PklNCIPLES OF VEGETATION. 345 



other plants, they are moft confpicuous toward the outer edge of the bark; 

 and in the dandelion they are difpofed in the form of concentric circles. 



The Wood of roots is that part which appears after the bark is 

 taken off, and is firmer and lefs porous than the bark or pith. It confifls 

 of two diftind fubftances j (i) the pulpy, or parenchymatous; (2) the lig- 

 neous. The wood is connedted to the bark by large portions of the 

 bark inferred into it, moftlyin the form of rays, tending to the centre 

 of the pith, eafily difcernible by the eye in a tranfverfe feiftion of moft 

 roots. Thefe infertions, like the bark, confift of many veflcls, moftly 

 round or oval. 



The ligneous veflcls are generally difpofed in collateral rows running 

 longitudinally through the root. Some of thefe contain air, others fap. 

 The air-veflels are diftinguifhed by being whiter. 



■ The Pith is the centrical part of the root. Some roots have no 

 pith, as the ftramonium, nicotiana, &c. ; others have little or none at the 

 extremities of the roots, but a confiderable quantity near the top. The 

 pith, like every other part of a plant, is derived from the feed ; but in 

 fome it is more immediately derived from the bark ; for the infertions 

 of the bark running in betwixt the rays of the wood, meet in the centre, 

 and conftitute the pith. It is owing to this circumftance, that among 

 roots which have no pith in their lower parts, they are amply provided 

 with it towards the top, as in columbine, lovage, &c. 



The bladders of the pith are of very different fizes, and generally of 

 a circular figure. Their pofition is more uniform than in the bark. Their 

 fides are not mere films, but a compofidon of fmall fibres or threads, 

 which gives the pith, when viewed with a microfcopCj the appearance 

 of a piece of fine gauze or net-work. 



We Ihall conclude the defcription of roots with obferving, that their 

 whole fubftance is nothing but a congeries of tubes and fibres, adapted 

 by nature for the abforption of nourifhment, and of courfe the extenfion 

 and augmentation of their parts. 



In defcribing the Trunks of plants, it is neceffary to premife, that 

 whatever is faid with regard to them applies equally to the branches. 



The trunk, like the root, confifts of three parts; the bark, wood, and 

 pith. Thefe, though fubftantially the fame in the trunk as in the root, 

 are often very different in texture and appearance, 



I, The fl^in of the bark is compofed of very minute bladders, inter- 

 .ipcrfed with longitudinal woody fibres, as in the nettle, thillle, and moft 



herbs. 



