OF NATURAL HISTORY. 30 



branches deprived of leaves, and that they do not rife at all 

 in dried plants. Hence the fap of vegetables is not taken 

 up in the fame manner as a fpunge imbibes water, but is for- 

 ced to afcend by an unknown a£lion of the veiTels. The 

 fpring of the tracheae may put in motion the air they con- 

 tain, and that air may have fome influence on the general 

 movement. But, by whatever powers the fap is moved, the 

 exiftence of the motion is certain ; and it is equally certain, 

 that this movement of the fap produces the fame efFe^s 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 cir- 

 culation fimilar to that of the blood in the more perfect ani- 

 mals. It afcends and defcends in the fame veffels ; and thefe 

 motions are evidently affected 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 ta- 

 jenia, the polypus, and many other infedls, which exhibit not 

 the fmalleft veftiges of circulation in their juices. 



The pith, or medullary fubftance of plants, has fome re- 

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

 the texture of the brain or fpinal-marrow is deftroyed, life is 

 extinguiflied ; and, when the pith of plants is deftroyed or 

 dried up by age, they no longer retain the power of vegeta- 

 ting. The leaves of plants are analogous to the lungs of an- 

 imals. It is by the lungs that the perfpiration of animals is 

 chiefly effected : and plants difcharge moft of their fuperflu- 

 ous moifture by the leaves. They expofe a large furface to 

 the a<5lion of the fun, which produces a tranfpiration fo copi- 

 ous, 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 



