The raisers of this fine race however a.s.sert 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 a.s.signed by physiologists, and the plans proposed by cultivators for the production of double flowers, are reducible to three heads, which may be cla.s.sed 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 ill.u.s.tration 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 ill.u.s.tration 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 circ.u.mstances 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 ill.u.s.trations, 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_, _Althaea_, _Paeonia_, _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 ill.u.s.trations 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]

Mr. Darwin[573] has thrown out the suggestion that the cause for the appearance of double flowers may be sought for in some previous state of things, bringing about sterility or imperfect formation, or functional activity of the genitalia of the flower, and consequent compensatory increase of the petaline element, either in the form of an increased number of bracts, petals, &c., or in the subst.i.tution of petals for stamens and pistils, &c.

In considering these points the question arises whether they can be reconciled one with another. And there is little doubt but that they may be. The production of a flower is preceded by an arrest of vegetation; this is obvious: the current of the plant's life becomes changed, the growth of the leaves is checked, the lengthening of the branches is arrested as the flower-bud forms; moreover, there is a close relationship in a large majority of flowers between the outer envelopes of the flower and the scales of a leaf-bud; this is especially so in regard to the venation, and is admitted by all morphologists. So far, then, it may be said that the production of a flower, like that of a bud, is due to a diminution of vegetative action; and as in double flowers we have, for the most part, merely a repet.i.tion and exuberant formation of floral envelopes, so we may attribute their formation to a continuance of the same feeble vegetative action as that which produced the first or normal series. How, then, can a copious supply of rich food, such as is provided by cultivation, produce double flowers? To this question, according to our theory, the reply would be that the quant.i.ty of food is excessive, more than the plant can properly digest; and hence vegetative action is stopped, at least partially--pretty much as it would be if the plant were placed in the opposite condition of starvation. The effect of supplying a plant (or an animal) with an excessive supply of food, which it cannot a.s.similate, is in many respects similar to that which results from partially cutting off the supplies. And the same reasoning applies to sterility. If by high culture, or the supply of an undue quant.i.ty of nourishment, the const.i.tution of the plant be impaired, or if the plant be pampered, it is no wonderful thing that sterility should ensue. Hence, then, may it not be a.s.serted as a general principle that in the production of double flowers a partial arrest of development, if not of growth, however produced, is an essential preliminary? All the attendant phenomena, such as the obliteration of the stamens, the augmentation in the number of floral whorls, the occurrence of prolification, are consistent with the supposition of a primary arrest of development, more or less complete, as the case may be: at one time permanent, at another time relaxed and intermittent, or in a third set of cases the vegetative activity or power of growth may be restored, and from the centre of the flower may spring a perfect branch with perfect leaves, the production of sheaths only being superseded by the development of leaves, in which all the parts--sheath, stalk, and blade--are present.

When once the disposition to form double flowers is established, that tendency becomes hereditary: there are races of single Stocks in which, out of hundreds of plants, scarcely one double-flowered form is met with; but when the tendency to produce double blooms is set up, single flowers become the exception: thus, in the Balsams, before mentioned, not one in fifty now produces single flowers, and the seeds of these double Balsams produce double-flowered seedlings, with scarcely a "rogue" among them.

The following list of plants producing double flowers of any kind is taken from that given in 'Seemann's Journal of Botany,' vol. ii, p. 177, and to which some additions have been made. Miscalled double flowers, such as those of the _Compositae_, _Viburnum Hydrangea_, &c., are excluded.

RANUNCULACEae.

Clematis Viticella, _Linn._, S. Europe.

florida, _Thunb._, j.a.pan.

Fortunei, _Moore_, j.a.pan.

patens, _Desne_, j.a.pan.

Anemone j.a.ponica, _Sieb. et Zucc._, j.a.pan.

coronaria, _Linn._, S. Europe, Asia Minor.

hortensis, var. _Linn._, S. Europe.

palmata, _Linn._, N. Africa, Spain, Portugal.

nemorosa, _Linn._, Europe, N. America, Siberia.

sylvestris, _Linn._, S. Europe, Siberia.

Hepatica triloba, _Chaix._, Europe.

Ranunculus bulbosus, _Linn._, Europe, N. Amer.

repens, _Linn._, Europe, Siberia, N. Amer.

acris, _Linn._, Europe, Siberia.

aconitifolius, _Linn._, Europe.

gramineus, _Linn._, Italy, France, Portugal, Switzerland.

bullatus, _Linn._, S. Europe.

asiaticus, _Linn._, The East.

Ficaria ranunculoides, _Moench._, Europe.

Thalictrum anemoides, _Michae._, N. America.

Caltha pal.u.s.tris, _Linn._, Europe, Asia, N. America.

Trollius europaeus, _Linn._, Europe.

nepalensis, Himalaya.

Nigella damascena, _Linn._, Mediterranean.

Aquilegia vulgaris, _Linn._, Europe.

canadensis, _Linn._, N. America.

Delphinium Ajacis, _Linn._, S. Europe.

grandiflorum, _Linn._, Siberia, N. America.

Consolida, _Linn._, Europe, N. America.

cheilanthum, _Fisch._, Siberia.

elegans, _D. C._, North America.

Adonis autumnalis, _Linn._, Europe.

vernalis, _Linn._, Europe, Asia.

Paeonia Moutan, _Sims_, China, j.a.pan.

officinalis, _Retz._, Europe.

tenuifolia, _Linn._, Tauria.

albiflora, _Pall._, Siberia.

paradoxa, _Andr._, S. Europe.

NYMPHaeACEae.

Nelumbium speciosum, _Willd._, Africa, Asia.

BERBERIDACEae.

Berberis, _sp. cult._

PAPAVERACEae.

Papaver Rhoeas, _Linn._, Europe.

bracteatum, _Lindl._, Russia.

somniferum, _Linn._, S. Europe, Asia Minor, Egypt.

Chelidonium majus, _Linn._, Europe, Asia.

Sanguinaria canadensis, _Linn._, N. America.

Podophyllum peltatum, _Linn._, N. America.

CRUCIFERae.

Mathiola incana, _R. Br._, Mediterranean.

glabrata, _D. C._ annua, _Sweet._, South Europe, Syria.

Cheiranthus Cheiri, _Linn._, Europe.

Iberis umbellata, _Linn._, Europe.

amara, _Linn._, Europe.

Cardamine pratensis, _Linn._, Europe, Asia, Africa, America.

Hesperis matronalis, _Linn._, Europe, Siberia.

Barbarea vulgaris, _R. Br._, Europe.

Sinapis arvensis, _Linn._, Europe.

Bra.s.sica oleracea. _Linn._, Europe.

CISTACEae.

Helianthemum vulgare, _s.p.a.ch._, Europe, N. Africa.