346 



NATURE 



[August 10, 1882 



THE COLOURS OF FLOWERS, AS ILLUS- 



TRATED BY THE BRITISH FLORA 1 



III. — Variegation and Retrogression 



SO far we have spoken for the most part as though 

 every flower were of one unvaried hue throughout. 

 We must now add a few considerations on the subject of 

 the spots and lines which so often variegate the petals 

 in certain species. In this connection a hint of Mr. 

 Wallace is full of meaning. Everywhere in nature, he 

 points out, spots and eyes of colour appear on the most 

 highly-modified parts, and this rule applies most notice- 

 ably to the case of petals. Simple regular flowers, like 

 the buttercups and roses, hardly ever have any spots or 

 lines ; but in very modified forms like the labiates and the 

 orchids they are extremely common. 



Structurally speaking, the spots and lines seem to be 

 the direct result of high modification ; but functionally, 

 they act as honey-guides, and for this purpose they have 

 no doubt undergone special selection by the proper in- 

 sects. The case is just analogous to that of the peacock's 

 plumes or the wings of butterflies. In either instance, 

 t'je spots and eye-marks tend to appear on the most 

 highly- modified surfaces; but they are perpetuated and 

 intensified by special selective action. Lines are com- 

 paratively rare on regular flowers, but they tend to appear 

 as soon as the flower becomes even slightly bilateral, and 

 they point directly towards the nectaries. Hence they 

 cannot be mere purposeless products of special modifica- 

 tion ; they clearly subserve a function in the economy of 

 the plant, and that function is the direction of the insect 

 towards the proper place for effecting the fertilisation of 

 the ovary. In the common rhodendron, the connection 

 can be most readily observed with the naked eye, and the 

 honey tested by. the tongue. In this case, one lobe of the 

 corolla secretes a very large drop of nectar in a fold near 

 its base, and the lines of dark spots appear on this lobe 

 alone, pointing directly towards the nectariferous surface. 



The Geraniacece afford an excellent illustration of the 

 general principle. They are on the whole a comparatively 

 high family of polypetals, for their ovary tends to become 

 compound and very complicated, and they have many 

 advanced devices for the dispersion of their seeds. Oralis 

 corniculata, our simplest English form, is pale yellow : 

 O. acetosella is white, with a yellow base, and its veins are 

 delicately tinged with lilac. The flowers of Erodium 

 and Geranium, which are much more advanced, are gene- 

 rally pink or purplish, often marked with paler or darker 

 lines. For the most part, however, these regular forms 

 are fairly uniform in hue; but the South African Pelar- 

 goniums, cultivated in gardens and hot-houses, are slightly 

 bilateral, the two upper petals standing off from the three 

 lower ones ; and these two become at once marked 

 with dark lines, which are in some cases scarcely visible. 

 and in others fairly pronounced. From this simple be- 

 ginning one can trace a gradual progress in heterogeneity 

 of colouring, till at last the most developei bilateral 

 Pelargoniums have the two upper petals of quite a dif- 

 ferent hue from the three lower ones, besides being deeply 

 marked with belts and spots of dappled colour. In the 

 allied Tropaolum or Indian cress (Fig. 21) this tendency 

 is carried still further. Here, the calyx is prolonged 

 into a deep spur, containing the honey, inaccessible to 

 any but a few large insects ; and towards this spur all the 

 lines on the petals converge. 



In most regular flowers, the lines are mere intensifica- 

 tions (or diminutions) of the general colouration along 

 the veins or ribs of the corolla ; and they point towards 

 the base or claw of the petal, where the honey is usually 

 secreted. But in irregular flowers, we often get a higher 

 modification of colour, so that one region of the petal is 

 yellow or white, while another is pink or blue; and these 



1 Continued from p. 326. 



regions often run transversely, not longitudinally. Such 

 modifications usually affect the most highly-altered parts 

 of the irregular flower. 



The common wild pansy, Viola tricolor, affords a good 

 example of complex variegation. Its flowers are purple, 

 white, or yellow ; or have these pigments variously inter- 

 mixed. The two upper pairs of petals are usually the 

 most coloured ; the lower one is broadest, and generally 

 yellow at the base, with dark lines leading towards the 

 spur. Viola palustris exhibits the same tendency in ,1 

 less degree ; it is pale blue, with purple streaks. The 

 whole family is immensely interesting from the present 

 point of view, and should be closely observed by the 

 student at first hand. 



Among regular Corolliflorce, variegation is not very 

 common, though it occurs much oftener than in the poly- 

 petalous classes, especially at the throat of the tube, as in 

 the forget-me-nots (Myosotis) ; but in irregular Corolli- 

 florce it is exceedingly frequent. The Lentibulacea and 

 other small families afford several examples. In the great 

 order of Labiata:, the highly modified lower lip is very 

 often spotted, especially where it is most developed. This 

 is the case in Stachys silvatica, Lamium purpureum, 

 Galeopsis tetrahit, Calamintha acinos, Nepeta eataria, A. 

 glechoma, Ajuga reptans, Scutellaria galericulata, and 

 many other species. Several exotic kinds show the same 

 tendency in a more marked degree. 



The Scrophularinece, however, form perhaps the best 

 example of any. We have noticed already that compara- 

 tively few of these are as blue or as purple as might be 

 expected from their high organisation. The explanation 

 is that they have mostly got beyond the monochromatic 

 stage altogether, and reached the level of intense varie- 

 gation. They are, in fact, a family with profoundly 

 modified flowers, most of which are very specially adapted 

 to very exceptional modes of insect fertilisation. The 

 Veronicas alone among our English genera are simply 

 blue, with white or pink lines ; the others are mostly 

 spotted or dappled. Antirrhinum ma/us is purple, some- 

 times crimson or white, with the curiously closed throat a 

 bright yellow. Linaria cymbalatia is blue or lilac, with 

 white patches, and the palate a delicate primrose. L. 

 spuria is yellow, with a purple throat. L. minor is purple, 

 with a white lower lip and yellow palate. The very 

 strange flowers of Scrophularia have a curious, inde- 

 scribable mixture of brown, green, dingy purple, and buff. 

 Sibthorpia is pink, with the two smaller lobes of the 

 corolla yellow. Digitalis purpurea, the foxglove, is 

 purple, spotted with red and white. Euphrasia, eye- 

 bright, is white or lilac, with purple veins, and the middle 

 lobe of the lower lip yellow. Melampyrum arvense is 

 red, with pink lips and a purple throat. As a rule, the 

 spots or patches of intrusive colour are developed trans- 

 versely near the palate or around the throat. Purple, red, 

 or blue appear to be the prevalent ground-tones, with 

 white and yellow introduced as contrasted tints. 



Among Monocotyledons, such plants as the highly 

 modified Iris genus show similar results. Our own /. 

 faiidissima has blue sepals, with yellow petals and spathu- 

 late stigmas, all much veined. The Orchidacece exhibit 

 the same tendency far more markedly. Orchis maseula, O. 

 maculata, O. laxiflora, and many other British species have 

 the lip spotted (Fig. 22). Inf. militaris and O. hireina, the 

 variegation is even more conspicuous. In O- ustulata, 

 the spots on the lip are raised. The problematical bee- 

 orchid, Ophrys apifera, is singularly dappled on the lip 

 and disk, and has the sepals different in colour from the 

 rest of the flower. Aeeras aul//ropop/tora, the man-orchid, 

 has green sepals and petals, edged with red, and a yellow- 

 lip, pink fringed. Cypripedium ealeeolus, the lady's 

 slipper, Ceplialanthera grandiflora, white helleborine, and 

 most other British species, are similarly very diversified 

 in colour. As to the exotic species, some of them are 

 more peculiarly tinted and blended with half a dozen dif- 



