"This, at that time, to us, enigmatical phenomenon, was kept in mind until we had an opportunity of instituting comparisons between the climate of Japan and China and our own, and we then concluded that in the case of a plant imported from thence, and exposed to such different climatical influences, the origin of the greater or less imperfection of its sexual organs was probably owing to this change, as we had experienced in Kerria and Camellia; and that the sterility of many other exotic plants might be attributed to the same cause. The difference in the climatical relations of Japan and Europe is very considerable. In Japan, previous to the new growth of Kerria and Camellia, a rainy season of three months' duration prevails; in Europe, on the contrary, dry winds prevail especially in the eastern part, where our plains are often transformed into deserts. Is it, therefore, remarkable that a plant introduced from Japan into Europe, exposed to the influences of this great diversity of climate, should produce imperfect sexual organs incapable of further propagating the plant from seeds? A rich soil, with the necessary amount of moisture, will never engender double flowers."[567]
Mr. Darwin[568] describes a peculiar form of Gentiana Amarella, in which the parts of the flower were more or less replaced by compact aggregations of purple scales in great numbers. A similar condition is, indeed, not uncommon in this plant, and, as Mr. Darwin also remarked, on hard, dry, bare, chalky banks, thus bearing out the views expressed by the writer in the 'Gartenzeitung' just cited. Some double flowers of Potentilla reptans found growing wild near York, and transmitted to the writer by a correspondent, were observed growing along a high wall, in a dry border, close to a beaten path, bordering on a gravel pit, others were found on a raised bank, which, from its elevation and exposure to the sun, was particularly dry.
On the other hand, the double-flowered Cardamine pratensis, which is occasionally found in a wild state, always grows in very wet places.
Of late years a remarkable double-flowered race of Primula sinensis has been obtained. In particular, Messrs. Windebank and Kingsbury, of Southampton, have succeeded in raising a set of plants in which the flowers are very double and very attractive in a florist's point of view. The corollas in these flowers are not merely duplicated, but from their inner surface spring, in some cases, funnel-shaped or tubular petals (p. 315), so regular in form as quite to resemble a perfect corolla. These tubes are attached to the inner side of the tube of the corolla, in the same way as are the stamens, these latter organs being, it appears, absent. The carpels are present, but open at the top, and bear numerous ovules, hence it was at first surmised that these plants were obtained and perpetuated, by the application of pollen from single flowers to these double-flowered varieties.
The raisers of this fine race however assert that "the double kinds are all raised from the seed obtained from single flowers; the double blooms do not produce seed, as a rule, and even if they did yield seed, and it were to germinate, the plants so raised would simply produce single flowers." Semi-double flowers will produce seed, but it is necessary that they should be fertilised with the pollen from the single blooms. They rarely, however, if ever, produce really double flowers when so fertilised, and the number of semi-double flowers, even, is always small, the remainder, and, consequently, the larger part, proving single. To obtain double varieties, the raiser fertilises certain fine and striking single flowers, with the pollen of other equally fine single blooms, and the desired result is obtained. This is Messrs. Windebank and Kingsbury's modus operandi, the exact process or mode of accomplishment being, however, a professional secret.[569]
From what has been said, as well as from other evidence which it is not necessary to detail in this place, it may be seen that the causes assigned by physiologists, and the plans proposed by cultivators for the production of double flowers, are reducible to three heads, which may be classed under Plethora, Starvation, and Sterility. These three seem inconsistent one with the other, but are not so much so as they at first sight appear to be.
Tho advocates of the plethora theory have much in their favour: for instance, the greater frequency of double flowers among cultivated plants than among wild ones. The great preponderance of double flowers in plants derived from the northern hemisphere, when contrasted with those procured from the southern, as alluded to by Dr. Seemann, seems also to point to the effect of cultivation in producing these flowers. Now, although this is, to a large extent, due to the selection that has been for so long a period practised by gardeners, still that process will not account for the appearance of double flowers where no such selection has been exercised; as in the case of wild plants. Some double peas, observed by Mr. Laxton, appeared suddenly; they had not been selected or sought for, but they were produced, as it would appear, as a result of high cultivation, and during the period when the plant was in greatest vigour; and as the energies of the plant failed, so the tendency to produce double flowers ceased. Indeed, in reference to this subject, it is always important to bear in mind the time at which double flowers are produced; thus, an annual plant subjected to cultivation, will, it may be, produce single flowers for the firet year or two, then a few partially double flowers are formed, and from these, by careful selection and breeding, a double-flowered race may be secured. Sometimes, as in the peas before alluded to, in the same season the earlier blossoms are single, while later in the year double blossoms are produced. This happens, not only in annuals, but also in perennials, and is not infrequent in the apple; an illustration of this occurrence in this tree is given in the 'Gardeners' Chronicle' for 1865, p. 554.[570] Sometimes the flowers on a particular branch are double, while those on the rest of the plant are single.[571] On these points, the evidence furnished by a double white hawthorn in the Royal Botanic Gardens at Edinburgh is important. Professor Balfour kindly wrote as follows in reply to an inquiry respecting this plant:—"A double white hawthorn in the Royal Botanic Gardens produced double flowers in spring. It retained its leaves during autumn and winter, until the following spring. It then flowered in the second spring, but produced weak single flowers only, and has continued to do so ever since. The flowering has been always weak, since this change of flowers from double to single. Mr. M'Nab attributes the change in the duration of the leaves to the filling up of the ground round the tree, to the height of a foot and a half on the stem. He is now trying the effect of extra manure in giving extra vigour to the plant." Here, at least, the production of single flowers would seem to be the result of debilitating causes, connected with the unusual persistence of the leaves, &c., for while the tree was healthy, double flowers were produced.
A similar illustration came under the writer's own notice. Some seedling balsams, of a strain which from long selection and hereditary tendency produces, year after year, double flowers were, in the spring (of 1866), allowed to remain in the seed-pans for many weeks after they were ready to be potted off; they were hence partly starved, and when they bloomed, they produced single flowers only. But these same plants, when more liberally treated, produced an abundance of double flowers. Moreover, other seedlings of the same batch, but sown later, and potted off at the usual time, produced double flowers as usual. Of a like character is the fact that the double Ranunculus asiaticus loses its doubleness if the roots are planted in a poor soil.
On the other hand, the way in which double stocks are stated to be produced at Erfurt, viz.: by giving the plants a minimum supply of water, and the other circumstances alluded to as showing the connection between the production of double flowers, and a deficiency of water, as well as the experiments of Mr. Monro, go to show that, so far from plethora, the inducing cause must be more nearly allied to inanition, though the impoverishing process is, to a certain extent, counteracted by only allowing a few of the seed-pods to ripen, and thus concentrating in a small number of flowers the nutriment intended for many.
Professor Edward Morren ('Bull. Acad. Roy. Belg.,' 2me ser., vol. xix, p. 224) considers the existence of true variegation in leaves, and the production of double flowers, as antagonistic one to the other; the former is a sign of weakness, the latter of strength. But it would seem that the exceptions are so numerous—so many cases of the co-existence of variegated leaves, and double flowers are known, at least in individual plants if not in species—that no safe inferences can be drawn as to this point. Since the above remarks were printed, Professor Morren has published a second paper on the subject, upholding his former views as to the incompatibility of variegated foliage (not mere colouration) and double flowers. In this paper he criticises the objections raised by the present writer and others, and examines some of the alleged exceptions. Some of these the Belgian savant finds to prove his rule, inasmuch as although there is a co-existence of variegated foliage and double flowers in these illustrations, yet the plants are weakly, the flowers ill formed, or fall off before expansion. Admitting all this, there still remain cases in which double flowers and variegated foliage do exist in conjunction, and where the plants are vigorous and the flowers well developed. Instances of this are known to cultivators in species of Dianthus, Hemerocallis, Althæa, Pæonia, Rosa, Ranunculus, Serissa, Saponaria, etc., and probably the art of the cultivator would speedily be successful in raising other examples, were it a matter of importance or interest to them to do so. At any rate, the existence of a few unimpeachable illustrations is sufficient to support the opinion of the present writer, and objected to so strongly by M. Morren that, in the present state of our knowledge, "no safe inferences can be drawn" from the facts alluded to by the Belgian professor.[572]