pores in the sides of the tissue. They have been, in particular, so described and represented lately by Mons. Brongniart in Cycadeæ, in which the tubes are large, and the appearance very conspicuous. (Annales des Sciences, vol. xvi. tab. 21.) But I think it possible to demonstrate that this is an optical deception, and that the supposed perforations are of the same nature as the similar punctuations in cellular tissue, viz. semitransparent granules. In the first place, no colourless light passes through the supposed pores in any case; on the contrary, they are dark, and have a solid appearance at all times, except when, at a certain distance out of the focus of the microscope, they become luminous. Secondly, if they were holes, they would, at least, be seen open when the tissue is dry and contracted, although they might close up when it becomes swollen with moisture. That, however, they never are: on the contrary, they are more opaque when dry than when wet. Thirdly, they become more and more opaque as the magnifying power with which they are viewed is increased; a circumstance which seems incompatible with perforations. Finally, and it is this which will possibly be regarded most conclusive, if the tissue of Zamia be allowed to remain macerating for some time in dilute nitric acid, the apparent pores disappear: that is to say, the granules that cause the appearance of perforations are dissolved. It has been thought that such appearances as these were confined to Cycades and Coniferæ; but I suspect that they are far from uncommon in other families. Such tissue constitutes a considerable part of the wood of Calycanthus (Plate II. fig. 4.), as has been already noticed; and it is abundant in an East Indian genus allied to Trichopus. This kind of tissue might be called granular woody fibre: it approaches very nearly to the character of ducts, into which, in Zamia, it seems to pass by almost insensible transitions. It may, however, be known from dotted ducts, either by its very acuminated extremities, or by its granules not being arranged in a spiral manner.

3. The third kind of woody fibre is the glandular. This has hitherto only been noticed in Coniferæ, in which it is uniformly found in every species. Its dimensions are more considerable than that of either of the last-mentioned forms;

and, like the second, it has been described as perforated with pores. It differs from granular woody fibre in the markings of the tube being vesicular, and usually transparent, with a darkened centre (Plate II. fig. 5, 6. 8.), which last is what has been described as a pore, the vesicle itself being considered a thickened rim. Kieser figures the glands as pores, both in Pine

ently found by examining a thin shaving of Pinus Strobus with a microscope, when they will be seen in the form of transparent globules, having a dark centre, and placed upon the walls of the tissue. That these globules are not pores, seems to me to be proved thus: they are flaccid when dry, and distend when moistened, which is not the property of a pore; their centre is more generally opaque than transparent, which is also not the property of a pore; they may be torn through the middle without any hole becoming visible; and, finally, they may sometimes be detached from the tissue (Plate II. fig. 7.), or fall away spontaneously. In the latter case they leave a hole in the tissue at the place where they grew; and holes thus occasioned misled Kieser into the belief that the woody fibre of Ephedra was really pierced with pores of considerable magnitude. An illustration of the manner in which these perforations are caused will be found in Plate II. fig. 7.

M. Adolphe Brongniart has rightly stated, that there exists in Gnetum Gnemon a form of tissue exactly the same as in Coniferæ. (Voyage de Freycinet.) In a species of that genus collected in Tavoy by Dr. Wallich, the glands are only different from what we commonly meet with in Coniferæ, in being arranged side by side, instead of being placed in single rows irregularly one above the other. (Plate II. fig. 5.)

Woody fibre constitutes a considerable proportion of the ligneous part of all plants; it is common in bark, and it forms the principal portion of the veins of leaves, to which it gives stiffness and tenacity.

VASCULAR tissue consists of simple membranous tubes tapering to each end, but often ending abruptly, either having a fibre generated spirally in their inside, or having their walls marked by dots or transverse bars arranged in a spiral direction.

Such appears to me to be the most accurate mode of describing this kind of tissue, upon the exact nature of which anatomists are, however, much divided in opinion; some believing that the fibre coheres independently of any membrane, others doubting or denying the mode in which the vessels terminate; some describing the vessels as ramifying; and a fourth class ascribing to them pores and fissures, as we have already seen has been done in cellular tissue and woody fibre. It will be most convenient to consider all these points separately, along with the varieties into which vascular tissue passes.

There are two principal kinds of vascular tissue; viz. spiral vessels (Plate II. fig. 9. 11.), and ducts (Plate II. fig. 13. 15, 16. 18. 20.).

SPIRAL VESSELS (fig. 6, 7.) (Vasa spiralia, Lat.; Trachea of many; Fistula spirales of Malpighi; Spiralgefässe or Schraubengefässe, Germ. ;) are membranous tubes with conical extremities; their inside being occupied by a fibre twisted spirally, and capable of unrolling with elasticity. To the eye they, when at rest, look like wire twisted round a cylinder that is after

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wards removed. For the purpose of finding them for examination, the stalk of a strawberry leaf, or a young shoot of the Cornus alba (common dogwood) may be conveniently used; in these they may be readily detected by gently pulling the specimen asunder, when they unroll, and appear to the naked eye like a fine cobweb.

Very different opinions have been entertained as to the exact structure of spiral vessels. They have been considered to be composed of a fibre only, twisted spirally, without any connecting membrane; or to have their coils connected by an extremely thin membrane, which is destroyed when the vessel unrolls; or to consist of a fibre rolled round a membranous cylinder; or even, and this was Malpighi's idea, to be formed by a spiral fibre kept together as a tube by interlaced fibres. Again, the fibre itself has been by some thought to be a flat strap, by others a tube, and by a third class of observers a kind of gutter formed by a strap having its edges turned a little inwards. Finally, the mode in which they terminate, although formerly stated by Mirbel to be continuous with the cellular tissue, is so little known, that the learned M. De Candolle, in his Organographie, published in 1827, remarks, "Personne jusqu' ici n'a vu d'une manière claire, ni l'origine, ni la terminaison d'un vaisseau." (P. 58.) As doubts upon these points arise from the extreme minuteness of the vessels, and from the different degrees of skill that observers employ in the use of the microscope, I can scarcely hope that any observations of mine will have much weight. Nevertheless I may be permitted to state briefly what arguments occur to me in support of the definition of the spiral vessel as given above.

With regard to the presence of an external membrane within which the spiral fibre is developed, it must be confessed that direct observation is scarcely sufficient to settle that point. It is easy to prove the existence of a membrane, but it is difficult to demonstrate whether it is external or internal with respect to the fibre. The best mode of examination is to separate a vessel entire from the rest of the tissue, which may be done by boiling the subject, and then tearing it in pieces with the points of needles or any delicate sharp

instrument; the real structure will then become much more apparent than if the vessel be viewed in connection with the surrounding tissue. From some beautiful preparations of this kind by Mr. Valentine and Mr. Griffiths, it appears that the membrane is external: in the root of the Hyacinth, for example, the coils of the spiral vessel touch each other, except towards its extremities; there they gradually separate, and it is then easy to see that the spiral fibre does not project beyond the membrane, but is bounded externally by the latter, which would not be the case if the membrane were internal : a representation of such a vessel is given at Plate II. fig. 9. Another argument as to the membrane being external may not unfairly be taken from the manifest analogy that a spiral vessel bears to that form of cellular tissue (p.11.), in which a spiral fibre is generated within a cellule: it is probable that the origin of the fibre is the same in both cases, and that its position with regard to the membrane is also the same.

It is much more difficult to determine whether the fibre is solid, or tubular, or flat like a strap; and Amici has even declared his belief that the question is not capable of solution with such optical instruments as are now in use. When magnified 500 times in diameter, a fibre appears to be transparent in 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 M. 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 I have been able to apply 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. I should have been induced to think, with Dr. Bischoff,