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.

"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.

b. The primary layers upon this.

c. The secondary layers.

"2. The first substance (1. a.) undergoes no apparent change by a short boiling in caustic potash.

"3. The second (1.6,) by short boiling in the caustic alkali is converted into starch, carbonic acid being evolved (granting that starch is the only substance upon which iodine acts so characteristically).

"4. The third (1. c.) by boiling in caustic potash is converted into a peculiar, as yet unknown (?), vegetable principle, which is coloured orange yellow by iodine, Whether in this case carbonic acid be also formed, I will not take upon myself to decide; at least in Experiment VIII., on the addition of sulphuric acid, I did not observe any effervescence. Moreover, this orange colour is as distant as heaven from earth, from the colour produced by adding iodine to vegetable mucus.

"Whether the carbonic acid be formed at the expense of the carbon of the vegetable substance uniting with the oxygen of the air, or by the decomposition of the water, remains still to be investigated; as, likewise, to discover whether by longer boiling, it could take up more carbon, and become converted into oxalic acid.

"The most interesting result is, however, without doubt, that, by the action of the caustic potash, one portion of vegetable matter becomes, by a retrograde metamorphosis, as it were, again converted into starch; a discovery, the extension of which gives promise of most interesting results for organic chemistry."

M. Payen selected with the utmost care the nascent tissue of the ovules of the Almond, Apple, and Sunflower; the halfformed tissue of the Cucumber; the sap of the same plant; the two months' old pith of the Elder; the pith of Æschynomene paludosa; the hairs of Cotton; and the new tissue of spongioles. They gave him the following results:

But when he came to analyse wood in which a deposit had

taken place, he found these proportions materially altered;

When, however, the tissue was acted upon by such agents as have the property of destroying the matter of lignification, the proportions of the three fundamental principles approached more nearly those of primitive tissue.

With reference to the discrepancy between the first and last of these tables, Payen remarks that, as alkalies do not remove all the matter of lignification, it is possible that this substance may consist of two kinds of matter, one of which only is capable of being acted upon by azotic acid. He also adds that, although concentrated sulphuric acid has the same power as nitric acid of separating from the primitive tissue of plants their sedimentary matter, yet it possesses this difference, that it gives it the property of becoming blue when acted upon by iodine; a circumstance which has doubtless given rise to the statement that lignine may be transformed into starch. (Comptes rendus, vii. 1055. 1125.)

CHAPTER II.

ELEMENTARY ORGANS.

THE general properties of the elementary organs are, elasticity, extensibility, contractibility, and permeability to fluids or gaseous matter. The first gives plants the power of bending to the breeze, and of swaying backwards and forwards without breaking. The second enables them to develope with great rapidity when it is necessary for them to do so, and also to give way to pressure without tearing. The third causes parts that have been overstrained to recover their natural dimensions when the straining power is removed, and it permits the mouths of wounded vessels to close up so as to prevent the loss of their contents. The fourth secures the free communication of the fluids through every part of a plant which is not choked up with earthy matter.

The special properties of the elementary organs must be considered separately.

That of these the CELLULAR TISSUE is the most important is apparent by its being the only one of the elementary organs which is uniformly present in plants; and by its being the chief constituent of all those compound organs which are most essential to the preservation of species.

It transmits fluids in all directions. In most cellular plants no other tissue exists, and yet in them a circulation of sap takes place; it constitutes the whole of the medullary rays, conveying the elaborated juices from the bark towards the centre of the stem; all the parenchyma in which the sap is diffused upon entering the leaf, and by which it is exposed to evaporation, light, and atmospheric action, consists of cellular tissue; much of the bark in which the descending current of the sap takes place is also composed of it; and in endogenous plants, where no bark exists, there appears to be no other route that the descending sap can take, than through the cellular substance in which the vascular system is em