also in the stem, in each separate twig, each 

 leaf, and every part of the surface; and that -the 

 motion of the sap continues from the root towards 

 the twigs and leaves, even when the stem has been 

 entirely stripped of bark, inner and outer. This 

 force acts not only from the roots in the direction of 

 the summit, but also from the summit in the direc- 

 tion of the root. 



From his experiments he deduces the presence 

 of a powerful attractive force, residing in every part 

 of the plant. 



We now know, that this attractive force, as such, The P res - 



' sure of the 



did not cause the rise of the mercury or water in atmosphere 



is the active 



his tubes, and it appears clearly from his experi- force. 

 ments, that the absorbent power of plants, of each 

 leaf, of each fibre of the root, is sustained by a f 

 powerful external force, which is nothing else than 

 the pressure of the atmosphere. 



By the evaporation of water at the surface of A partial 



vacuum is 



plants, a vacuum arises within them, in consequence caused 



i within 



of which water and matters soluble in water are plants by 

 driven inwards and raised from without with facility, S. " 

 and this external pressure, along with capillary 

 attraction, is the chief cause of the motion and 

 distribution of the juices. 



With respect to the absorbent power of the sur- The surface 

 face of the plant for gases, under a certain external absorbs 

 pressure, his experiments offer the most beautiful 8 



