38 MOTION OF THE JUICES OF THE ANIMAL BODY. 



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 direction of the root. 



From his experiment he deduces the presence of a powerful attractive force resi- 

 ding in every part of the plant. 



We now know, that this attractive force, as such, did not cause the rise of the 

 mercury or water in his tubes, and it appears clearly from his experiments, that 

 the absorbent power of plants., of each leaf, of each fibre of the root, is sustained 

 by a powerful external force which is nothing else than the pressure of the atmos- 

 phere.* 



By the evaporation of water at the surface of plants, a vacuum arises within 

 them, in consequence of which water and matters soluble in water are driven 

 inwards and raised from without with facility, and this external pressure, along with 

 capillary attraction, is the chief cause of the motion and distribution of the juices.t 



With respect to the absorbent power of the plant for gases, under a certain exter- 

 nal pressure, his experiments offer the most beautiful evidence.;}: HALES says, in 

 his experiment XXII., " This height of the mercury did in some measure show 

 the force with w f hich the sap was imbibed, though not near 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 e r, 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 the 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 rise at all ; 

 because there would be no room for it in the tube. But if 9 parts of 12 in the 

 water be imbibed by the branch, and in the mean time, but three such parts of air 

 issue into the tube, then the mercury must needs rise near 6 inches, and so pro- 

 portionably in different cases." 



When, in his experiments, the root, the stem, or a twig had been injured at any 

 part, by the cutting off of buds, root-fibres, or small twigs, the absorbent power 

 of the remainder was diminished in a very obvious degree (because, from these 

 places, by the entrance of air the difference of air was more easily equalized ;) the 

 absorbent power was greatest on freshly-cut surfaces, on which, however, it gra- 

 dually decreased, till, after several days, it was not greater in these places than in 

 the uninjured surface of the plant. 



The evaporation, further, argues HALES, is the powerful cause which provides 

 food for the plant and its vicinity. Disease and death of the plant follow, when 

 the proportion between evaporation and supply is interrupted or destroyed in any 

 way. || 



When, in hot summers, the earth cannot supply, through the roots, the moisture 

 which during the day has evaporated through the leaves and surface of the tree, when 

 the tree, or a twig of it, dries up, the motion of the sap is arrested at these points. 

 When once dried, capillary action alone cannot restore the original activity ; the 

 evaporation is the chief condition of the life of plants ; by its means a permanent 

 motion, a continually repeated change in the quality of the. juice (sap) is effected. 



" By comparing," says HALES, *' the surface of the roots of plants, with the 

 surface of the same plant above ground, we see the necessity of cutting off many 

 branches from a transplanted tree : for if 256 square inches of root in surface was 

 necessary to maintain this cabbage in a healthy natural state; suppose upon dig- 

 ging it up, in order to transplant, half the roots be cut off (which is the case of 

 most young transplanted trees,) then it is plain that but half the usual nourishment 

 can be carried up, through the roots, on that account ; and a very much less pro- 



* The pressure rf the atmosphere is the active force. 



t A partial vacuum is caused within plants by evaporation. 



j The surface of plants absorbs gases. 



$ The absorbent power diminished by injury to the plant. 



|| Evaporation provides food for the plant. 



