4 fbff Stem, Wood, Pith, fcfr. [Book VIIL 



ftrufture more difficult ro be underftocd. It is how- 

 ever generally fuppofed to confift of two fubftances, the 

 parenchymatous or cellular, and the ligneous. The 

 ligneous parts are no more than a congeries of old, 

 dried lymp-ducts. Between ' the bark and the wood 

 a new ring of thefe du6ls is formed every year, which 

 gradually lofes its ibftnefs as the cold fea-fon approaches, 

 and towards the middle of winter is condenfed into a 

 folid ring of wood. Thefe annual rings, which are 

 vifible in moft trees when cut tranfverfely, ferve as 

 marks to determine their age. They feem to decreafe 

 in breadth, as the tree advances in age; and as they 

 are found to be very unequal in fize throughout, their 

 breadth probably varies according as the feafon is fa- 

 vourable or otherwife. 



The wood differs from the bark, not merely in the 

 degree of hardnefs ; its ftructure is efientially different, 

 and the apparent converfion of bark into wood is en- 

 tirely a deception. One ftriking difference between 

 the wood and the bark is, that the former is poffeffed 

 of fpiral veiTels which run from one end of the tree to" 

 the other. From the great refemblance of thefe air- 

 vefiels to thofe of infects, they are fuppofed to be fub- 

 fervient to the fame function. The item of fome 

 plants is entirely hollow, partly, it is fuppofed, from 

 thefe plants, which are generally of a quick growth, 

 requiring a more thari ordinary fupply of air. 



3d. The pith is fuuated in the center of the ilem, 

 and in young plants it is very abundant. It is faid by 

 fome authors to confift of exactly the famefubftanceas 

 the parenchyma or cellular fubftance of the bark j and 

 to be compofed of fmall cells or bladders, generally of 

 a circular figure, though in fome plants, as the oorage 

 and thiftle, they are angular. In moft plants the pith 

 i gra- 



