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APPENDIX.

Page 10.

THE only case of undoubtedly perforated parenchyına with which I am acquainted is in Sphagnum, where it was first noticed by Mr. Valentine (Muscologia Nottinghamiensis, No. 1. 1833). He correctly describes this genus as having the exterior cells of its branches furnished with an aperture communicating with the external air. "The aperture is tolerably distinct in S. acutifolium; it is situated at the upper end of the cell, and stands off obliquely, appearing like a minute truncated cone. An easy way to observe it is, to press out the air contained in the cells, which escapes from the aperture in a minute bubble." This curious contrivance might have been supposed analogous to the air passages into the trunk, below the insertion of the leaves of Tree Ferns, if it did not equally exist in all the parenchyma of the leaves themselves. Mr. Valentine does not notice the latter fact, and I believe he considers the circles in the leaves of Sphagnum not to be apertures: but I had ascertained, by Mr. Reade's ingenious charring process, that they undoubtedly are openings, before I saw John Röper's paper upon the subject in the Annales des Sciences (n. s. x. 314.). This writer determined that the circular spaces in Sphagnum leaves are openings, by observing the exit and entrance through them of the Rotifer vulgaris, and of minute granular matter. He considers the openings intended" to guarantee the organs of respiration from the too great influence of the air." But I do not perceive in what way such an effect is to be accomplished.

Page 32.

Spiral vessels certainly exist in the roots of many exogens; in the Parsnep and the Beet they are large, and readily extracted entire.

With regard to their existence in seeds, Mr. Quekett has favoured me with the following memorandum (March 13. 1839) :"If you place almonds in boiling water, and separate the testa,

and while thus softened you scrape or remove some of the veins which figure its surface, it will be found that they are almost wholly spiral vessels, which are of rather minute dimensions.

"There is something else curious respecting this seed. On its surface are numerous projecting cells which have very thick parietes these can be found burst and their contents emitted; in fact they look more like eggs of some minute insect, which however they are not, as I have examined seeds of unopened almonds, where they exist likewise they appear to me to be analogous to the cells which exist on the seeds of Cobæa scandens, which Don describes as mealiness, but which is, instead, a beautiful example of fibro-cellular tissue."

To this I may add, that in the seed of Soymida febrifuga there lies in the middle of the wing a thick stratum of fibro-cellular bodies, which would be regarded as spiral vessels if they were longer and more cylindrical; but which seem to be a curious and distinct form of fibro-cellular tissue.

Page 43.

In addition to the remarks here made upon Raphides, I have the satisfaction to insert in this place the following interesting communication from Mr. Quekett, upon the same subject:

"General Appearance.-Raphides are most frequently observed under two forms, appearing in one instance as transparent acicular crystals, which are either distinct from each other or united into a compact fibrous bundle, and in the other instance as small bodies composed of many crystals which radiate from the same centre, thereby forming a more or less spherical mass.

"Besides these two usual kinds, there are other forms, but of more rare occurrence, some of which are observed of regular crystalline figure; as the rhombohedron in some cells of Calla æthiopica, and bark of Cascarilla; octohedron, according to Meyen, in the stem of Tradescantia virginica; the rectangular prism in Quillaja saponaria; and oblique prisms, which occur with acicular crystals, in Scilla maritima: but still there are a few varieties which present an irregular crystalline figure, some of which can be observed also in Tradescantia virginica, and in the inner layers of the bark of the Lime tree, where they seem very thin and pointed at the extremities, appearing like slices cut longitudinally from the middle of a square prism, which may be imagined to possess a foursided pyramid at each end.

"Form. With respect to the form of the acicular Raphides,

some difference of opinion exists between Raspail and Mohl's description; the former asserting that they are six-sided prisms, terminated at each end by a pyramid with the same base; whilst the latter describes them as right-angled four-sided prisms, vanishing into points. It is a difficult matter to decide between these two opinions, if an entire crystal be the subject of examination; for, even if magnified 1000 times, the figure is not clearly defined: but, by having recourse to some delicate manipulation, the proper shape can be then ascertained, when it can be shown that neither of the two opinions is correct in all points.

"Raspail's reason for considering the acicular crystals to be hexagonal prisms arises from the appearance they present with transmitted light, when some (but not all) are seen to exhibit two dark margins and a streak of light between them, which extends the whole length of the crystal: from this he reasons that its figure is six-sided, the lateral planes reflecting the light which impinges upon them, and consequently are seen as darkened margins, whilst the surfaces which are superior and inferior, being in a position favourable for the transmission of the rays, are transparent. This is the argument brought forward in Raspail's Organic Chemistry, in favour of the hexagonal figure, but there are no attempts to prove if any other form or position of an acicular crystal could not present a similar aspect, which is the case, as will be shown hereafter.

"As respects the summit being a six-sided pyramid, its existence does not seem discoverable, for the crystal from about its centre gradually vanishes to a point, having no angular interruption, such as is observed in the large crystals in the root of Iris florentina or the wood of Quillaja, where a regular right-angled prism is surmounted by a pyramid. Mohl's idea of the shape certainly approximates the truth more than Raspail's; but it can be shown, though the acicular crystals are four-sided, they are not always right-angled prisms, as he asserts. To witness these facts the crystals, must be obtained, by lengthened maceration, free of cellular tissue, and then crushed into fragments, when many will present an obliquely transverse fracture, which exhibits tour sides, some having the angles right angles, and others acute and obtuse angles; in fact, the transverse section of such would resemble frequently a rhombus To examine more satisfactorily the fractured ends of these minute crystals, which scarcely measure the To.6 part of an inch, it is most convenient to place their fragments in a watch glass, with a small portion of Canada balsam, and to heat the whole

till the balsam has acquired a viscid consistence, then let it be removed from the source of heat, and be stirred whilst the mass is cooling till sufficiently hard, when the broken Raphides will be sustained in it in all directions; and frequently a few can be observed erect, which cannot fail to give a true outline of their real form, which by this method appears to be a four-sided prism, the angles being frequently oblique, and in other instances to put on the rectangular condition. Occasionally some of the crystals are observed to exhibit a triangular section of the isosceles shape, which seems to be an anomaly: but, to account for this, it is to be remembered that it is not opposed to the laws of crystallisation for certain bodies to crystallise in halves, consequently the half of a four-sided prism, taken from one angle to the opposite one, would be triangular; and it is conceived that when a crystal exhibits this triangular section it has belonged to the compact bundle of crystals in which it has been constrained to assume this form, as being fitted to fill up certain spaces better than one of the oblique or rectangular shape.

"This being the observed figure, it is not difficult to account for the black margins observed in certain crystals, which may be occasioned by a rectangular prism resting obliquely, or by an oblique or triangular prism, as well as by the hexagonal; for, as the sides of either are not in the plane of direction of transmitted light, the rays are reflected, and that portion which reflects the light appears dark.

"Besides these methods of determining the form of acicular raphides, if a little alcohol be added to water containing them, and examined immediately after by the microscope, the crystals can be often seen to roll over and over, and some of them will, whilst revolving, present alternately a pointed and then a flat extremity, just as if a wedge were revolving on its axis; showing that such crystals are four-sided, but two of the sides only attenuated to form the pointed appearance, the other two suffering no diminution of their breadth at any point.

"That the four-sided prism is the ultimate figure of these minute crystals is rendered more probable by the occurrence of rhombohedral and rhombic prisms without pyramids, of the same composition, in the same plant, but of much greater widths; and there can be no doubt that these latter bodies and the acicular are two modifications of crystal of the same substance. The most decided proof of their being four-sided is obtained by pressing lightly on the piece of glass which covers them, whilst examined under the

microscope, when those which appear six-sided instantly appear four-sided, owing to the square crystal resting obliquely this can be seen in the minute crystals of Squill, or in the large square ones of Quillaja saponaria.

"The rounded masses, which may be termed Conglomerate Raphides in opposition to the acicular variety, seldom present more than the pyramid of each little crystal composing them; but in a few cases, where an opportunity is afforded of examining the prism, it can be seen to be rectangular and terminated by a foursided pyramid.

"Classes of Plants in which they are found.—No division of the vegetable kingdom seems without more or less quantities of these crystalline formations, which are found in a great number of Exogens and Endogens, and likewise in Acrogens, being visible in Ferns and Mosses, and, according to Unger, in the lowest of the Algaceæ, as Nostoc Muscorum, and Conferva crystallifera.

"The frequency of occurrence of these bodies is such, that it appears that, instead of those plants containing them being exceptions, those are to be considered such which have none in their tissues.

"It does not appear, from numerous observations, that the acicular and conglomerate Raphides are equally common in the several classes of Plants; but that Exogens contain perhaps the one kind as often as the other, while Endogens undoubtedly contain most often the acicular variety.

"Situation.—The position of these bodies has been a subject of controversy; Raspail asserting that they are always in the intercellular passages, whilst Turpin, Meyen, and Unger maintain that they are universally in the interior of the cells, which latter opinion is easily proved to be nearly correct by a little careful dissection of any plant containing them.

"Raspail's advice to see these bodies is to tear a piece of the Hyacinth stem in a drop of water placed on the stage of the microscope, when numbers of acicular crystals will be visible (this method is not likely to show them in the interior of the cells); and from measuring he finds the length of the crystal longer than the ordinary cells of the tissue; and therefore he decides from this, that they cannot be contained in the interior of the cells, while he overlooks the fact that the cell in which they are contained may be often dilated to five or six times the size of those composing the ordinary tissue of the plant. The square crystals in Quillaja saponaria appear as if loose in the plant, but they are really in a