are for the most part irregular; in Endogens they are usually arranged in bands that follow the course of the principal veins. In these places it is clear that chromule, or colouring matter, is either not found at all or in very small quantity; but the cause of the deficiency is entirely unknown. It is, however, interesting to remark, that variegations of this kind are best preserved in sterile, and are soonest lost in fertile, soil, as if they were in reality an unhealthy state of a plant; a supposition, however, which there are no sufficient grounds otherwise for entertaining.

"We have already seen that all parts which either are green or susceptible of becoming so, decompose the carbonic acid of the sap or of the atmosphere, when they are exposed to solar light; that they part with the oxygen, and fix the carbon in their own tissue. Hence it was natural to conclude that this operation is connected with the formation of a green colour. In fact, when it takes place greenness does ensue; when it does not take place, the organ that developes in darkness preserves the primitive white colour of the tissue; and when it has taken effect incompletely, the results are intermediate between the two preceding cases.

"The deposition of carbon thus induced does not act upon the vegetable membrane; which always retains its original pearly lustre. But it forms a peculiar matter called chlorophyll or green chromule; the abundance or scantiness of which is what causes the different tints of leafy surfaces. The action of the membrane produces some effect, no doubt, either by reason of its own pallid hue, or its transparency or density; or of the hairs with which it is often covered; or of the air which it contains; or, finally, of the waxy matter by which they are protected. But how does it happen that carbon, which is black, is capable of producing a green appearance in vegetation? The old physiologists supposed that it is in reality an intense blue, and not a black; and that, shining through the yellow sides of the cells, the combination of the two colours produced green. This notion, however, is disproved by the most casual inspection, for the colouring matter may be separated from the tissue with the greatest facility, and it still preserves its colour; and, besides, the yellow of tissue, if any, is

so excessively feeble, as to be wholly insufficient to overcome the blueness of the carbon, if it were blue. The fact is, that the cause of carbon in the system of vegetation being green, belongs to that numerous class of facts of which no explanation can be given, in the existing state of human knowledge.

66 Although we are justified by the mass of evidence, in asserting that the green colour of plants is owing to the fixation of carbon in their tissue, in consequence of the power that light possesses of decomposing their carbonic acid, yet there are some exceptions that deserve attention. Humboldt found Poa annua and compressa, Plantago lanceolata, Trifolium arvense, Wallflower, and the Rhizomorpha verticillata, green in the subterranean galleries of the mines of Freyburg, although born in total darkness, but in atmosphere highly charged either with hydrogen or nitrogen. Ferns and Mosses, again, will be green where other plants are blanched; and Humboldt found near the Canaries a Fucus which was bright grass-green, although it had grown at the depth of from 25 to 32 fathoms (190 feet). Now, as light, according to the experiments of Bouguer, after traversing 180 feet, is weakened in the proportion of 1 to 1477-8, this Fucus must have been illuminated where growing by a power 203 times less than that of a candle at a foot's distance. Are we to suppose that this feeble degree of illumination was sufficient to decompose the carbonic acid of such a plant, or was not the decomposition rather owing to the operation of some unknown cause? "Leaves, which, as we very well know, are usually green, may assume different colours in special cases. It is common to see in the autumn this green change to yellow, as in the Lombardy Poplar, &c.; or to red, as in the Berberry, the Sumach, the Virginian Creeper, and many kinds of Oaks. It is remarked that red colours are most common in leaves which contain some kind of acid, as the Vine, the Pear, the Viburnum, the Sorrel, &c. The red colouring matter obtained from leaves forms infusions which, like those from flowers, become more intense when acted upon by acids. Yellow leaves act in this manner like yellow flowers. It is supposed by some, that, while red is owing to the developement of acid,

other colours may be ascribed to the presence of an alkali. This is, however, far from proved.

"The same colours which stain leaves in the autumn may also be produced by certain accidents. Thus the puncture of an insect, the attacks of parasitical fungi, or injury from early frosts, produce partially or entirely yellow or red colours; and, what is remarkable, the colours thus accidentally assumed are the same as the plant would have taken of itself in the autumn: thus accidents turn the leaves of the Poplar and the Lilac yellow, of the Sumach or the Pear tree red, as they become in the autumn.

"Certain leaves offer naturally, on one or both their surfaces, marks coloured in a particular manner, from the moment when they first unfold. Tradescantia discolor, and several Begonias, have their under surface red; certain Arums are irregularly blotched with red; there are species of Amaranth which, in an apparently healthy and natural state, have leaves banded with both yellow and red. It is worthy of note, that in regular and natural colourations red is very common, and yellow comparatively rare, although one would have thought that the latter, caused, as it seems to be, by a slighter kind of change than red, would have been the most common. Blue seems altogether excluded from changes of the leaves, except in the case of certain Eryngoes.

“In many plants, the leaves which grow in the vicinity of flowers are accustomed to offer various tints, which are almost uniformly in unison with the colours of the flowers they accompany; such floral leaves or bracts are yellow in many Euphorbias, scarlet in Sages, violet in Clary, and blue in particular states of the Hydrangea.

"Why then should it be different with petals and the petal-like parts of a flower? These organs are in truth nothing but modified leaves; they are capable in particular cases, such as Hesperis matronalis, of transforming themselves into genuine leaves, green, and capable of exhaling oxygen."

With regard to the exact relation that colours really bear to one another, and to the causes that are supposed to influence them, a memoir upon the colours of flowers, published

at Tubingen, in 1825, by Messrs. Schübler and Funk, is deserving of attention. From their account it appears that flowers may be divided into two great series: those having yellow for their type, and which are capable of passing into red or white, but never into blue; and those of which blue is the type, which can pass into red or white, but never yellow. The first of these series is called by these observers oxidised, and the second disoxidised; and they consider greenness as a state of equilibrium between the two series. De Candolle calls the first series xanthic, and the last cyanic. Upon this principle they admit the following scale, leaving white out of consideration:

It will be at once remarked, in considering these tables, that almost all flowers susceptible of changing colour only do it in general by rising or descending in the series to which they belong. Thus in the xanthic series, the flowers of Marvel of Peru may be yellow, orange-yellow, or red; those of the Austrian Rose, orange-yellow or orange-red; those of the Nasturtium vary from yellow to orange and orange-red; those of the Garden Ranunculus pass through every gradation in the series, from red, to green. As to the cyanic series, the Anemone varies from blue to violet and red; the Hyacinth from green to red through all the gradations; the Lithospermum purpureo-cæruleum from blue to violet-red; and the China Aster from violet-blue to violet, violet-red, and red.

Although there are certain exceptions to these rules, particularly in the Hyacinths, some of whose varieties approach

the xanthic series, yet they are so far conformable to nature as to help us either in searching for the causes of colour, or in predicting the possible varieties of colour in flowers of the same species, and sometimes of the same genus.

Messrs. Schübler and Funk, considering green as the common colour of plants, have attempted to show that other colours are modifications of it, regarding all deviations as owing to the admixture of acid or alkaline secretions, an opinion in which they have been supported by Macaire Prinsep, whose views are adopted by De Candolle and others. But Macquart asserts (Die Farben der Blüthen, 1835) that the chemical theory of Macaire Prinsep is erroneous, and offers quite another explanation of the nature of the changes in vegetable living colours. To understand this, it is necessary to consider the general nature of vegetable fluids, and especially what is really the colouring matter of plants.

Crude sap is colourless, and to a certain extent it remains so to the end of the existence of a plant, filling the cells and many of the intercellular passages. But by degrees it becomes altered, and gains the green colour which is called chlorophyll. This is in some cases a mere gelatinous mass lining the cells, or arranging itself in certain definite forms in different plants: in transverse zones in Conferva zonata and others, in spiral bands in Spirogyra, and in the form of gelatinous threads in many succulent plants; or it acquires a distinctly granular appearance. In the latter case, Mohl states that it is uniformly collected round a grain of starch, which forms its nucleus, or round several grains, as may be ascertained by testing the chlorophyll with iodine. That this is often so, may be easily seen by any one accustomed to delicate microscopical investigations. But I do not find the starch constantly inside the grains of chlorophyll; on the contrary, it is certain that in some instances, as that of Cattleya Forbesii, they are external to the chlorophyll. I have counted as many as nine embedded in the circumference of a single grain of chlorophyll in this plant.

This chlorophyll, although so abundant in plants as to be the exclusive cause of their green colour, is nevertheless,

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