22 THE PHILOSOPHY 



fap is moved, the exiftence of the motion is certain ; and It is equal- 

 ly certain, that this movement of the fap produces the fame effefts 

 in the vegetable, that the force of the heart and arteries does in the 

 animal. 



The motion of the fap, in vegetables, is not properly a circula- 

 tion fimilar to that of the blood in the more perfect animals. It 

 afcends and defcends in the fame veffels ; and thefe motions are evi- 

 dently affeded by heat and cold. The fap rifes copioufly in a warm 

 day, and defcends during the night, nearly in the fame manner as 

 the mercury rifes and falls in the thermometer. But, though the 

 analogy here fails with regard to man and the larger animals, yet 

 it holds in the taenia, the polypus, and many other infeds, which 

 exhibit not the fmallell veftiges of circulation in their juices. 



The pith, or medullary fubftance of plants, has fome refemblance 

 to the brain and fpinal-marrow of animals. When the texture of 

 the brain or fpinal-marrow is deftroyed, life is extinguifhed ; and, 

 when the pith of plants is deftroyed or dried up by age, they no 

 longer retain the power of vegetating. The leaves of plants are 

 analogous to the lungs of animals. It is by the lungs that the per- 

 fpiration of animals is chiefly effeded ; and plants difcharge moft of 

 their fuperfluous moifture by the leaves. They espofe a large fur- 

 face to the aftion of the fun, which produces a tranfpiration fo co- 

 pious, that fome plants throw out fifteen or twenty times more in a 

 given period, than is difcharged from the human body. When a 

 plant is deprived of its leaves in fummer, inftead of ripening its fruit, 

 it is in great danger of dying for want of thofe organs which carry 

 oflF the fuperfluous juices that arife from the root. A plant, in this 

 fituation, may be confidered as labouring under an afthma, or dy- 

 ing of a fuffocation. 



•5 Belide 



