Chap, i.] Air fa/els cf Plants. 5 



gradually dies away as they approach to maturity ; an4 

 in old trees it is almofl entirely obliterated. 



Such are the folid parts of plants; but to render 

 their organization more clearly underftood, in plate I. 

 fig. i. is the fection of a branch of afh, cut tranf- 

 verfely as it appears to the eye. Fig. 2. is the fame 

 feftion magnified. A. A. the bark. B. B. B. an 

 arched ring of fap-veffels next the cuticle. C.C.C. the 

 cellular fubftance of the bark, with another arched 

 row of fap-vc-ffels. D. D. a circular line of lymph- 

 duels next the wood. E. fL the wood. F. the firft 

 year's growth. G. the fecond. H. the third. 

 1. 1. 1. the true wood. K. K. the great air-veffels. 

 L. L. the lefTer air-veiTels. M. M. M. parenchyma- 

 tous infertions of the bark, reprefe.nted by white rays. 

 N. O. the pith. 



The name of air-veffels, as was before remarked, 

 has been given to certain tubes fituated in the wood, 

 leaves, and petals, but not in the bark of trees. They 

 are formed by a number of fmall filaments, fpirally 

 rolled up fo as to form a cavity in the middle, and are 

 fuppofed to be the inftruments of refpiration in plants j 

 but how this function is performed, is not clearly un- 

 derftood. Trees and fhruhs only are poffeffed of air- 

 veffels ; and when a plant is placed under the exhauftecl 

 receiver of an air-pump, the air only iffues from the 

 wood, in which the air-veffels are fituated. 



There is rcafon to believe that the air's proper en- 

 trance to plants is through the cuticle, which is proved 

 to be a vafcular fubftance, fince, when under an ex- 

 haufted receiver, it iffues dire&ly through the cuticle. 

 That the air is neceffary to the fuftenance of plants, 

 appears from the experiments of Dr. Bell *. In the 



* See his excellent Thefis on the Phyfiology of Plants, Manch. 

 Mem. vol. ii. 



B 3 winter 



