the middle, and more or less opaque at the edges; a circumstance which has no doubt given rise to the idea that it is a strap or riband, with the edges either thickened, according to De Candolle, or rolled inwards, according to Mirbel. But it is also the property of a transparent cylinder to exhibit this appearance when viewed by transmitted light, as any one may satisfy himself by examining a bit of a thermometer tube. A better mode of judging is, perhaps, to be found in the way in which the fibre bends when the vessel is flattened. If it were a flat thread, there would be no convexity at the angle of flexure, but the external edge of the bend would be straight. The fibre, however, always maintains its roundness, whatever the degree of pressure that may be applied to it. (Plate II. fig. 10.) This I think conclusive as to the roundness of the fibre; but it does not determine the question of its being tubular or solid. Bischoff, who has investigated the nature of spiral vessels, asserts (De verâ vasorum plantarum spiralium Structurâ et Functione Commentatio, 1829), that it is solid, and this agrees with my own observations. But M. Girou de Buzareingues states that it is hollow and contains fluid, and he gives numerous excessively magnified figures to illustrate his statement. Hedwig also long since believed that, when coloured fluids rise in spiral vessels, he saw them follow the direction of the spires. This last fact may, however, be explained upon the supposition that they rise in the channels formed by the approximation of cylindrical fibres, and not in the fibres themselves; in which case there could be little doubt that the fibres are really solid; and I must declare that I can find no such appearances as those described by M. de Buzareingues.

The last-mentioned physiologist states, that the fibre often runs between two cylindrical tubes, so that there is not only an outer membrane, but an inner one also. He adds that the inner tube contains air, but that fluid is lodged in the space between the two tubes. These observations cannot be repeated, for the learned author on no occasion names the plants in which he has remarked these peculiarities of structure, which have hitherto escaped the most skilful vegetable anatomists.

Link contends, that the fibre, although simple at first, soon forks and forks again, and that the branches thus produced all follow the direction of the spire.

The termination of spiral vessels is, beyond all doubt, conical. This was stated by Nees von Essenbeck, in his Handbuch der Botanik, published in 1820; and in 1824 Dutrochet asserted, that they end in conical spires, the point of which becomes very acute; but one would not suppose, judging from the figure given by the latter writer, that he had seen the terminations very clearly. If the point of a spiral vessel in the Hyacinth (Plate II. fig. 9.) be examined, it will be seen that the end of the spiral fibre lies just within the acute point of the vessel, and that the spires become gradually more and more relaxed as they approach the extremity, as if their power of extension gradually diminished, and the membrane acquired its pointed figure by the diminution of elasticity and extensibility in the fibre. It is not, however, always in a distinct membrane that the spiral vessel ends. In Nepenthes the fibres terminate in a blunt cone, in which no membrane is discoverable. (Plate II. fig. 11.)*

A spiral vessel is formed by the convolutions either of a single spire, or of many, always turning in the same direction. In the first case it is called simple, in the latter compound. The simple is the most common. (Plate II. fig. 9.) Kieser finds from two to nine fibres in the BaDe la Chesnaye as many as twenty-two in the same plant. There are four in Nepenthes (Plate II. fig. 11.), five in Liparis pendula. In general, compound spiral vessels are thought to be almost confined to Endogenous

nana.

* A singular change occurs in the appearance of the spiral vessels of Nepenthes, after long maceration in dilute nitric acid, or caustic potash : the extremities cease to be conical and spirally fibrous, but become little transparent oblong sacs, in which the spires of the fibres gradually lose themselves. This alteration, which is a very likely cause of deception, is perhaps owing to the extremities of the vessels being more soluble than the other part, the sac being composed of the confluent dissolved fibres. This is in some measure confirmed by the subsequent disappearance of all trace of fibres in any part of the vessels, under the influence of those powerful solvents.

plants, where they are very common in certain families, especially Marantaceæ, Zingiberaceæ, and Musaceæ; but their existence in Nepenthes, and, according to Rudolphi, in Heracleum speciosum, renders it probable that future observations will show them to be. not uncommon among Exogens also.

In Coniferæ the spiral vessels have in some cases their spires very remote, and even have glands upon their membrane between the spires. Link speaks of a peculiar kind of spiral found in Coniferous plants "fibris tenuissimis distincta," and calls them vasa spiralia fibrosa.

In size, spiral vessels, like other kinds of tissue, are variable; they are generally very small in the petals and filaments. Mirbel states them to be sometimes as much as the 288th of an inch in diameter; Hedwig finds them, in some cases, not exceeding the 3000th; a very common size is the 1000th. According to the observations of Link, they may be found of extremely different size in one and the same bundle of tissue in the stem of Canna, the largest being, the middle size, and the smallest of an inch in dia

meter.

An irritability of a curious kind has been noticed by Malpighi in the fibre of a spiral vessel. He says (Anat. p. 3.), that in herbaceous plants, and some trees, especially in the winter, a beautiful sight may be observed, by tearing gently asunder a portion of a branch or stem still green, so as to separate the coils of the spires. The fibre will be found to have a peristaltic motion, which lasts for a considerable time. An appearance of the same nature has been described by Don in the bark of Urtica nivea. These observations are, however, not conformable to the experience of others. De Candolle is of opinion that the motion seen by Malpighi is due to a hygrometrical quality combined with elasticity; and as spiral vessels do not exist in the bark of Urtica nivea, it seems that there is some inaccuracy in Don's remark.

The situation of spiral vessels is in that part of the axis of the stem surrounding the pith, and called the medullary sheath, and also in every part the tissue of which originates from

it; such as the veins of leaves, and petals, and of all other modifications of leaves. It has been supposed that they are never found either in the bark, the wood, or the root; and this appears to be generally true. But there are exceptions to this: Mirbel and Amici have noticed their existence in roots; and Mr. Valentine and Mr. Griffith have both extracted them from the root of the Hyacinth; they do not, however, appear to have been hitherto seen in the roots of Exogens. I know of no instance of their existence in bark, except in Nepenthes, where they are found in prodigious quantities, not only between the alburnum and the liber, embedded in cellular tissue, as was first pointed out to me by Mr. Valentine, but also sparingly both in the bark and wood. They have been described by myself as forming part of the testa of the seed of Collomia, and Brown has described them as existing abundantly in that of Casuarina. In the former case, the tissue was rather the fibro-cellular, as has been already explained (p. 18.); in the latter, they are apparently of an intermediate nature between the fibro-cellular and the vascular; agreeing with the former in size, situation, and general appearance, but differing in being capable of unrolling. In the stem of Endogens, spiral vessels occur in the bundles of woody tissue that lie among its cellular substance; in the leaves of some plants of this description they are found in such abundance, that, according to De la Chesnaye, as quoted by De Candolle, they are collected in handfuls in some islands of the West Indies for tinder. The same author informs us that about a drachm and a half is yielded by every plantain, and that the fibres may be employed either in the manufacture of a sort of down, or may be spun into thread. In Coniferous plants they are few and very small, and in Flowerless plants they are for the most part altogether absent; the only exceptions being in Ferns and Lycopodiaceæ, orders occupying a sort of middle place between flowering and flowerless plants: in these they no doubt exist. My friend, Mr. Griffith, has succeeded in unrolling them in the young shoots of Lycopodium denticulatum, and Mr. Quekett in Diplagium seramporense.

Some have thought that the spiral vessels terminate in those little openings of the cuticle called stomates; but there does not seem to be any foundation for this opinion.

DUCTS (Plate II. fig. 12. and 15.) are membranous tubes, with conical or rounded extremities; their sides being marked with transverse lines, or rings, or bars, and being incapable of unrolling without breaking.

These approach so nearly to the spiral vessel that it is impossible to doubt their being a mere modification of it. Some writers confound all the forms under the common name of spiral vessels, but it is more convenient to consider them as distinct, not only on account of their peculiar appearances, but because they occupy a station in plants in which true spiral vessels are not often found; and it is therefore probable that their functions are different. They vary between the

and

All the forms of the duct seem reducible to the following varieties:

1. The Closed (Plate II. fig. 15.), which are absolutely the same as spiral vessels, except that they will not unroll.

2. The Annular (Plate II. fig. 12. d). These are well described by Bischoff as being formed of fibrous rings, placed at uncertain intervals; or, to speak more accurately, they, like spiral vessels, are formed of a spiral thread, but it is often broken, so as, in some parts, to separate into a number of distinct rings. These rings are included within a membranous tube, by which they are held together. Annular ducts are common in the soft parts of plants, especially in such as grow with much rapidity; in the Garden Balsam they are particularly abundant. They are among the largest kinds of vessels.

3. The Reticulated (Plate II. fig. 12. f). In these the spiral fibre, instead of separating into a number of distinct rings, is continuous in some places, and anastomoses in others, so as to form a sort of netted appearance. Vessels of this kind, like the last, are found in the stem of some herbaceous plants; as, for example, the Garden Balsam, in which they may be seen in a great variety of states.

D

U. B. GENT