Page:Encyclopædia Britannica, first edition - Volume I, A-B.pdf/62

 A G R I C U L T U R E.‘ In all roots, there are lignous veffels dilperfed in different proportions through the parenchyma of the bark. S e c t. II. Of the Root. Thefe lignous vefiels run longitudinally through the form of fmall threads, which are tubular, as In examining the root of plants, the firft thing that bark in thefrom the rifing of the fap in them when a root prefents itfelf is the flcin, which is of various colours in is evident tranfverfely. Thefe lignous fap-veffels do not rundifferent plants. Every root, after it has arrived at a. isin cut certain age, has a double Ikin. The firft is coeval with dired lines through the bark, but, at fmall diftances, the other parts, and exifts in the feed: but afterwards incline towards one another in fiich a manner, that they there is a ring fent off from the bark, and forms a fe- appear to the naked eye to be inofculated; but the micond'lkin; e g. in the root of the dandelion, towards crofcope difeovers them to be only contiguous, and brathe end of May, the original or outer (kin appears thri- ced together by the parenchyma. Thefe braces -or coveted, and is eafily feparated from the new one, which ardations are very various both in fize and number in but in all plants they are moft numeis frefher, and adheres more firmly to the bark. Peren- different-roots; nial plants are fupplied in this manner with a new fkin rous towards the inner edge of the bark. Neither are veffels Angle tubes, but, like the nerves in animals, every year; the outer one always falls off in the autumn thefe are bundles of twenty or thirty fmall contiguous cylinor winter, and a new one is formed from the bark in drical tubes, which, uniformly run from the extremity, the fucceedteg fpring. The Ikin has numerous cells of the root, without fending off any branches, or fufor veffels, and is a continuation of. the parenchymatous fering any change in their fize or ftiape. part of the radicle. However, it does not confift folcly In fome roots, as parfnip, efpecially in the ring next, of parenchyma; for the microfcOpe fhews that there are the inner extremity of the bark, thefe vefiels contain a many tubular lignous veffels interfperfed through it. lymph, which is fweeter than the fap contained When the fkm is removed, the true cortical fubftance inkindtheofbladders From this circumor bark appears, which is alfo a continuation of the pa- ftance they haveofgotthetheparenchyma. name of lymph-dufis. renchymatous part of the radicle, but greatly augmentThefe lymph-duds fometimes yield a mucilaginous, ed. The bark is of very different fizes. In molt trees, lymph, as in the comphrey; and fometimes a white milky it is exceeding thin in proportion to the wood and pith. On the other hand,"In carrots, it is almoft one half of the glutinous lymph, as in the angelica, fonchus, burdock, femidiameter of the root; and, in dandelion, it is near- fcorzonera, dandelion, ere. The lymph-duds are fuppofed to be the veflels from which the gums and bally twice as thick as the woody part. i. The Bark is compofed of two fubftances ; the pa* fams are fecerned. The lymph of fennil, when expofed renchyma, or pulp, which is the principal part; and a few, to the air, turns into a clear tranfparent balfam; and 'fcoody fibres. The parenchyma is exceedingly porous, that of the fcorzonera, dandelion, <bc. condenfes into a and has a great refemblance to a fpunge; for it Ihrivels gum. confiderably when dried, and dilates to its former dimen- The fituation of the veffels is various. In fome plants,’ fions when infufed in water. Thefe pores or veffels are they ftand in a ring or circle at the inner edge of the not pervious fo as to communicate with each other, but bark, as in afparagus ; in others, they appear in lines, confift of diftindt little cells or bladders, fcarcely vifible or rays, as in berage; in the parfnip, and feveral other without the afiiftance of the microfcope. In all roots, thefe plants, they are moft confpicuous toward the outer edge cells are conftantly filled with a thin watery liquor. They of the bark; and in the dandelion, they are difpofed are generally of a fpherical figure; though in fome roots, in the form of concentric circles. as die buglofs and dandelion, they are oblong. In many 2. The Wood of roots is that part which appears roots, as the horfe-raddilh, peony, afparagus, potatoe, <fac, after the bark is taken off, and is firmer and lefs porous the parenchyma is of one uniform ftrufture. But in o- than the bark or pith. It confifts of two diftind fubthers it is more diverfified, and puts on the fhape of rays ftances, viz. the pulpy, or parenchymatous, and the running from the centre towards the circumference of lignous. The wood is conneded to the bark by laVge the bark. Thefe rays fometimes run quite through the portions of the bark inferred into it. Thefe infertions bark, as in loyage ; and Ibiwetimes advance towards the are moftly in the form of rays, tending to the centre of middle of it, as in melilot and moft of the Leguminous and the pith,, which are eafily difcernible by the eye in a tranfumbelliferous plants. Thefe rays generally ftand at an verfe fedion of moft roots. Thefe infertions, like the equal diftance from each other in the fame plant; but bark, confift of many veffels, moftly of a round or, oval the diftance'varies greatly in different plants. Neither are figure. , t hey of equal fizes : In carrot they are exceedingly fmall, The lignous veffels are generally difpofed in collateraland fcarcely difcernible ; in melilot and cherval, they rows running longitudinally through the root. Some of aie thicker. They are likewife more numerous in fome thefe contain air, and others fap. The a:r-veffels are lo plants than in' others. Sometimes they are of"the fame called, becaufe they contain.no liquor. 7’^tfe air-vefthjeknefs from one edge of the bark to the other; and fels are diftinguiftied by being whiter than the others. fonie grow wifer as they approach towards the Ikin. Thn 3. The Pith is the centrical part of the root. Some veffels with which thefe rays are amply furnilhed, are roots have no pith, as the ftramonium, nicotiana, fuppofed to be air-veffels, becaufe they are always found others have little or none at the extremities of the roots, to be dry, and not fo tranfparent as the veffels which e- but have a confiderable quantity of it near the top. vidcntly contain the lap. The pith, like every other part of a plant, is derived from

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