ON TflE STEM OF THEE3» 337 



air to promote this purpose ? The thickness of these vessels 

 is such, as almost to conceal the darkness of the liquid 

 that runs through them. To see their forms well, it is some- 

 times necessary to clear out their contents, which is best 

 done by placing the specimens in a basket fastened down 

 in a ruuning stream, or boiling them thoroughly, and then 

 throwing them into green wax perfectly melted. "When 

 this succeeds, it makes excellent specimens for the 

 cabinet. 



Though half fearful to give an opinion absolutely con- 

 tradictory to oue whose abilities I so much respect as 

 MirbeVs, yet 1 must think he is mistaken, when he says : 

 11 y a des plantes qui ont les m£mes sues dans toutes leur par- 

 ties." I never could find this; and though the potent Liquids peculiar 

 smell of the liquid belonging to the bark will often extend l0 each part * 

 to other parts of the plant, it generally vanishes if kept 

 separate for a day, or grows so faint in comparison with 

 the real liquid, as to prove it is not an ingredient. Nor can 

 I understand why he should suppose, that the tubes or 

 cylinders of the inner bark are merely vacancies of the 

 ordinary vessels; for they are exactly the same, and occupy 

 the same place ; their peculiar shape and office attend them 

 every where ; and there are no vessels like them in any 

 other part of the tree or shrub. I hav€ mentioned only 

 the vessels of the inner bark, because their form is unusual; 

 but the vessels of the bark are more simple and smaller, 

 and divided by aline or two, running longitudinally between 

 them. How ihe gums, resins, oils, milk, &c. are formed^ 

 I am not chymist sufficient to give any clear idea concern- 

 ing; but the labours of Dr. Thomson seem more to eluci- 

 date this subject, than those of any other author 1 am 

 acquainted with. Nothing can be more admirable than the 

 manner in which he accounts for sugar in plants ; it is ex- 

 emplified each day in those that are out of health.. 

 Mirbel has also a very valuable paper on the subject. 



3d I now turn to the wood of the stem. This is marked Formation 

 by nature with such strong lines, it is hardly possible to and " se of th<5 

 mistake its parts. Place the stem of any plant in a colour- 

 ed liquor, and every vessel which conveys the sap from the 

 earth to the top of the tree will be marked and tinged. 



Vol. XXIII.— Supplement. 2 Th« 



