FOOTNOTES:
[366] 'Nature-printed Ferns,' 8vo edition, vol. ii, p. 197.
[367] 'Gard. Chron.,' 1845. p. 790.
[368] 'A New Arrangement of Phænog. Plants,' p. 36.
[369] 'Bull. Soc. Bot. France,' 1856, t. iii, p. 569.
[370] The reader will find an abstract of Mr. Darwin's views in his work on the 'Variation of Animals and Plants,' vol. ii, p. 181.
[371] Loc. cit., i, 336.
CHAPTER III.
ALTERATIONS OF COLOUR.[372]
Changes in the colour of the several organs of plants are more often either pathological or the result of variation than of malformation properly so called.
Alterations in colour arise from a diminished or an increased amount of colouring matter, or from an unusual distribution of the solid or fluid matters on which the colour depends. The superposition of cells containing colouring material of different tints produces naturally a very different set of hues from those which are manifested when the colours are not blended. Referring the reader to the ordinary text-books on vegetable physiology and chemistry for details as to the nature and disposition of colouring materials in plants under natural circumstances, it will only be necessary to cite a few instances of deviation from the general colour of plants or their organs.
Albinism.—This change is due to the deficient formation of green colouring matter or chlorophyll, and is more a pathological condition than a deformity.
It seems necessary to draw a distinction between this state and ordinary blanching or etiolation. In the former case chlorophyll seems never to be formed in the affected parts, even if they be exposed to light, while an etiolated organ, when placed under favorable circumstances, speedily assumes a green colour. In Richardia æthiopica one or more leaves become occasionally as white as the spathe is usually.
Virescence.—Engelmann[373] pointed out that, so far as flowers were concerned, there are two ways in which they assume a green colour, either by a simple development of chlorophyll in place of the colouring matter proper to the flower, or by an actual development of leaf-like organs in the room of the petals—frondescence. Morren[374] judiciously proposed to keep these two conditions separate, calling the one virescence, the other frondescence ([see p. 241]).
Many of the cases recorded as reversions of the parts of the flower to leaves are simply instances of virescence; indeed, it is not in all cases easy to distinguish between the two states. The examination of the arrangement of the veins is often of assistance in determining this point; for instance, if, under ordinary circumstances, the venation of the petal be such as is characteristic of the sheath of the leaf, while in the green-coloured flower of the same species the venation is more like that which belongs to the blade of the leaf, the inference would, of course, be that the green colour was due to frondescence or phyllody.
The persistence or duration of petals is often increased when they are subject to this change; instead of falling off speedily they become persistent when so affected.
Some flowers are more liable to virescence than others. The common honeysuckle, Lonicera Periclymenum, is one of these, and it is noticeable in this plant that the calyx remains unaffected—a circumstance which Morren says shows the distinctness of virescence from frondescence; for, in this instance, we have the most foliaceous portion of the flower remaining unchanged, while the corolla and other organs, usually less leaf-like in their nature, assume a green colour; but this may rather be attributed to the axial nature of the so-called adherent calyx. The stamens in these green-flowered honeysuckles are usually green also, but with abortive anthers, and the pistil also is in a rudimentary condition. Umbelliferæ are not unfrequently subject to this change, e.g., Torilis Anthriscus, Daucus Carota, Heracleum Sphondylium, Carum carui, &c. Primulaceæ, again, are frequently subject to virescence. Among Compositæ the following species are recorded as having had green flowers—Cirsium tricephalodes, Senecio vulgaris, Calendula officinalis, Pyrethrum Parthenium, Carduus crispus, Hypochæris radicata, Hieracium prealtum, Cirsium arvense, Coreopsis Drummondi.[375] In Ranunculaceæ virescence has been observed in Delphinium elatum, crassicaule and Ajacis, Anemone hortensis and nemorosa, Aquilegia vulgaris, Ranunculus Philonotis.
Many of these cases, and others that might be cited, are probably instances of frondescence or phyllody ([see p. 241]).
Chromatism.—This term is here intended to apply specially to those cases in which any organ of a plant assumes a colour approximating to that of the petals, or in which the normal green is replaced by tints of some other colour. To a certain extent the change in question is the same as that spoken of under the head of petalody ([see p. 283]), but there are cases in which, while the ordinary situation and form are those of leaves, the coloration is that of the petals. Such was the case in the Gesnera mentioned by Morren ([see p. 88]), and in which a leaf occupied the position of an inflorescence, and became brightly coloured. In tulips the presence of a highly coloured leaf on the flower-stalk, below the flower, is not uncommon. So also the bracts or leaves below the perianth in Anemone coronaria and hortensis not unfrequently assume the coloration usually confined to the parts of the perianth. A similar illustration has presented itself, as this sheet is passing through the press, in which two of the leaflets of the compound leaf of a rose were brightly coloured like the petals, the others being of their ordinary green colour.
The occurrence of coloured bracts, as in Poinsettia, Bougainvillea, &c., is very common under natural conditions, and need not here be further alluded to.
Increased intensity of colour often accompanies teratological changes; an instance has just been alluded to in the Gesnera; the feather hyacinth, Muscari comosum, furnishes another illustration, the adventitious pedicels being brightly coloured.
In fasciated stems, also, of herbaceous plants, it not unfrequently happens that the upper portions of the stem are brightly coloured.
The occurrence of flowers or fruits of different colours on the same plant, or even in the same cluster, is a phenomenon which does not come within the scope of the present book; the reader may, however, be referred to the excellent summary on this subject published by Mr. Darwin in his work on the 'Variation of Animals and Plants under Domestication.'