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OF THE CHEMICAL CONSTITUTION OF THE ELEMENTARY
The tissue of plants, as it is first generated, and before it is incrusted with the peculiar secretions formed by the leaves, consists exclusively of oxygen, hydrogen, and carbon: and it is probable that none of the kinds of tissue differ originally in their proportions of these three principles; for the microscope does not show a difference in the action of chemical agents
I mention this because Mr. Rigg has arrived at a different conclusion, the accuracy of which has been insisted upon by the Rev. J. B. Reade, in a paper printed in Taylor's Magazine (Nov. 1837). It must be, however, apparent to any person conversant with vegetable anatomy, that such a separation of tissue as in this case is supposed to have been obtained is physically impossible, and, consequently, the results given are fallacious.
The subject has been subsequently taken up by Schleiden and Payen, whose experiments, made independently of each other, and in entirely different ways, both lead to the conclusion that the original tissue of plants is in all cases of the same chemical constitution, or nearly so, but that the sedimentary deposit which is formed inside each sac of tissue is of some other chemical nature; the lignine of chemists is therefore composed of two or more different substances, viz. the primitive tissue and its subsequent incrustations. As this subject is important with reference to many phenomena in vegetable physiology, I give at some length the results of both Schleiden and Payen.
The former makes a statement in Wiegman's Archives to the following effect:
“ In the manuals which treat of organic chemistry, we generally find woody fibre treated of as a proximate element
amongst the indifferent vegetable substances, along with starch, gum, sugar, &c. It is only lately that Reade has endeavoured to earn the merit of analysing the different forms of organised vegetable substances when separated, but I doubt whether any thing available for science has resulted therefrom. I will, however, in no wise intimate that Mr. Reade has not made use of really isolated spiral vessels in his analysis, since he expressly asserts it; but he does not once mention the remotest attempt to separate the interior matter from the cells and vessels, which necessarily must have been done if any value were to be attached to the result of the elementary analysis.
“ This question arises with every one who knows that the greater part of vegetable tissue consists of a pellucid membrane, and the formations deposited on its inner surface: Are this membrane and the subsequent deposits formed of the same chemical substance? In fact, as we know from Mohl and Meyen that the increased thickness of the walls of cells consists of several layers, that even the spiral fibres are composed of an original fibre, and a subsequently deposited covering surrounding it, which I have found confirmed in innumerable instances, the further question arises whether both the single layers of incrustation, and the (additional ?) parts of the spiral fibres, are not different from each other. As there can be no mechanical separation of such closely combined and microscopic parts, nothing can be done further than to superadd to chemical examination the use of the microscope, and by this means to observe the action of chemical reagents on the different elementary parts of the vegetable structure.
66 I. I had made fine sections of an internodium of Arundo Donax an inch in diameter, and boiled them for some minutes in a solution of caustic potash. On bringing the section again under the microscope I was surprised by a peculiar appearance. A few ringed and spiral vessels were cut through, so that one could plainly see the section of their very thick fibre. By the boiling in caustic potash the spiral vessel was acted upon in its different parts in a very peculiar manner. The exterior enveloping membrane (the original wall of the
cell) was apparently not in the slightest degree altered ; it was still firm, close, transparent, and clear as water. The fibre itself consisted of two component parts; namely, of a (primary ?) fibre lying close to the wall of the cell, and of an enveloping membrane surrounding the fibre on the three free sides in the interior of the cell. The caustic potash had coloured this enveloping membrane of a somewhat darker yellow, otherwise it was firm and apparently unaltered; the primary fibre, on the contrary, was changed into a gelatinous mass, so that on the plane of the section it was swelled up into a pretty considerable elevation. Unfortunately I did not follow up or vary these interesting observations, till after I had thrown away the remainder of the fresh piece of the Donax.
“II. The next experiment I instituted was on the leaves of Pleurothallis ruscifolia. The greater part of the cells of this plant contain beautiful spiral fibres, which appear to grow firmly against the walls of the cells. These fibres are all very broad and flat, like a riband, their thickness varying according to their position. Those cells which are situated vertically, immediately under the epidermis of the under side of the leaf, contain a thicker fibre than the less regularly formed cells, which are separated from the latter by a layer of green parenchyma, and from the upper epidermis of the leaf by an occasionally broken layer of colourless cells, mostly with plain walls. After I had boiled fine sections of this leaf in caustic potash for a few minutes, and again examined them, I found that the spires of the first-mentioned layer had become entirely separated from the walls of the cells. Under the simple microscope I could easily tear up single cells with a needle, and isolate the whole spiral fibre uninjured. Moreover, all the fibres were tumefied, and had acquired a gelatinous appearance from the action of the caustic potash. I now added a drop of sulphuric acid, which neutralised the potash with effervescence, and I then added an alcoholic solution of iodine. On again bringing the object under the microscope I was most agreeably surprised. All the spiral fibres, according to the varying thickness of the section (hence the unequal action of the caustic potash), appeared of different shades, from claret colour to the deepest violet. In those places where the section was not more than a single cell in thickness, a difference between the fibres in the above-mentioned layers was visible, inasmuch as those of the under side of the leaf (the thickest), even where they were most deeply coloured, did not appear of a pure violet colour, but redder, somewhat as if there had been a slight addition of orange. These fibres were also evidently less tumid, and the boundaries were more clearly defined. Those in the middle of the leaf, on the contrary, appeared quite gelatinous, and were coloured of a light blue. The membrane of the cells was in all cases clear as water, and colourless. This was not all: those cells which contained no spiral fibres, and which before, when magnified 230 times, appeared to consist of quite simple walls, even those of the green parenchyma, appeared now completely pitted; the primitive membrane and its pits were clear as water, and colourless, whilst the pits of the thickening layer were of a violet colour.
“ III. I now took for comparison a woody stalk of Rosmarinus officinalis, and treated it in precisely the same manner. The result differed slightly from the above. The cells of the pith are here very thick-sided and pitted, as are also the exterior cells. The wood consists of the medullary sheath, of spiral vessels, and of prosenchymatous cells, the walls of which are just like the woody cells of very young coniferous wood. Here, in every part except the youngest annual rings, the original membrane (even of the spiral vessels) was not coloured, whilst those parts superposed, and even the spiral fibres, were deep orange. The cells of the youngest annual ring, on the contrary, appeared slightly pitted and very pale blue.
“ IV. A species of Pelargonium, when submitted to the same action, gave the same results, only that the thin-walled but pitted exterior cells were also coloured blue.
“ V. In the Teltow Turnip and Carrot, the primitive walls of the cells remained colourless; the incrusting layers of the same became blue; whilst, on the contrary, the fibres of the spiral and reticulated vessels became deep orange.
• VI. The spiral fibres, in the cells of a leaf of Oncidium altissimum, which had been preserved for seven months in weak alcohol (of about 30°), were coloured orange. The spires here consist, however, of two parts, which on the plane of the section could be easily distinguished, as in Arundo Donax; and I imagine that the spire in Pleurothallis consists only of the inner original fibre. I was not able to institute any experiments with fresh leaves of this plant, and have therefore not been able to decide this question with certainty. The original cellular membrane remained here, as in the first-mentioned cases, colourless, and the layers of increase became blue.
“ VII. Opuntia monacantha gave the same results. In all the cells which were completely converted into wood, the additional layers, whether spiral or pitted, became of a deep orange colour, those of the pith and bark blue, and the primary cellular membrane still remained clear as water.
“ An Echinocactus gave the same result.
66 VIII. The wood of Betula alba and Populus tremula, when submitted to the above manipulation, showed nothing but pitted formations, the primitive membrane of which remained colourless, whilst the layers of increase were coloured dark orange.
“ IX. A five years' old shoot of the trunk of Pinus silvestris gave, as regards the original walls of the cells, confirmation of the former constant results. The layers of increase were coloured orange, the cells of the bark and the youngest annual rings light blue.
“ It is of course to be understood, that, by comparative experiments on all these plants, I had previously satisfied myself of the absence of starch in the cells in question.
“ The foregoing, though only preliminary experiments, seem to indicate the following results:
“1. Vegetable tissue consists of three distinct chemical substances :
a. The original membrane of the cells.
c. The secondary layers. “ 2. The first substance (1. a.) undergoes no apparent change by a short boiling in caustic potash.