which consequently have truncated extremities; and to the latter, forms of tissue in which the cellules taper to each end, and, consequently, overlap each other at their extremities. (Plate II. fig. 8. 19.)

FIBROUS CELLULAR or FIBRO-CELLULAR TISSUE is that in which the sides are composed either of both membrane and fibre together, or of fibre only.

It is only lately that this kind has been recognised. The first observation with which I am acquainted is that of Moldenhauer, who, in 1779, described the leaves of Sphagnum as marked by fibres twisted spirally. (Fig. 1. a, p. 4.) Link afterwards stated, that the supposed fibres were nothing but the lines where small cells contained in a larger one unite together; and his opinion was received. It is nevertheless certain, that the tissue of Sphagnum is as Moldenhauer described it. In November, 1827, I described the tissue of Maurandya Barclayana as consisting of cellules formed of spiral threads crossing each other, interlaced from the base to the apex, and connected by a membrane. A few other solitary cases of the observation of this kind of tissue had subsequently occurred, when the admirable investigation of a modern anatomist suddenly threw an entirely new light upon the subject.

Instead of being very rare, cellular tissue of this kind appears to be found in various parts; it has been already mentioned as existing in the leaves of Sphagnum; it is also found in the pith of Rubus odoratus. I originally discovered it in the parenchyma of the leaves of Oncidium altissimum, and in the testa of various seeds. Mr. Griffiths has detected it abundantly in the aerial roots of Orchideous plants, observations since confirmed by Mr. Brown; and Dr. Purkinje has shown, by a series of excellent observations and drawings, that it forms the lining of the valves of almost all anthers. The forms under which it exists in these parts are far more various than those of membranous cellular tissue. The principal varieties are these:

A. Membrane and Fibre combined.

1. Fibres twisted spirally, adhering to a spheroidal or angular membrane, and often anastomosing irregularly, without the

spires touching each other. (Plate I. fig. 12.) This is what is found in Oncidium altissimum leaves, in the aerial roots of some Orchideous plants, in the lining of many anthers, and is what Mohl has figured (Ueber die Poren, &c. tab. i. fig. 9.), from the pith of Rubus odoratus. It approaches very nearly to the nature of spiral vessels, hereafter to be described, and appears only to be distinguishable by the spires of the fibres not being in contact, being incapable of unrolling, having no elasticity or tenacity; and by not being cylindrical and tapering to each end, but spheroidal.

2. Fibres crossing each other spirally, and forming a reticulated appearance by their anastomosing in oblong or botuliform cells. Of this nature are the reticulated cells of the testa of Maurandya Barclayana, Wightia gigantea, and the like. (Plate I. fig. 11.)

3. Fibres running straight along the sides of truncated cylindrical cells in the anthers of Calla æthiopica and many other plants. (Plate I. fig. 13.)

4. Fibres running transversely in parallel lines round three of the sides of prismatical right-angled cells, in the anthers of Nymphæaceæ, &c.

5. Fibres very short, attached to the sides of cells of various figures, to which they give a sort of toothed appearance, as in the anther of Phlomis fruticosa and other Labiatæ. (Plate I. fig. 15.)

The last three were first noticed by Dr. Purkinje.

6. The fibre twisted spirally, in the open membranous tubes that form the elaters of Jungermannia, apparently constitutes another form of tissue of this order. (Plate I. fig. 17.)

B. Fibre without Membrane.

7. Spiral fibres repressed by mucus, but having sufficient elasticity to uncoil when the mucus is dissolved, and then breaking up into rings. (Plate I. fig. 16.) These are what are found in the testa of Collomia linearis. They approach spiral vessels so very nearly, that when I originally discovered them I mistook them for such. They are known by their roundish or depressed figure when at rest, and by the want of an inclosing membrane, and by their brittleness when uncoiled.

8. Fibres short, straight, and radiating, so as to form little starlike appearances, found in the lining of the anthers of Polygala Chamæbuxus, &c. by Dr. Purkinje. (Plate I. fig. 19.)

9. Fibres originating in a circle, curving upwards into a sort of dome, and uniting at the summit, observed by the same anatomist in the anthers of Veronica perfoliata, &c.

10. Fibres standing in rows, each distinct from its neighbour, and having its point hooked, so that the whole has some resemblance to the teeth of a currycomb, in the anthers of Campanula; first noticed by Dr. Purkinje. (Plate I. fig 18.)

11. Fibres forming distinct arches, as seen in the anthers of Linaria cymbalaria, &c. by Dr. Purkinje. (Plate I. fig. 4.)*

In the centre of some of the cellules of the cellular tissue of many plants there is a roundish nucleus, apparently consisting of granular matter, the nature of which is unknown. It was originally remarked by Mr. Francis Bauer, in the cellules of the stigma of Phaius Tankervilliæ. A few other vegetable anatomists subsequently noticed its existence; and Mr. Brown, in his recent Memoir on the mode of impregnation in Orchideæ and Asclepiadeæ, has made it the subject of more extended observation. According to this gentleman, such nuclei not only occasionally appear on the cuticle of some plants (Plate III. fig. 9.), in the pubescence of Cypripedium and others, and in the internal tissue of the leaves, but also in the cells of the ovulum before impregnation. It would also seem that Mr. Brown considers stomata to be formed by the juxtaposition of two of these nuclei.

SECT. II. Of Woody Fibre.

THIS (Vasa fibrosa, Lat.; Petits tubes, Mirb.; Tissu cellulaire allongé or ligneux, Fr.; Vaisseaux propres fasciculaires, Mirb.; Ligneæ fistula, Malpighi; Fasergefässe, or Baströhren,

* According to the last mentioned author, the fibres themselves are generally tubular, and either perfectly round or somewhat compressed, or even three or four sided. He considers it proved, that they are hollow, by their appearance when compressed, by their occasionally containing bubbles of air, and by the difference between their state when dried and when

recent.

Germ.; perhaps the Vital vessels of Schultz;) consists of very slender transparent membranous tubes, tapering acutely to each end, lying in bundles, and, like the cellular tissue, having no direct communication with each other, except by invisible pores. (Plate II. fig. 1. a, b; 2. 5. a, &c.)

Many vegetable anatomists consider it a mere form of cellular tissue, in an elongated state; but scarcely with justice; for if this mode of viewing the subject were pushed a little farther, it would be necessary to refer every modification of tissue to the cellular, which would be obviously improper.* Woody fibre may be at all times known by its elongated figure and extremely attenuated character; usually it has no sort of markings upon its surface, except occasionally a particle or two of greenish matter in its inside; but sometimes it is covered with spots that have been mistaken for pores, and that give it a peculiar character (Plate II. fig. 3. and 4.); and I have remarked an instance, in Oncidium altissimum, of its having tubercles on its surface. (Plate II. fig. 2.) Generally, while cellular tissue is brittle, and has little or no cohesion, woody fibre has great tenacity and strength; whence its capability of being manufactured into linen. Every thing prepared from flax, hemp, and the like, is composed of woody fibre.

That even the most delicate of it consists of tubes, may be readily seen by examining it with a high magnifying power, and also by the occasional detection of particles of greenish matter in its inside. (Plate II. fig. 2. b.) A very different opinion has nevertheless been held by some physiologists, who have thought that the woody fibre is capable of endless divisibility. "When," says Duhamel, "I have examined under the microscope one of the principal fibres of a pear tree, it seemed to me to consist of a bundle of yet finer fibres; and when I have detached one of those fibres, and submitted it to a more powerful magnifying power than the first, it has still

* The distinction between cellular tissue and woody fibre is more pronounced in the long club-shaped aerial radicle of Rhizophora Candelaria, than in any plant with which I am acquainted. It there consists of large, very long, transparent tubes, lying imbedded in fine brownish granular matter, which is minute cellular tissue.

appeared to be formed of a great number of yet more delicate fibres." (Physique des Arbres, i. 57.) To this opinion Du Petit Thouars assents, conceiving the tenuity of a fibre to be infinite, as well as its extensibility. (Essais sur la Végétation, p. 150.) These views have doubtless arisen from the use of very imperfect microscopes, under low powers of which such appearances as Duhamel describes are visible; but with modern glasses, and after maceration in nitric acid, or even in pure water, each particular fibre can be separated with the greatest facility. Their diameter is often very much less than that of the finest human hair; the tubes of hemp, for example, when completely separated, are nearly six times smaller. It must, however, be observed, that the fibres of this plant, as used in linen making, are by no means in a state of final separation, each of the finest fibres that meet the naked eye being in reality a bundle of tubes. While, however, some are of this extremely small size, others have a diameter as considerable as that of ordinary cellular tissue itself; in Coniferæ the tubes are often or ʊ of an inch in diameter, and in the Lime they average about 10. Link states (Elementa, p. 85.) that they are very large in trees of hot countries, as, for instance, the Brazilian coffee.

It has been asserted by some writers, that the tubes of the woody fibre are occasionally divided internally by transverse septa or partitions; but the fact is denied by Link, who declares that "ejusmodi septa non existunt." It is no doubt true that, in general, there is no trace of such septa; but I think it is impossible to deny their existence in the tissue of the Lime tree, at least.

There are three distinct kinds of woody fibre:

1. That in which the walls are not occupied with either granules or glands sticking to them, or in which the former are of very rare occurrence. (Plate II. fig. 1.) This is the finest and the commonest of all; and is also the most genuine state of woody fibre.

2. That in which the walls have uniformly considerable numbers of granules of regular size sticking to them in a scattered manner. (Plate II. fig. 3, 4, 5.) These granules have been, and are still considered by many anatomists as