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5. Fibres forming distinct arches, as seen in the anthers of Linaria cymbalaria, &c. by Purkinje. (Plate I. fig. 4.) *
In the centre of some of the bladders 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 Francis Bauer, in the vesicles of the stigma of Phaius Tankervilliæ. A few other vegetable anatomists subsequently noticed its existence; and Brown, in his Memoir on the mode of impregnation in Orchidaceæ and Asclepiadaceæ, has made it the subject of more extended observation. According to this botanist, 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 ovule before impregnation. It would seem that Brown considers stomates to be formed by the juxtaposition of two of these nuclei. (See also Slack, in the Trans. Soc. Arts, xlix.)
Dr. Schleiden has published some extremely interesting observations upon this body, which he regards as a universal elementary organ, and calls Cytoblast.+ According to this observer, the form varies from oval to lenticular and round, the colour from yellowish to a silvery white, and changing to pale yellow up to darkest brown upon the application of iodine: in size it varies between ozuo of a Paris inch in diameter, in Fritillaria pyrenaica, where it attains its largest size, and to doo in the embryonal end of the pollen tube of Linum pallescens. In structure it is usually granular; in consistence it varies between extreme softness and such a degree of toughness, as enables it to resist the pressure of the compressorium without altering its form. In the interior of the Cytoblast, or sunk in its surface, is a small, well-defined
* 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.
+ I regret very much that my imperfect acquaintance with the German language is insufficient to enable me to give the valuable observations of this excellent observer more in detail.
body, which, to judge from its shadow, represents a thick ring or a thick-sided hollow spherule: there is generally but one such spherule to each Cytoblast, but occasionally there are two or even three. The spherule varies in size from half the diameter of the Cytoblast to a point too small to be measured; and Dr. Schleiden has ascertained that this minute body is formed earlier than the Cytoblast itself. It is sometimes darker, sometimes clearer than the rest of the Cytoblast; and is usually of a firmer consistence, remaining well defined when the latter is crushed by pressure into amorphous mucus.
If the gum which is found in the youngest albumen of a plant be examined, it will be found turbid with molecules of extreme minuteness. Of these some acquire a larger size and a more definite outline than others, and by degrees Cytoblasts appear, which seem to be a granular coagulation round each molecule. As soon as the Cytoblast has attained its full size, there appears upon it a fine transparent vesicle; this is a young cell, which at first represents a very flat segment of a sphere whose flat side is formed of the Cytoblast and convex side of the young vesicle, which is fixed upon it like the half of an hour-glass (wie ein Uhrglas auf einer Uhr). The space lying between the convexity of the vesicle and the Cytoblast is as clear and transparent as water, and is apparently filled with an aqueous fluid. If these young cells are isolated, we may, by shaking the field of the microscope, wash the mucous molecules almost clean; but they cannot be long observed, because they dissolve in distilled water in a few minutes, and leave nothing but the Cytoblast behind. The vesicles continue to swell out, and their lining becomes formed of jelly, with the exception of the Cytoblast, which soon becomes a part of their wall: the cell keeps increasing in size, till at last the Cytoblast is only a minute body imbedded in the side of the cavity, or sometimes loose in the cavity. It is, however, in time absorbed, and it is only after its absorption has occurred that, as Schleiden believes, the process of depositing secondary layers begins. The Cytoblast appears, however, sometimes to have a permanent existence, as in the pollen of Larix europæa, and in those hairs in which a circulation of
the sap is observable. In those Schleiden has remarked (and my own observations coincide with his) that all the currents proceed from the Cytoblast and return to it.
SECT. II. Of Pitted Tissue, or Bothrenchyma. *
This, which has had a variety of names, (Tubes poreux, Vaisseaux en chapelet, Tubes corpusculiféres, Vasiform Tissue, Dotted Ducts,) consists of tubes, often of considerable size, appearing when viewed by transmitted light as if riddled full of holes. Upon a more accurate inspection, however, it is found to receive that appearance from its sides being filled with little pits sunk in the thickness of the lining. (See Plate II. fig. 2.) Of this there are two kinds.
1. Articulated Bothrenchyma.— This is very common in wood. The holes which are so evident to the naked eye, in a transverse section of the oak or the vine, are its mouths; and the large openings in the ends of the woody bundles of Monocotyledonous stems, as in the Cane, are also almost always caused by the section of it. The stems of Arundo Donax, or of any larger grass, is an excellent subject for seeking it in; it can be readily extracted from them when boiled. It is composed of truncated cylinders, placed one upon the other, and so forming a long cylinder, which be
* Bo@poc, a little pit.
comes a tube, open from end to end when the partitions between the cells are absorbed. The cylinders in some plants, as in Phytocrene, are regular in size, and easily separate from each other, as has been observed by Griffith; in other cases, and this is the most common case, their ends are oblique and produce the appearance of bands when they come in contact (fig. 6.1.): hence they have been looked upon as a modification of the spiral vessel.
2. Continuous Bothrenchyma.—This (fig. 6. a a a) forms the Vasa spiroidea porosa of Link, and is still less of the nature of vascular tissue than the last. It consists of long, slender, uninterrupted, pitted tubes, resembling Pleurenchyma in form, but not tough, nor collected into solid bundles. This is common in the roots of plants, and is often found in connection with spiral vessels.
It has been said by Bischoff that tissue of this kind is an alteration of a spiral vessel, whose fibre is broken into short pieces, which stick to the sides of the tube and cause the pitted appearance. Mr. Slack adopted this idea : he considered them to be transparent spaces in the sides of the cells, and caused by the separation, at intervals, of a spiral fibre whose convolutions are partially and firmly united in the spaces between the dots; and he represents a case of vasiform tissue from Hippuris in illustration of his position. But I have sought in vain for any proof of the correctness of these views. On the contrary, it is probable that the functions of Taphrenchyma are to convey fluid, which is an additional reason for regarding it entirely distinct from vascular tissue.
The granular woody tissue of former editions of this work I now regard as a form of continuous Bothrenchyma. Renewed observation with better instruments satisfy me that the marks on the sides of the tubes of Cycadaceæ, taken by Adolphe Brongniart for pores, and by myself for granules, are neither the one nor the other, but short oblique furrows in the lining of the tubes.
Sect. III. Of Woody Tissue, or Pleurenchyma. Tuis, which Meyen calls Pleurenchyma, consists of very slender, tough, transparent, membranous tubes, tapering acutely to each end, lying in bundles, and, like the cellular tissue, generally having no direct communication with each other, except by invisible pores. Slack states, that they are often met with open at their extremities; “ which probably arises either from the membrane being obliterated where it was applied to another fibre, or ruptured by the presence of an adjoining tube, as we sometimes find the conical extremity of another tube inserted into the aperture.”
Many vegetable anatomists consider it a mere form of cellular tissue, in an elongated state. However true this may be in theory, woody tissue may be known by its toughness and extremely attenuated character. The distinction between cellular and woody tissue is particularly well seen in the long club-shaped aërial radicle of Rhizophora Candelaria. It there consists of large, very long, transparent tubes, lying imbedded in fine brownish granular matter, which is minute cellular tissue (fig. 7.)
Usually it has no markings upon its fig. 7.
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 which give it a peculiar character (Plate II. fig. 3. 5. and 20.); and I have remarked an instance, in Oncidium altissimum, of its having tubercles on its surface. It often contains amylaceous granules in abundance. Generally, while cellular tissue is brittle, and has little or no cohesion, woody tissue 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 tissue; but cotton, which is cellular tissue, bears no comparison as to strength, with either flax or hemp.
Alphonse De Candolle gives the following as the result obtained by Labillardière, as to the relative strength of different