OF NATURAL HISTORY. 19 



Ikin, the body, and the liber, or inner circle ; which laft, about the 

 end of autumn, afTumes the fame texture and firmnefs with the 

 wood. The fubftance of the bark is compofed of a number of lon- 

 gitudinal fap and air veiTels, which have the appearance of fine 

 threads, running from the root to the trunk and branches. Befide 

 thefe veflels, the bark is furniflied with a parenchymatous or pulpy 

 fubftance, in which there are a vaft variety of folhcnliy or fmall 

 bladders. The bark is connected to the wood by tranfverfe infer- 

 tions of the parenchyma. 



The wood confifts of two diftin£l fubflances ; the one is denfe, 

 and compaft, and conftitutes what is termed the ligneous body ; the 

 other is porous, moift, and pulpy, and is therefore called the paren- 

 chymatous part of the wood. A portion of wood is placed alter- 

 nately between a fimilar portion of parenchyma. Thefe alternate 

 portions proceed from the edges of the pith, as radii from the cen- 

 ter of a circle, widening proportionally as they approach the cir- 

 cumference. Both of them, however, like the bark, are furniflied 

 with numberlefs fap and air veflels. 



The pith or heart is bounded on all fides by the wood, and is 

 compofed of the fame materials : It is nothing but a vaft congeries 

 of air and fap veflels, interwoven with the parenchyma and blad- 

 ders, not unlike the tilTue of gauze or lace. This texture is com- 

 mon to every part of the trunk, being only more clofe and compa£l 

 in the bark and wood than in the pith. It is well known, that the 

 pith of plants diminifties in proportion to their age. The reafon is 

 obvious : Every year the ring of veflels, which lies contiguous to 

 the wood, dries, condenfes, and becomes wood. 



The leaves of vegetables confift of a fine fkin, which inclofes the 

 parenchyma or pulp. This flcin, like that of animals, is an organic- 



C 2. body*,. 



