Plant.

Plate CCCXCIV

It is here worth remarking, that every plant is really poffeffed of two roots, both of which are contained in the feed. The plume and radicle, when the feed is firft deposited in the earth, derive their nourishment from the feminal root; but, afterwards, when the radicle begins to shoot out its filaments, and to absorb fome moisture, not, however, in a fufficient quantity to fupply the exigencies of the plume, the two lobes, or main body of the feed, rife along with the plume, affume the appearance of two leaves, refembling the lobes of the feed in fize and shape, but having no refemblance to thofe of the plume, for which reason they have got the name of diffimilar leaves.

Thefe diffimilar leaves defend the young plume from the injuries of the weather, and at the fame time, by absorbing dew, air, &c. affift the tender radicle in nourifhing the plume, with which they have ftill a connection by means of the feminal root above defcribed. But when the radicle or fecond root has defcended deep enough into the earth, and has acquired a fufficient number of filaments or branches for abforbing as much aliment as is proper for the growth of the plume; then the feminal or diffimilar leaves, their utility being entirely fuperfeded, begin to decay and fall off.

Fig. 1. A, the foramen or hole in the bean through which the radicle shoots into the foil.

Fig. 2. A tranfverfe fection of the bean; the dots being the branches of the feminal root.

Fig. 3. A, the radicle. B, the plume or bud. Fig. 4. A, a longitudinal section of one of the lobes of the bean a little magnified, to fhow the fmall bladders of which the pulpy or parenchymatous part is compofed.

Figs. 5. 6. A, a transverse section of the radicle. B, a tranfverfe fection of the plume, fhowing the organs or veffels of the feminal root.

Plate Fig. 4. A view of the feminal root branched out upCCCXCI. on the lobes.

Plate

Fig. 7. The appearance of the radicle, plume, and feCCCXCIV minal root, when a little further advanced in growth. Having thus briefly defcribed the feed, and traced its evolution into three principal organic parts, viz. the plume, radicle, and feminal leaves, we fhall next take an anatomical view of the root, trunk, leaves, &c.

2. Of the root.] In examining the root of plants, the first thing that prefents itfelf is the fkin, which is of various colours in different plants. Every root, after it has arrived at a certain age, has a double skin. The firft is coeval with the other parts, and exifts in the feed; but afterwards there is a ring fent off from the bark, and forms a fecond fkin; e. g. in the root of the dandelion,. towards the end of May, the original or outer skin appears fhrivelled, and is eafily feparated from the new one, which is fresher, and adheres more firmly to the bark. Perennial plants are supplied in this manner with a new skin every year; the outer one always falls off in the autumn and winter, and a new one is formed from the bark in the fucceeding fpring. The fkin has numerous cells or veffels, and is a.continuation of the parenchymatous part of the radicle. However, it does not confift folely of parenchyma; for the microfcope fhows that there are many tubular ligpeous veffels interfperfed through it.

When the fkin is removed, the true cortical fubftance or bark appears, which is also a continuation of the pa

renchymatous part of the radicle, but greatly augmented. The bark is of very different fizes. In moit trees it is exceeding thin in proportion to the wood and pith. On the other hand, in carrots, it is almoft one-half of the femidiameter of the root; and, in dandelion, it is nearly twice as thick as the woody part.

The bark is compofed of two fubitances; the parenchyma or pulp, which is the principal part, and a few woody fibres. The parenchyma is exceedingly porous, and has a great refemblance to a sponge; for it fhrivels confiderably when dried, and dilates to its former dimenfions when infufed in. water. Thefe pores or veffels are not pervious, fo as to communicate with each other; but confift of diftinct little cells or bladders, fcarcely vifible without the affiftance of the microfcope. In all roots, these cells are conftantly filled with a thin watery liquor. They are generally of a spherical figure; though in fome roots, as the buglofs and dandelion, they are oblong. In many roots, as the horse-radifh, peony, afparagus, potatoe, &c. the parenchyma is of one uniform structure. But in others it is more diverfified, and puts on the fhape of rays, running fro the centre towards the circumference of the bark. Thefe rays fometimes run quite through the bark, as in lovage; and fometimes advance towards the middle of it, as in melilot and most of the leguminous and umbelliferous plants. Thefe rays generally ftand at an equal distance from each other in the fame plant; but the distance varies greatly in different plants. Neither are they of equal fizes: in carrot they are exceedingly fmall, and fcarcely difcernible; in melilot and chervil, 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 towards the fkin. The veffels with which thefe rays are amply furnished, are fuppofed to be air-veffels, because they are always found to be dry, and not fo tranfparent as the veffels which evidently contain the fap.

In all roots there are ligneous veffels difperfed in different proportions through the parenchyma of the bark. Thefe ligneous veffels run longitudinally through the bark in the form of fmall threads, which are tubular, as is evident from the rifing of the fap in them when a root is cut tranfverfely. These ligneous fap-veffels do not run in direct lines through the bark, but at small diftances incline towards one another, in fuch a manner that they appear to the naked eye to be inofculated ; but the microscope difeovers them to be only contigu ous, and braced together by the parenchyma. These braces or coarctations are very various both in fize and number in different roots; but in all plants they are most numerous towards the inner edge of the bark. Neither are thefe veffels fingle tubes; but, like the nerves in animals, are bundles of 20 or 30 fmall conti guous cylindrical tubes, which uniformly run from the extremity of the root, without fending off any branches or fuffering any change in their fize or shape.

In fome roots, as parfnep, efpecially in the ring next the inner extremity 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 lymph-du&s. .

Thefe lymph-ducts fometimes yield a mucilaginous lymph, as in the comphrey; and fometimes a white

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Place

milky glutinous lymph, as in the angelica, fonchus,
burdock, fcorzonera, dandelion, &c. The lymph-ducts
are fuppofed to be the veffels from which the gums and
balfams are fecerned. The lymph of fennel, when ex-
posed to the air, turns into a clear transparent balfam;
and that of the scorzonera, dandelion, &c. condenses in-
to a gum.
The fituation of the veffels is various. In fome plants
they stand 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; in the parfnep, and feveral
other plants, they are most confpicuous towards the outer
edge of the bark; and in the dandelion, they are difpo-
fed 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 confifts of two distinct substances, viz. the pulpy or parenchymatous, and the ligneous. The wood is connected to the bark by large portions of the bark inferted into it. These insertions are mostly in the form of rays, tending to the centre of the pith, which are easily discernible by the eye in a transverse section of moft roots. Thefe infertions, like the bark, confift of many veffels, moftly of a round or oval figure.

The ligneous veffels are generally difpofed in collateral rows running longitudinally through the root. Some of these contain air, and others fap. The air-veffels are fo called, because they contain no liquor. Thefe airveffels are distinguished by being whiter than the others. 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 have a confiderable quantity of it 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 compofition of small fibres or threads; which gives the pith, when viewed with a microscope, the appearance of a piece of fine gauze or net-work.

We fhall 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 nourishment, and of course the extenfion and augmentation of their parts.

Fig. 8. A tranfverfe fection of the root of wormCCCXCIV wood as it appears to the naked eye.

Fig. 9. A fection of fig. 8. magnified. AA, the fkin, with its veffels. BBBB, the bark. The round holes CCC, &c. are the lymph-ducts of the bark: All the other holes are little cells and fap-veffels. DDD, parenchymatous infertions from the bark, with the cells, &c. EEEE, the rays of the wood, in which the holes are the air-veffels. N. B. This root has no pith...

3. Of the Trunk, Stalk, or Stem.] In defcribing the trunks of plants, it is neceffary to premife, that what ever is faid with regard to them applies equally to the branches.

The trunk, like the root, confifts of three parts, viz. Plant. the bark, wood, and pith. Thefe parts, though fubftantially the fame in the trunk as in the root, are in many cafes very different in their texture and appear

ance.

The fkin of the bark is composed of very minute bladders, interfperfed with longitudinal woody fibres, as in the nettle, thistle, and moft herbs. The outfide of the fkin is vifibly porous in fome plants, particularly the cane.

The principal body of the bark is compofed of pulp or parenchyma, and innumerable veffels much larger than thofe of the fkin. The texture of the pulpy part, though the fame fubftance with the parenchyma in roots, yet feldom appears in the form of rays running towards the pith; and when thefe rays do appear, they do not extend above half way to the circumference. The veffels of the bark are very differently fituated, and defined for various purposes in different plants. For example, in the bark of the pine, the inmost are lymph ducts, and exceedingly fmall; the outmoft are gum or refiniferous veffels, deftined for the fecretion of turpentine; and are fo large as to be distinctly visible to the naked eye.

The wood lies between the bark and pith, and confifts of two parts, viz. a parenchymatous and ligneous. In all trees, the parenchymatous part of the wood, though much diverfified as to fize and confiftence, is uniformly difpofed in diametrical rays, or infertions running betwixt fimilar rays of the ligneous part.

The true wood is nothing but a congeries of old dried lymph-ducts. Between the bark and the wood a new ring of thefe ducts is formed every year, which gradually lofes its foftnefs as the cold feafon approaches, and towards the middle of winter is condensed into a folid ring of wood. These annual rings, which are distinctly visible in most trees when cut through, ferve as natural marks to distinguish their age (fig. 10. 11.) The rings Plate of one year are fometimes larger, fometimes lefs, than CCCXCV, thofe of another, probably owing to the favourableness or unfavourableness of the feafon.

The pith, though of a different texture, is exactly of the fame fubftance with the parenchyma of the bark, and the infertions of the wood. The quantity of pith is various in different plants. Inftead of being increased every year like the wood, it is annually diminished, its veffels drying up, and affuming the appearance and structure of wood; infomuch that in old trees there is fearce fuch a thing as pith to be difcerned.

The

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 fmall cells or bladders, of the fame kind with thofe of the bark, only of a larger fize. general figure of thefe bladders is circular; though in fome plants, as the thistle and borage, they are angular. Though the pith is originally one connected chain of bladders, yet as the plant grows old they fhrivel, and open in different directions. In the walnut, after a certain age, it appears in the form of a regular tranfverfe hollow divifion. In fome plants it is altogether wanting; in others, as the fonchus, nettle, &c. there is only a tranfverfe partition of it at every joint. Many other varieties might be mentioned; but these muit be left to the obfervation of the reader. A 2 Fig.