Before giving a summary on Bud-variation I will discuss some singular and anomalous cases, which are more or less closely related to this same subject. I will begin with the famous case of Adam's laburnum or Cytisus Adami, a form or hybrid intermediate between two very distinct species, namely, C. laburnum and purpureus, the common and purple laburnum; but as this tree has often been described, I will be as brief as I can.
Throughout Europe, in different soils and under different climates, branches on this tree have repeatedly and suddenly reverted to both parent-species in their flowers and leaves. To behold mingled on the same tree tufts of dingy-red, bright yellow, and purple flowers, borne on branches having widely different leaves and manner of growth, is a surprising sight. The same raceme sometimes bears two kinds of flowers; and I have seen a single flower exactly divided into halves, one side being bright yellow and the other purple; so that one half of the standard-petal was yellow and of larger size, and the other half purple and smaller. In another flower the whole corolla was bright yellow, but exactly half the calyx was purple. In another, one of the dingy-red wing-petals had a bright yellow narrow stripe on it; and lastly, in another flower, one of the stamens, which had become slightly foliaceous, was half yellow and half purple; so that the tendency to segregation of character or reversion affects even single parts and organs.[[899]] The most remarkable fact about this tree is that in its intermediate state, even when growing near both parent-species, it is quite sterile; but when the flowers become pure yellow or pure purple they yield seed. I believe that the pods from the yellow flowers yield a full complement of seed; they certainly yield a large number. Two seedlings raised by Mr. Herbert from such seed[[900]] exhibited a purple tinge on the stalks of their flowers; but several seedlings raised by myself resembled in every character the common laburnum, with the exception that some of them had remarkably long racemes: these seedlings were perfectly fertile. That such purity of character and fertility should be suddenly reacquired from so hybridized and sterile a form is an astonishing phenomenon. The branches with purple flowers appear at first sight exactly to resemble those of C. purpureus; but on careful comparison I found that they differed from the pure species in the shoots being thicker, the leaves a little broader, and the flowers slightly shorter, with the corolla and calyx less brightly purple: the basal part of the standard-petal also plainly showed a trace of the yellow stain. So that the flowers, at least in this instance, had not perfectly recovered their true character; and in accordance with this, they were not perfectly fertile, for many of the pods contained no seed, some produced one, and very few contained as many as two seeds; whilst numerous pods on a tree of the pure C. purpureus in my garden contained three, four, and five fine seeds. The pollen, moreover, was very imperfect, a multitude of grains being small and shrivelled; and this is a singular fact; for, as we shall immediately see, the pollen-grains in the dingy-red and sterile flowers on the parent-tree, were, in external appearance, in a much better state, and included very few shrivelled grain. Although the pollen of the reverted purple flowers was in so poor a condition, the ovules were well-formed, and, when mature, germinated freely with me. Mr. Herbert also raised plants from seeds of the reverted purple flowers, and they differed very little from the usual state of C. purpureus; but this expression shows that they had not perfectly recovered their proper character.
Prof. Caspary has examined the ovules of the dingy-red and sterile flowers in several plants of C. adami on the Continent,[[901]] and finds them generally monstrous. In three plants examined by me in England, the ovules were likewise monstrous, the nucleus varying much in shape, and projecting irregularly beyond the proper coats. The pollen-grains, on the other hand, judging from their external appearance, were remarkably good, and readily protruded their tubes. By repeatedly counting, under the microscope, the proportional number of bad grains, Prof. Caspary ascertained that only 2.5 per cent. were bad, which is a less proportion than in the pollen of three pure species of Cytisus in their cultivated state, viz. C. purpureus, laburnum, and alpinus. Although the pollen of C. adami is thus in appearance good, it does not follow, according to M. Naudin's observations[[902]] on Mirabilis, that it would be functionally effective. The fact of the ovules of C. adami being monstrous, and the pollen apparently sound, is all the more remarkable, because it is opposed to what usually occurs not only with most hybrids,[[903]] but with two hybrids in the same genus, namely in C. purpureo-elongatus, and C. alpino-laburnum. In both these hybrids, the ovules, as observed by Prof. Caspary and myself, were well-formed, whilst many of the pollen-grains were ill-formed; in the latter hybrid 20.3 per cent., and in the former no less than 84.8 per cent. of the grains were ascertained by Prof. Caspary to be bad. This unusual condition of the male and female reproductive elements in C. adami has been used by Prof. Caspary as an argument against this plant being considered as an ordinary hybrid produced from seed; but we should remember that with hybrids the ovules have not been examined nearly so frequently as the pollen, and they may be much oftener imperfect than is generally supposed. Dr. E. Bornet, of Antibes, informs me (through Mr. J. Traherne Moggridge) that with hybrid Cisti the ovarium is frequently deformed, the ovules being in some cases quite absent, and in other cases incapable of fertilisation.
Several theories have been propounded to account for the origin of C. adami, and for the transformations which it undergoes. These transformations have been attributed by some authors to simple bud-variation; but considering the wide difference between C. laburnum and purpureus, both of which are natural species, and considering the sterility of the intermediate form, this view may be summarily rejected. We shall presently see that, with hybrid plants, two different embryos may be developed within the same seed and cohere; and it has been supposed that C. adami might have thus originated. It is known that when a plant with variegated leaves is budded on a plain stock, the latter is sometimes affected, and it is believed by some that the laburnum has been thus affected. Thus Mr. Purser states[[904]] that a common laburnum-tree in his garden, into which three grafts of the Cytisus purpureus had been inserted, gradually assumed the character of C. adami; but more evidence and copious details would be requisite to make so extraordinary a statement credible.
Many authors maintain that C. adami is a hybrid produced in the common way by seed, and that it has reverted by buds to its two parent-forms. Negative results are of little value; but Reisseck, Caspary, and I myself, tried in vain to cross C. laburnum and purpureus; when I fertilised the former with pollen of the latter, I had the nearest approach to success, for pods were formed, but in sixteen days after the withering of the flowers they fell off. Nevertheless, the belief that C. adami is a spontaneously produced hybrid between these two species is strongly supported by the fact that hybrids between these species and two others have spontaneously arisen. In a bed of seedlings from C. elongatus, which grew near to C. purpureus, and was probably fertilised by it, through the agency of insects (for these, as I know by experiment, play an important part in the fertilisation of the laburnum), the sterile hybrid C. purpureo-elongatus appeared.[[905]] Thus, also, Waterer's laburnum, the C. alpino-laburnum,[[906]] spontaneously appeared, as I am informed by Mr. Waterer, in a bed of seedlings.
On the other hand, we have a clear and distinct account given by M. Adam, who raised the plant, to Poiteau,[[907]] showing that C. adami is not an ordinary hybrid. M. Adam inserted in the usual manner a shield of the bark of C. purpureus into a stock of C. laburnum; and the bud lay dormant, as often happens, for a year; the shield then produced many buds and shoots, one of which grew more upright and vigorous with larger leaves than the shoots of C. purpureus, and was consequently propagated. Now it deserves especial notice that these plants were sold by M. Adam, as a variety of C. purpureus, before they had flowered; and the account was published by Poiteau after the plants had flowered, but before they had exhibited their remarkable tendency to revert into the two parent-species. So that there was no conceivable motive for falsification, and it is difficult to see how there could have been any error. If we admit as true M. Adam's account, we must admit the extraordinary fact that two distinct species can unite by their cellular tissue, and subsequently produce a plant bearing leaves and sterile flowers intermediate in character between the scion and stock, and producing buds liable to reversion; in short, resembling in every important respect a hybrid formed in the ordinary way by seminal reproduction. Such plants, if really thus formed, might be called graft-hybrids.
I will now give all the facts which I have been able to collect illustrative of the above theories, not for the sake of merely throwing light on the origin of C. adami, but to show in how many extraordinary and complex methods one kind of plant may affect another, generally in connection with bud-variation. The supposition that either C. laburnum or purpureus produced by ordinary bud-variation the intermediate and the other form, may, as already remarked, be absolutely excluded, from the want of any evidence, from the great amount of change thus implied, and from the sterility of the intermediate form. Nevertheless such cases as nectarines suddenly appearing on peach-trees, occasionally with the fruit half-and-half in nature,—moss-roses appearing on other roses, with the flowers divided into halves, or striped with different colours,—and other such cases, are closely analogous in the result produced, though not in origin, with the case of C. adami.
A distinguished botanist, Mr. G. H. Thwaites,[[908]] has recorded a remarkable case of a seed from Fuchsia coccinea fertilised by F. fulgens, which contained two embryos, and was "a true vegetable twin." The two plants produced from the two embryos were "extremely different in appearance and character," though both resembled other hybrids of the same parentage produced at the same time. These twin plants "were closely coherent, below the two pairs of cotyledon-leaves, into a single cylindrical stem, so that they had subsequently the appearance of being branches on one trunk." Had the two united stems grown up to their full height, instead of dying, a curiously mixed hybrid would have been produced; but even if some of the buds had subsequently reverted to both parent-forms, the case, although more complex, would not have been strictly analogous with that of C. adami. On the other hand, a mongrel melon described by Sageret[[909]] perhaps did thus originate; for the two main branches, which arose from two cotyledon-buds, produced very different fruit,—on the one branch like that of the paternal variety, and on the other branch to a certain extent like that of the maternal variety, the melon of China.
The famous bizzarria Orange offers a strictly parallel case to that of Cytisus adami. The gardener who in 1644 in Florence raised this tree, declared that it was a seedling which had been grafted; and after the graft had perished, the stock sprouted and produced the bizzarria. Gallesio, who carefully examined several living specimens and compared them with the description given by the original describer P. Nato,[[910]] states that the tree produces at the same time leaves, flowers, and fruit, identical with the bitter orange and with the citron of Florence, and likewise compound fruit with the two kinds either blended together, both externally and internally, or segregated in various ways. This tree can be propagated by cuttings, and retains its diversified character. The so-called trifacial orange of Alexandria and Smyrna[[911]] resembles in its general nature the bizzarria, but differs from it in the sweet orange and citron being blended together in the same fruit, and separately produced on the same tree: nothing is known of its origin. In regard to the bizzarria, many authors believe that it is a graft-hybrid; Gallesio on the other hand thinks that it is an ordinary hybrid, with the habit of partially reverting by buds to the two parent-forms; and we have seen in the last chapter that the species in this genus often cross spontaneously.
Here is another analogous, but doubtful case. A writer in the 'Gardener's Chronicle'[[912]] states that an Æsculus rubicunda in his garden yearly produced on one of its branches "spikes of pale yellow flowers, smaller in size and somewhat similar in colour to those of Æ. flava." If as the editor believes Æsculus rubicunda is a hybrid descended on one side from Æ. flava, we have a case of partial reversion to one of the parent-forms. If, as some botanists maintain, Æ. rubicunda is not a hybrid, but a natural species, the case is one of simple bud-variation.
The following facts show that hybrids produced from seed in the ordinary way, certainly sometimes revert by buds to their parent-forms. Hybrids between Tropæolum minus and majus[[913]] at first produced flowers intermediate in size, colour, and structure between their two parents; but later in the season some of these plants produced flowers in all respects like those of the mother-form, mingled with flowers still retaining the usual intermediate condition. A hybrid Cereus between C. speciosissimus and phyllanthus,[[914]] plants which are widely different in appearance, produced for the first three years angular, five-sided stems, and then some flat stems like those of C. phyllanthus. Kölreuter also gives cases of hybrid Lobelias and Verbascums, which at first produced flowers of one colour, and later in the season flowers of a different colour.[[915]] Naudin[[916]] raised forty hybrids from Datura lævis fertilised by D. stramonium; and three of these hybrids produced many capsules, of which a half, or quarter, or lesser segment was smooth and of small size like the capsule of the pure D. lævis, the remaining part being spinose and of larger size like the capsule of the pure D. stramonium: from one of these composite capsules, plants were raised which perfectly resembled both parent-forms.
Turning now to varieties. A seedling apple, conjectured to be of crossed parentage, has been described in France,[[917]] which bears fruit, with one half larger than the other, of a red colour, acid taste, and peculiar odour; the other side being greenish-yellow and very sweet: it is said scarcely ever to include perfectly developed seed. I suppose that this is not the same tree with that which Gaudichaud[[918]] exhibited before the French Institute, bearing on the same branch two distinct kinds of apples, one a reinette rouge, and the other like a reinette canada jaunâtre: this double-bearing variety can be propagated by grafts, and continues to produce both kinds; its origin is unknown. The Rev. J. D. La Touche sent me a coloured drawing of an apple which he brought from Canada, of which half, surrounding and including the whole of the calyx and the insertion of the footstalk, is green, the other half being brown and of the nature of the pomme gris apple, with the line of separation between the two halves exactly defined. The tree was a grafted one, and Mr. La Touche thinks that the branch which bore this curious apple sprung from the point of junction of the graft and stock: had this fact been ascertained, the case would probably have come into the small class of graft-hybrids presently to be given. But the branch may have sprung from the stock, which no doubt was a seedling.
Prof. H. Lecoq, who has made a great number of crosses between the differently coloured varieties of Mirabilis jalapa,[[919]] finds that in the seedlings the colours rarely combine, but form distinct stripes; or half the flower is of one colour and half of a different colour. Some varieties regularly bear flowers striped with yellow, white, and red; but plants of such varieties occasionally produce on the same root branches with uniformly coloured flowers of all three tints, and other branches with half-and-half coloured flowers and others with marbled flowers. Gallesio[[920]] crossed reciprocally white and red carnations, and the seedlings were striped; but some of the striped plants also bore entirely white and entirely red flowers. Some of these plants produced one year red flowers alone, and in the following year striped flowers; or conversely, some plants, after having borne for two or three years striped flowers, would revert and bear exclusively red flowers. It may be worth mentioning that I fertilised the Purple Sweet-pea (Lathyrus odoratus) with pollen from the light-coloured Painted Lady: seedlings raised from one and the same pod were not intermediate in character, but perfectly resembled both parents. Later in the summer, the plants which had at first borne flowers identical with those of the Painted Lady, produced flowers streaked and blotched with purple; showing in these darker marks a tendency to reversion to the mother-variety. Andrew Knight[[921]] fertilised two white grapes with pollen of the Aleppo grape, which is darkly variegated both in its leaves and fruit. The result was that the young seedlings were not at first variegated, but all became variegated during the succeeding summer; besides this, many produced on the same plant bunches of grapes which were all black, or all white, or lead-coloured striped with white, or white dotted with minute black stripes; and grapes of all these shades could frequently be found on the same footstalk.
In most of these cases of crossed varieties, and in some of the cases of crossed species, the colours proper to both parents appeared in the seedlings, as soon as they first flowered, in the form of stripes or larger segments, or as whole flowers or fruit of two kinds borne on the same plant; and in this case the appearance of the two colours cannot strictly be said to be due to reversion, but to some incapacity of fusion, leading to their
segregation. When, however, the later flowers or fruit, produced during the same season or during a succeeding year or generation, become striped or half-in-half, &c., the segregation of the two colours is strictly a case of reversion by bud-variation. In a future chapter I shall show that, with animals of crossed parentage, the same individual has been known to change its character during growth, and to revert to one of its parents which it did not at first resemble. From the various facts now given there can be no doubt that the same individual plant, whether a hybrid or a mongrel, sometimes returns in its leaves, flowers, and fruit, either wholly or by segments, to both parent-forms, in the same manner as the Cytisus adami, and the Bizzarria Orange.
We will now consider the few facts which have been recorded in support of the belief that a variety when grafted or budded on another variety sometimes affects the whole stock, or at the point of junction gives rise to a bud, or graft-hybrid, which partakes of the characters of both stock and scion.
It is notorious that when the variegated Jessamine is budded on the common kind, the stock sometimes produces buds bearing variegated leaves: Mr. Rivers, as he informs me, has seen instances of this. The same thing occurs with the Oleander.[[922]] Mr. Rivers, on the authority of a trustworthy friend, states that some buds of a golden-variegated ash, which were inserted into common ashes, all died except one; but the ash-stocks were affected,[[923]] and produced, both above and below the points of insertion of the plates of bark bearing the dead buds, shoots which bore variegated leaves. Mr. J. Anderson Henry has communicated to me a nearly similar case: Mr. Brown, of Perth, observed many years ago, in a Highland glen, an ash-tree with yellow leaves; and buds taken from this tree were inserted into common ashes, which in consequence were affected, and produced the Blotched Breadalbane Ash. This variety has been propagated, and has preserved its character during the last fifty years. Weeping ashes, also, were budded on the affected stocks, and became similarly variegated. Many authors consider variegation as the result of disease; and on this view, which however is doubtful, for some variegated plants are perfectly healthy and vigorous, the foregoing cases may be looked at as the direct result of the inoculation of a disease. Variegation is much influenced, as we shall hereafter see, by the nature of the soil in which the plants are grown; and it does not seem improbable that whatever change in the sap or tissues certain soils induce, whether or not called a disease, might spread from the inserted piece of bark to the stock. But a change of this kind cannot be considered to be of the nature of a graft-hybrid.
There is a variety of the hazel with dark-purple leaves, like those of the copper-beech: no one has attributed this colour to disease, and it apparently is only an exaggeration of a tint which may often be seen on the leaves of the common hazel. When this variety is grafted on the common hazel,[[924]] it sometimes colours, as has been asserted, the leaves below the graft; but I should add that Mr. Rivers, who has possessed hundreds of such grafted trees, has never seen an instance.
Gärtner[[925]] quotes two separate accounts of branches of dark and white-fruited vines which had been united in various ways, such as being split longitudinally, and then joined, &c.; and these branches produced distinct bunches of grapes of the two colours, and other bunches with grapes either striped or of an intermediate and new tint. Even the leaves in one case were variegated. These facts are the more remarkable because Andrew Knight never succeeded in raising variegated grapes by fertilising white kinds by pollen of dark kinds; though, as we have seen, he obtained seedlings with variegated fruit and leaves, by fertilising a white variety by the variegated dark Aleppo grape. Gärtner attributes the above-quoted cases merely to bud-variation; but it is a strange coincidence that the branches which had been grafted in a peculiar manner should alone have thus varied; and H. Adorne de Tscharner positively asserts that he produced the described result more than once, and could do so at will, by splitting and uniting the branches in the manner described by him.
I should not have quoted the following case had not the author of 'Des Jacinthes'[[926]] impressed me with the belief not only of his extensive knowledge, but of his truthfulness: he says that bulbs of blue and red hyacinths may be cut in two, and that they will grow together and throw up a united stem (and this I have myself seen), with flowers of the two colours on the opposite sides. But the remarkable point is, that flowers are sometimes produced with the two colours blended together, which makes the case closely analogous with that of the blended colours of the grapes on the united vine-branches.
Mr. E. Trail stated in 1867, before the Botanical Society of Edinburgh (and has since given me fuller information), that several years ago he cut about sixty blue and white potatoes into halves through the eyes or buds, and then carefully joined them, destroying at the same time the other eyes. Some of these united tubers produced white, and others blue tubers; and it is probable that in these cases the one half alone of the bud grew. Some, however, produced tubers partly white and partly blue; and the tubers from about four or five were regularly mottled with the two colours. in these latter cases we may conclude that a stem had been formed by the union of the bisected buds; and as tubers are produced by the enlargement of subterranean branches arising from the main stem, their mottled colour apparently affords clear evidence of the intimate commingling of the two varieties. I have repeated these experiments on the potato and on the hyacinth on a large scale, but with no success.
The most reliable instance known to me of the formation of a graft-hybrid is one, recorded by Mr. Poynter,[[927]] who assures me, in a letter of the entire accuracy of the statement, Rosa Devoniensis had been budded some years previously on a white Banksian rose; and from the much enlarged point of junction, whence the Devoniensis and Banksian still continued to grow, a third branch issued, which was neither pure Banksian nor pure Devoniensis, but partook of the character of both; the flowers resembled, but were superior in character to those of the variety called Lamarque (one of the Noisettes), while the shoots were similar in their manner of growth to those of the Banksian rose, with the exception that the longer and more robust shoots were furnished with prickles. This rose was exhibited before the Floral Committee of the Horticultural Society of London. Dr. Lindley examined it, and concluded that it had certainly been produced by the mingling of R. Banksiæ with some rose like R. Devoniensis, "for while it was very greatly increased in vigour and in the size of all the parts, the leaves were half-way between a Banksian and Tea-scented rose." It appears that rose-growers were aware that the Banksian rose sometimes affects other roses. Had it not been for this latter statement, it might have been suspected that this new variety was simply due to bud-variation, and that it had occurred by a mere accident at the point of junction between the two old kinds.
To sum up the foregoing facts: the statement that Cytisus adami originated as a graft-hybrid is so precise that it can hardly be rejected, and, as we have just seen, some analogous facts render the statement to a certain extent probable. The peculiar, monstrous condition of the ovules, and the apparently sound condition of the pollen, favour the belief that it is not an ordinary or seminal hybrid. On the other hand, the fact that the same two species, viz. C. laburnum and purpureus, have spontaneously produced hybrids by seed, is a strong argument in support of the belief that C. adami originated in a similar manner. With respect to the extraordinary tendency which this tree exhibits to complete or partial reversion, we have seen that undoubted seminal hybrids and mongrels are similarly liable. On the whole, I am inclined to put trust in M. Adam's statement; and if it should ever be proved true, the same view would probably have
to be extended to the Bizzarria and Trifacial oranges and to the apples above described; but more evidence is requisite before the possibility of the production of graft-hybrids can be fully admitted. Although it is at present impossible to arrive at any certain conclusion with respect to the origin of these remarkable trees, the various facts above given appear to me to deserve attention under several points of view, more especially as showing that the power of reversion is inherent in Buds.