346 PRINCIPLES OF VEGETATION. 



herbs. The outfide of the ikin is vifibly porous in fome plants, parti- 

 cularly the cane. 



The principal body of the bark is compofed of pulp or parenchyma,, 

 and innumerable vefTels much larger than thofe of the fkin. The tex- 

 ture of the pulpy part, though the fame fubftance with the parenchyma 

 in roots, feldom appears in the form of rays running towards the pith i 

 and when thefe rays da appear, they do not extend above half way to 

 the circumference. The vefTels of the bark are very differently fituated, 

 and deftined for various purpofes in different plants. For example, in 

 the bark of the pine, the inmoft are lymph-dufts and exceedingly fmall ; 

 the outmoft are gum or refiniferous vefTels, deftined for the fecretion of 

 turpentine j and are fo large as to be diftindlly vifible to the naked eye. 



2. The Wood lies betwixt the bark and pith, and confifls of two 

 parts; (i) parenchymatous, (2) ligneous. In all trees, the parenchyma- 

 tous part of the wood, though much diverfified as to fize and confift- 

 ence, is uniformly difpofed in diametrical rayS, or infertions running 

 betwixt fim.ilar rays of the ligneous part. 



The true wood is nothing but a congeries of old dried lymph-dudls. 

 Between the bark and the wood a new ring of thefe is formed every year, 

 which gradually lofes its foftnefs as the cold feafon approaches, and, to- 

 wards the middle of winter, is condenfed into a folid ring of wood. Thefe 

 annual rings, which are diftindly vifible in moft trees when cut through, 

 ferve as natural m/arks to diftinguifh their age. The rings of one year 

 are fometimes larger, fometimes lefs, than thofe of another, probably 

 owing to the favourableneis or unfavourablenefs of the feafon. 



3. The Pith, though of a different texture, is exaftly of the fame 

 fubftance with the parenchyma of the bark, and the infertions of the 

 wood. The quantity of pith is various indifferent plants. Inftead of 

 being increaftd fvery year like the wood,, it is annually diminifhed, its 

 vefTels drying up and afTuming the appearance and ftrudure of wood ; 

 infomuch that in old trees there is fcarce fuch a thing as pith to be dii- 

 cerned. 



A ring of fap-veffels are ufually placed at the outer edge of the pith, 

 next the wood. In the pine, fig, and walnut, they are very large. The 

 parenchyma of the pith is compofed of fmjiU cells or bladders, of the 

 fame kind v.ith thofe of the bark, only of a larger fize. The general 

 figure of thefe bladders is circular; though in fome plants, as the thidle, 

 and borage, they are angular. Though the pith is originally one con- 

 ne(5led chain of bladders; yet, as the plant grows old, they Ihrivel, and 



open. 



