Z4^ PRINCIPLES OF V E G E T A T I O N. 



and umbelliferous plants. Thefe rays generally ftand at an equal dlf- 

 tance from each other in the fame ^lant j but the diftance varies greatly 

 in different plants. Neither are they of equal fizes : in carrot they arc 

 exceedingly fmall and fcarcely difcerniblej in melilot and cherval they are 

 thicker. They are likewife more numerous in fome plants than in others. 

 Sometimes they are of the fame thicknefs from one edge of the bark to 

 the other; and fome grow wider as they approach toward the Ikin. 

 The veifels with which thefe rays are amply furnifhed are fuppofed to be 

 air-veffels, becaufe they are always found to be dry, and not fo tranfpa- 

 rent as the veffels which evidently contain the fap. 



In all roots there are ligneous veffels difperfed in different proportions 

 through theparenchymaot the bark. Thefe run longitudinally through the 

 barkinthe form of fmall threads, tubular,as is evidentfrom the riling of the 

 fap in them when a root is cut trafnverfely, Thefe ligneous fap-veffels do 

 not run in direft lines through the bark, but, at fmall diftances, incline 

 towards one another in iuch a manner, that they appear to the naked eye 

 to be inofculated ; but the microfcope difcovers them to be only conti- 

 guous, and braced together by the parenchyma. Thefe braces are very 

 various both in fize and number in different roots ; but in all plants they 

 are mod numerous toward the inner edge of the bark. Neither are 

 thefe veffels fingle tubes, but, like the nerves in animals, are bundles of 

 twenty or thirty fmall contiguous cylindrical tubes, which uniformly run 

 from the extremity of the root, without fending off any branches, or fuf- 

 fering any change in their fize or Ihape. 



In fome roots, as parfnips, efpecially in the ring next the inner extre- 

 mity of the bark, thefe veffels contain a kind of lymph, which is fweeter 

 than the fap contained in the bladders of the parenchyma. From this 

 circumftance they have got the name of Iyntfh-du5is, 



Thefe lymph-duels fometimes yield a mucilaginous lymph, as in the 

 comphrey ; and fometimes a white milky glutinous lymph, as in the an- 

 gelica, fonchus, burdock, fcorzonera, dandelion, &c. The lymph-dufts 

 are iuppofed to be the veffels from which the gums and balfams are fe- 

 creted. The lymph of fennel, when cxppfed to the air, turns into a clear 

 tranfparent balfam ; and that of the fcorzonera, dandelion, &c. condenfes 

 into a gum. 



1 he fituation of the veffels Is various. In fome plants they ftand in 

 a ring or circle at the inner edge of the bark> as in afparagus ; in others 

 they appear in lines or rays, as in borage i in the^ parfnip, and feveral 



other 



