of the vessel. Reticulated vessels are not found in young plants, being the result of a change produced by the advanced growth of the plant. They are found in a few plants only, but in some, as the balsam, (impatiens balsamina,) at its full growth, they are the only kind of vessel contained in the root, in which part they occur more frequently than in the stem. In the fasciculi, (bundles of connected vessels,) they are situated towards the bark."-(Treatise on Veg. Phys., 8.)

(4.) The beaded vessel is a variety or modification of the punctuated vessel; it is described in the Treatise as resembling a chain of oblong, ovate (i. e. egg-shaped) cells, or beads, and as found only in knots of the stem and tubercles of the roots, and intended to unite the other vessels with one another.

In the new Treatise on Botany (1835), the wood-cut, fig. 13 (1), is thus alluded to; the reader will do well to compare this illustration with that extracted from the original Treatise (No. 1) above:"In the closed duct, the fibre is placed exactly as in the spiral vessel (see b); in the annular (see a), the spires seem broken into rings, which join at their extremities, and give the organ the appearance of a tube partially filled with rings, lying irregularly in the inside."

The office of ducts (for so it appears all the tubes are now styled), is, "to convey fluid at one period of their existence, whatever it may originally have been; for they are certainly filled with sap in such plants as those to which we have just referred" (the balsam, and touch me not). "Considering their relation, in structure, to spiral vessels, one cannot avoid suspecting that to convey air was their original destination; but, if so, this must have ceased soon after their first creation; for the thinness of their sides, and the very imperfect manner in which they are guarded internally, by fibrous spires, are such, that no resistance would be offered by them to the infiltration of fluid from the tissue that surrounds them."

Fig. 14.

a

312. Situation of the Vessels and Cells.-That of herbaceous plants has been described at paragraph 308, wherein it was shown, that the stems consisted of a cuticle and bark, surrounding a parenchymatous, or pulpy mass of cells. This cellular tissue generally involves bundles of conducting sap-vessels, ascending in a vertical direction the whole length of the stem. Sometimes two or more varieties of vessels are observed in the same stem, the spiral vessels changing to the punctuated, while other simple spirals are formed in a position nearer to, and often surrounding, the pith. At fig. 14 is shown the spiral tubes (a) ranged within

the substance of the cellular tissue of simple cells (b), as in the sugar-cane. In the stems of exogenous plants, the spirals are situated round the medulla, or pith: they are found, also, in the leaves, calyx, corolla, stamens, and pistil, but rarely in the bark. In endogens they are found in the centre of every vascular and fibrous bundle; also in the leaves, &c. The Treatise on Vegetable Physiology observes, "If the stem of a gourd be examined at different periods of its growth, or in different parts of the plant, between the root and the top of the stem, we shall find that, in the young plant, or at the newly unfolded extremity of the old plant, the vessels are all simple spirals. As the plant advances in age, each of the vessels already formed becomes punctuated, and new spiral vessels appear on the opposite side of the fasciculus; and this transformation of spiral into punctuated vessels proceeds, until all the vessels in the oldest parts of the stem, that are next the root, are punctuated vessels."

313. Vessels of the woody Stem.-This species of stem, peculiar to trees and shrubs, has been described at No. 242, as consisting of concentric layers. The layers indicate, in general, the age of the plant, since a new layer of alburnum is added every year in that part of the stem which is contiguous to the bark. But besides these concentric layers, there are other membranous, or cellular masses, which diverge in a horizontal direction from the layer of wood next the pith, to the liber, or inner layer of the bark: these rays are termed medullary rays, (radii medullares,) because they formerly were supposed to be derived from the medulla, marrow, or pith of the tree*. They appear to be composed of cellular membrane or tissue. Fig. 15,

Fig. 15.

exhibits a woody stem, wherein a represents the bark surrounding three layers of wood, that next the bark being the alburnum, or layer of the present year; b, b, show the convergent or medullary rays proceeding from the innermost layer of wood, that next the pith. But the most beautiful exemplification of the structure of woody stems, is to be found in those natural specimens which are sold by mathematical instrument makers. There is, or was, a set of these prepared specimens, known by the name of "Custance's vegetable cuttings:" it consists of twelve sliders of box wood, each containing four cuttings or slices of some root or

* Mr. Knight has asserted that the term ought to be convergent, and not divergent; because, the bark, being evidently the source from whence these processes proceed, and by which they are formed, they must, of necessity, be considered as converging-i. e., tending towards each other, instead of diverging, or spreading more widely apart.

stem, between two fine and transparent pieces of talc. These slices are so prepared as to exhibit the natural vascular structure;-the woody fibre, that encloses the vessels, lying in concentric layers, and the medullary, or convergent processes, crossing them in a horizontal direction. Wood-cuts and drawings are, at best, but tolerable substitutes for these real specimens of Nature's works; though it is not denied, that correct figures may assist the vegetable anatomist in his researches, particularly if they be accompanied by luminous descriptions. It is to be lamented that the descriptions which are given in SIR HUMPHRY DAVY's Lectures on Agricultural Chemistry, are unworthy of his three fine plates. They are, at a remote distance from the figures, very meagre in their allusions, and in fact, do not enter into a minute definition of those parts which are so admirably laid down. The plates represent highly magnified portions of the section: 1st. Of an elm branch, "exhibiting the tubular structure, and silver and spurious grain;” 2nd. Of a part of the branch of an oak; 3rd. Of that of the branch of an ash. The vessels and cells of the bark,-the conducting vessels, or channels through which the vessels passed,-the fibrous tissue of the wood,-the convergent layers, and the cells of the pith,—all are admirably depicted, and with so much nicety, that had the drawings been accompanied by equally minute and precise descriptions, the student could not have failed to acquire most satisfactory information. As it is, however, the plates do little more than exhibit the skill of the draughtsman and engraver, and excite curiosity without gratifying it.

314. If the stem of a young seedling tree, or a tender twig of the growth of the current year, be transversely divided, and an extremely thin slice of it be placed under a magnifying glass of high power, it will exhibit the pith in the centre, surrounded by a single ring of wood, in which may be traced the conducting vessels of the sap. This is the first layer of wood of the yearling shoot in its herbaceous state, beyond and exterior of which, is a circle of bark containing a cellular tissue, and another set of vessels that convey the returning juices: finally, this bark is enclosed, and defended by the epidermis or cuticle. Here then is a young twig or yearling shoot of a tree, in a state little differing from that of an herbaceous plant.

But an object vastly more minute is well worthy of observation; for, if the stalk of a bunch of currants, nay, even if the little pedicle which supports one individual currant be carefully divided, and a thin slice of either be examined by a power of at least two hundred and fifty times, the structure will be found astonishingly complete; cuticle, bark, cellular tissue, vessels, pith, all are there.

In fact, the latter object exhibits an organization of great symmetrical beauty: it contains within the cuticle and bark, a cellular tissue of oblong, or lozenge-shaped cells, arranged in the form of a ring around the wood, if so it may be termed; and this wood exhibits the sap-vessels which supply the berry. They appear like dots in the midst of a slender ring that encloses the pith.

The structure of this minute atom is as complete as that of the young shoot of an oak, or ash tree, in its earliest developement; but let that young shoot attain a year's growth, and the case will be altered, it will then exhibit the perfect structure of a tree or shrub, somewhat resembling that represented in fig. 16, which is a magnified specimen of an Ash of the growth of three years. The drawing has been taken from one of those finely executed engravings in Davy's Lectures, alluded to in par. 313; but here it is greatly diminished.

No. 1 is the transverse cutting as it appears to the eye; No. 2 represents the same, magnified; a, is the pith; b, the inner concentric ligneous layer of the first year, now the true wood; the medullary sheath is situated between a and b just at the point when the convergent rays approach the pith; c, is the layer of wood less matured of the second year; the sap-vessels therein are more apparent; d, is the outermost, or recently formed concentric layer, or the alburnum, in which the large sap-vessels, s, are very visible; f, is the liber, or new bark, with the reducent vessels; g, the parenchyma, or cellular tissue of the bark; h, the outer bark, and

its vessels; m, m, m, m, point to the direction of the medullary, or convergent layers, 25 of which are drawn in the figure.

Of these rays, or layers, Radii medullares, the Treatise on Vegetable Physiology observes: "The flattened masses of cellular tissue, which compose these, consist of oblong cells, resembling the oblong cells which surround the vessels, except that they are horizontally arranged. These stretch from the first layer in the centre to the surface of the wood; and are there opposed by others situated in the liber, or inner bark. In these rays the cells are more numerous in the middle of the mass; so that in a vertical section of a stem, they appear to be of a lozenge shape." The cellular texture of the medullary rays has led them to be regarded as processes of the pith; "but that this is an error seems evident, for the first layer of wood, the one next the pith, has no medullary rays traversing it, although every subsequent layer is crossed by them. It consists of small oblong cells imbedding a circle of simple spiral vessels, and a cellular lining or sheath which is interposed between the pith and the spiral tubes." If the reader who possesses the last edition of DAVY's Agricultural Lectures, that of 1827, compare the plates therein, which have been alluded to, with fig. 14 above, and the foregoing definition; and particularly, if he attentively examine the beautiful natural specimens in CUSTANCE'S Cuttings, he will not fail to acquire some practical information concerning that astonishing structure, which may be traced throughout the whole of the works of the vegetable kingdom.

315. Vessels of the Leaves.-Leaves exhibit a vascular, cellular, and cuticular system or texture. In the first, the vascular; bundles of sap vessels enter the leaf through its footstalk, and are distributed throughout the minutest fibres which compose the network: the spiral vessels in particular, may be detected in the midrib, and in all its ramifications. In the cellular system, a mass of cells, which constitutes the chief bulk, and forms the colouring matter, may be traced in all leaves. These cells vary much in their form; in some leaves they are globular, in others oval, tubular, or of a cylindrical figure. They are connected with pores which open through the cuticle, and these pores, and the cuticle that covers or lines them, form the cuticular system of the leaf. The following comprehensive view of the structure of the leaves has been taken from SIR HUMPHRY DAVY's Third Agricultural Lecture, wherein also is a fine plate of a portion of a vine leaf highly magnified, “cut so as to exhibit the trachea" (spiral vessels), and "copied from GREW'S Anatomy of Plants."

"The leaves, the great sources of the permanent beauty of vege