430 ^f' ^^^^ on the Phyfiology of Plants. 



be concluded, that fometimes the quantity per* 

 fpired exceeds, fometimes it equals, and fome* 

 times it is lefs than, the quantity abforbed. 



Plants are poflefled of a power of forming 

 their different parts, and this is done by fecre- 

 tion. We may conjedure what the agents arc 

 ■which produce this efFed, but in refped to the 

 manner of their operation, we are entirely in the 

 dark. In animals, where the vital power is 

 ftrong, this is the principal agent in producing 

 the new arrangement of parts, vvhich is made in 

 every fecretion ; but in plants, where this 

 power is weaker, it would be unequal to per- 

 form the function, if it were not afTifted by ab- 

 forption and fermentation. Wherever any firm 

 matter is to be fecreted, the veflels have a con- 

 voluted courfe, to allow the juice to be ferment- 

 ed, and the thinner parts to be abforbed. In 

 this manner, the ftones and kernels of fruits are 

 fupplied with nourifhment by fibres, which arc 

 much convoluted. The proper juice feems to 

 be formed only when the fap has afcended to- 

 wards the leaves, and is defcending to the roots. 

 The wood alfo is formed during the defcent of 

 the fap ; for when a ligature is made round the 

 ftem of a tree, the wood above the ligature be- 

 comes much thicker, while that below, remains 

 of its former fize. 



The pabula, from which vegetables receive 

 the matter of fecretion, are contained in the 



furround- 



