Dr. Bell on the PhyfioUgy of Plants. 4 1 9 



in vegetables, named air-vefTels, or va/a aeria. 

 Thcfe are chiefly fituated in the wood, leaves, 

 and petals J but are wanting in the bark of 

 trees, and in the herbaceous plants. They 

 are formed by a mjmber of fmall filaments, 

 fpirally rolled up, fo as to form a cavity in 

 the middle. The name of va/a aeriu has been 

 given them, becaufe they are empty of liquor, 

 and becaufe a great quantity of air is certainly 

 found in the wood of plants, where thefe veffds 

 are chiefly placed, and where there is no peculiar 

 organization. They are fuppofed to be the 

 inftruments of refpiiation in vegetables j but 

 in what manner this fundion is performed, is 

 not clearly underfl:ood. 



I. Some imagine that the air enters the plants 

 by the roots, in a non-elaftic ftate, and gradually 

 recovers its elafticity in its paflage through 

 them. 1 o this opinion it is objeded. i. Th^at 

 a great number of va/a aeria is found in the 

 roots of trees, where the juice has undergone 

 little or no circulation, and where of courfe 

 little or no air can be fuppofed to be evolved. 

 2. That the roots are very incommodioufly 

 placed for abforbing air, being generally fo 

 deeply buried in the earth, as to be entirely out 

 of Its reach. 



II. Others fuppofe the air is abforbed by the 

 Jeaves, and thence carried into the body of the 

 plant. There arc indeed many air-velTels in the 



iL t 2 leaves. 



