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.
On the direct or immediate action of the Male Element on the Mother Form.—Another remarkable class of facts must be here considered, because they have been supposed to account for some cases of bud-variation: I refer to the direct action of the male element, not in the ordinary way on the ovules, but on certain parts of the female plant, or in the case of animals on the subsequent progeny of the female by a second male. I may premise that with plants the ovarium and the coats of the ovules are obviously parts of the female, and it could not have been anticipated that they would be affected by the pollen of a foreign variety or species, although the development of the embryo, within the embryonic sack, within the ovule, within the ovarium, of course depends on the male element.
Even as long ago as 1729 it was observed[928] that white and blue varieties of the Pea, when planted near each other, mutually crossed, no doubt through the agency of bees, and in the autumn blue and white peas were found within the same pods. Wiegmann made an exactly similar observation in the present century. The same result has followed several times when a variety with peas of one colour has been artificially crossed by a differently-coloured variety.[929] These statements led Gärtner, who was highly sceptical on the subject, carefully to try a long series of experiments: he selected the most constant varieties, and the result conclusively showed that the colour of the skin of the pea is modified when pollen of a differently coloured variety is used. This conclusion has since been confirmed by experiments made by the Rev. J. M. Berkeley.[930]
Mr. Laxton of Stamford, whilst making experiments on peas for the express purpose of ascertaining the influence of foreign pollen on the mother-plant, has recently[931] observed an important additional fact. He fertilised the Tall Sugar pea, which bears very thin green pods, becoming brownish-white when dry, with pollen of the Purple-podded pea, which, as its name expresses, has dark-purple pods with very thick skin, becoming pale reddish-purple when dry. Mr. Laxton has cultivated the tall sugar-pea during twenty years, and has never seen or heard of it producing a purple pod; nevertheless, a flower fertilised by pollen from the purple-pod yielded a pod clouded with purplish-red, which Mr. Laxton kindly gave to me. A space of about two inches in length towards the extremity of the pod, and a smaller space near the stalk, were thus coloured. On comparing the colour with that of the purple-pod, both pods having been first dried and then soaked in water, it was found to be identically the same; and in both the colour was confined to the cells lying immediately beneath the outer skin of the pod. The valves of the crossed pod were also decidedly thicker and stronger than those of the pods of the mother-plant, but this may have been an accidental circumstance, for I know not how far their thickness in the Tall Sugar-pea is a variable character.
The peas of the Tall Sugar-pea, when dry, are pale greenish-brown, thickly covered with dots of dark purple so minute as to be visible only through a lens, and Mr. Laxton has never seen or heard of this variety producing a purple pea; but in the crossed pod one of the peas was of a uniform beautiful violet-purple tint, and a second was irregularly clouded with pale purple. The colour lies in the outer of the two coats which surround the pea. As the peas of the purple-podded variety when dry are of a pale greenish-buff, it would at first appear that this remarkable change of colour in the peas in the crossed pod could not have been caused by the direct action of the pollen of the purple-pod: but when we bear in mind that this latter variety has purple flowers, purple marks on its stipules, and purple pods; and that the Tall sugar-pea likewise has purple flowers and stipules, and microscopically minute purple dots on the peas, we can hardly doubt that the tendency to the production of purple in both parents has in combination modified the colour of the peas in the crossed pod. After having examined these specimens, I crossed the same two varieties, and the peas in one pod, but not the pods themselves, were clouded and tinted with purplish-red in a much more conspicuous manner than the peas in the uncrossed pods produced at the same time by the same plants. I may notice as a caution that Mr. Laxton sent me various other crossed peas slightly, or even greatly, modified in colour; but the change in these cases was due, as had been suspected by Mr. Laxton, to the altered colour of the cotyledons, seen through the transparent coats of the peas; and as the cotyledons are parts of the embryo, these cases are not in any way remarkable.
Turning now to the genus Matthiola. The pollen of one kind of stock sometimes affects the colour of the seeds of another kind, used as the mother-plant. I give the following case the more readily, as Gärtner doubted similar statements with respect to the stock previously made by other observers. A well-known horticulturist, Major Trevor Clarke, informs me[932] that the seeds of the large red-flowered biennial stock (Matthiola annua; Cocardeau of the French) are light brown, and those of the purple branching Queen stock (M. incana) are violet-black; and he found that, when flowers of the red stock were fertilised by pollen from the purple stock, they yielded about fifty per cent. of black seeds. He sent me four pods from a red-flowered plant, two of which had been fertilised by their own pollen, and they included pale brown seed; and two which had been crossed by pollen from the purple kind, and they included seeds all deeply tinged with black. These latter seeds yielded purple-flowered plants like their father; whilst the pale brown seeds yielded normal red-flowered plants; and Major Clarke, by sowing similar seeds, has observed on a greater scale the same result. The evidence in this case of the direct action of the pollen of one species on the colour of the seeds of another species appears to me conclusive.
In the foregoing cases, with the exception of that of the purple-podded pea, the coats of the seeds alone have been affected in colour. We shall now see that the ovarium itself, whether forming a large fleshy fruit or a mere thin envelope, may be modified by foreign pollen, in colour, flavour, texture, size, and shape.
The most remarkable instance, because carefully recorded by highly competent authorities, is one of which I have seen an account in a letter written, in 1867, by M. Naudin to Dr. Hooker. M. Naudin states that he has seen fruit growing on Chamærops humilis, which had been fertilised by M. Denis with pollen from the Phœnix or date-palm. The fruit or drupe thus produced was twice as large as, and more elongated than, that proper to the Chamærops; so that it was intermediate in these respects, as well as in texture, between the fruit of the two parents. These hybridised seeds germinated, and produced young plants likewise intermediate in character. This case is the more remarkable as the Chamærops and Phœnix belong not only to distinct genera, but in the estimation of some botanists to distinct sections of the family.
Gallesio[933] fertilised the flowers of an orange with pollen from the lemon; and one fruit thus produced bore a longitudinal stripe of peel having the colour, flavour, and other characters of the lemon. Mr. Anderson[934] fertilised a green-fleshed melon with pollen from a scarlet-fleshed kind; in two of the fruits "a sensible change was perceptible; and four other fruits were somewhat altered both internally and externally." The seeds of the two first-mentioned fruits produced plants partaking of the good properties of both parents. In the United States, where Cucurbitaceæ are largely cultivated, it is the popular belief[935] that the fruit is thus directly affected by foreign pollen; and I have received a similar statement with respect to the cucumber in England. It is known that grapes have been thus affected in colour, size, and shape: in France a pale-coloured grape had its juice tinted by the pollen of the dark-coloured Teinturier; in Germany a variety bore berries which were affected by the pollen of two adjoining kinds; some of the berries being only partially affected or mottled.[936] As long ago as 1751[937] it was observed that, when differently coloured varieties of maize grow near each other, they mutually affect each other's seeds, and this is now a popular belief in the United States. Dr. Savi[938] tried the experiment with care: he sowed yellow and black-seeded maize together, and on the same ear some of the seeds were yellow, some black, and some mottled,[939] the differently coloured seeds being arranged in rows or irregularly. Mr. Sabine states[940] that he has seen the form of the nearly globular seed-capsule of Amaryllis vittata altered by the application of the pollen of another species, of which the capsule has gibbous angles. Mr. J. Anderson Henry[941] crossed Rhododendron Dalhousiæ with the pollen of R. Nuttallii, which is one of the largest-flowered and noblest species of the genus. The largest pod produced by the former species, when fertilised with its own pollen, measured 1-2/8 inch in length and 1½ in girth; whilst three of the pods which had been fertilised by pollen of R. Nuttallii measured 1⅝ inch in length and no less than 2 inches in girth. Here we see the effect of foreign pollen apparently confined to increasing the size of the ovarium; but we must be cautious in assuming, as the following case shows, that in this instance size has been directly transferred from the male parent to the capsule of the female plant. Mr. Henry fertilised Arabis blepharophylla with pollen of A. Soyeri, and the pods thus produced, of which he was so kind as to send me detailed measurements and sketches, were much larger in all their dimensions than those naturally produced by either the male or female parent-species. In a future chapter we shall see that the organs of vegetation in hybrid plants, independently of the character of either parent, are sometimes developed to a monstrous size; and the increased size of the pods in the foregoing cases may be an analogous fact.
No case of the direct action of the pollen of one variety on another is better authenticated or more remarkable than that of the common apple. The fruit here consists of the lower part of the calyx and of the upper part of the flower-peduncle[942] in a metamorphosed condition, so that the effect of the foreign pollen has extended even beyond the limits of the ovarium. Cases of apples thus affected were recorded by Bradley in the early part of the last century; and other cases are given in old volumes of the Philosophical Transactions;[943] in one of these a Russeting apple and an adjoining kind mutually affected each other's fruit; and in another case a smooth apple affected a rough-coated kind. Another instance has been given[944] of two very different apple-trees growing close to each other, which bore fruit resembling each other, but only on the adjoining branches. It is, however, almost superfluous to adduce these or other cases, after that of the St. Valery apple, which, from the abortion of the stamens, does not produce pollen, but, being annually fertilised by the girls of the neighbourhood with pollen of many kinds, bears fruit, "differing from each other in size, flavour, and colour, but resembling in character the hermaphrodite kinds by which they have been fertilised."[945]
I have now shown, on the authority of several excellent observers, in the case of plants belonging to widely different orders, that the pollen of one species or variety, when applied to a distinct form, occasionally causes the coats of the seeds and the ovarium or fruit, including even in one instance the calyx and upper part of the peduncle of the mother-plant, to become modified. Sometimes the whole of the ovarium or all the seeds are thus affected; sometimes only a certain number of the seeds, as in the case of the pea, or only a part of the ovarium, as with the striped orange, mottled grapes and maize, are thus affected. It must not be supposed that any direct or immediate effect invariably follows the use of foreign pollen: this is far from being the case; nor is it known on what conditions the result depends. Mr. Knight[946] expressly states that he has never seen the fruit thus affected, though he has crossed thousands of apple and other fruit-trees. There is not the least reason to believe that a branch which has borne seed or fruit directly modified by foreign pollen is itself affected, so as subsequently to produce modified buds: such an occurrence, from the temporary connection of the flower with the stem, would be hardly possible. Hence but very few, if any, of the cases of sudden modifications in the fruit of trees, given in the early part of this chapter, can be accounted for by the action of foreign pollen; for such modified fruits have commonly been afterwards propagated by budding or grafting. It is also obvious that changes of colour in the flower which necessarily supervene long before it is ready for fertilisation, and changes in the shape or colour of the leaves, can have no relation to the action of foreign pollen: all such cases must be attributed to simple bud-variation.
The proofs of the action of foreign pollen on the mother-plant have been given in considerable detail, because this action, as we shall see in a future chapter, is of the highest theoretical importance, and because it is in itself a remarkable and apparently anomalous circumstance. That it is remarkable under a physiological point of view is clear, for the male element not only affects, in accordance with its proper function, the germ, but the surrounding tissues of the mother-plant. That the action is anomalous in appearance is true, but hardly so in reality, for apparently it plays the same part in the ordinary fertilisation of many flowers. Gärtner has shown,[947] by gradually increasing the number of pollen-grains until he succeeded in fertilising a Malva, that many grains are expended in the development, or, as he expresses it, in the satiation, of the pistil and ovarium. Again, when one plant is fertilised by a widely distinct species, it often happens that the ovarium is fully and quickly developed without any seeds being formed, or the coats of the seeds are developed without an embryo being formed within. Dr. Hildebrand also has lately shown in a valuable paper[948] that, with several Orchideæ, the action of the plant's own pollen is necessary for the development of the ovarium, and that this development takes place not only long before the pollen-tubes have reached the ovules, but even before the placentæ and ovules have been formed; so that with these orchids the pollen apparently acts directly on the ovarium. On the other hand, we must not overrate the efficacy of pollen in this respect; for in the case of hybridised plants it might be argued that an embryo had been formed and had affected the surrounding tissues of the mother-plant before it perished at a very early age. Again, it is well known that with many plants the ovarium may be fully developed, though pollen be wholly excluded. And lastly, Mr. Smith, the late Curator at Kew (as I hear through Dr. Hooker), observed the singular fact with an orchid, the Bonatea speciosa, the development of the ovarium could be effected by mechanical irritation of the stigma. Nevertheless, from the number of the pollen-grains expended "in the satiation of the ovarium and pistil,"—from the generality of the formation of the ovarium and seed-coats in sterile hybridised plants,—and from Dr. Hildebrand's observations on orchids, we may admit that in most cases the swelling of the ovarium, and the formation of the seed-coats, are at least aided, if not wholly caused, by the direct action of the pollen, independently of the intervention of the fertilised germ. Therefore, in the previously-given cases we have only to add to our belief in the power of the plant's own pollen on the development of the ovarium and seed-coats, its further power, when applied to a distinct species or variety, of influencing the shape, size, colour, texture, &c., of these same parts.
Turning now to the animal kingdom. If we could imagine the same flower to yield seeds during successive years, then it would not be very surprising that a flower of which the ovarium had been modified by foreign pollen should next year produce, when self-fertilised, offspring modified by the previous male influence. Closely analogous cases have actually occurred with animals. In the case often quoted from Lord Morton,[949] a nearly purely-bred, Arabian, chesnut mare bore a hybrid to a quagga; she was subsequently sent to Sir Gore Ouseley, and produced two colts by a black Arabian horse. These colts were partially dun-coloured, and were striped on the legs more plainly than the real hybrid, or even than the quagga. One of the two colts had its neck and some other parts of its body plainly marked with stripes. Stripes on the body, not to mention those on the legs, and the dun-colour, are extremely rare,—I speak after having long attended to the subject,—with horses of all kinds in Europe, and are unknown in the case of Arabians. But what makes the case still more striking is that the hair of the mane in these colts resembled that of the quagga, being short, stiff, and upright. Hence there can be no doubt that the quagga affected the character of the offspring subsequently begot by the black Arabian horse. With respect to the varieties of our domesticated animals, many similar and well-authenticated facts have been published,[950] and others have been communicated to me, plainly showing the influence of the first male on the progeny subsequently borne by the mother to other males. It will suffice to give a single instance, recorded in the 'Philosophical Transactions,' in a paper following that by Lord Morton: Mr. Giles put a sow of Lord Western's black and white Essex breed to a wild boar of a deep chesnut colour; and the "pigs produced partook in appearance of both boar and sow, but in some the chesnut colour of the boar strongly prevailed." After the boar had long been dead, the sow was put to a boar of her own black and white breed,—a kind which is well known to breed very true and never to show any chesnut colour,—yet from this union the sow produced some young pigs which were plainly marked with the same chesnut tint as in the first litter. Similar cases have so frequently occurred, that careful breeders avoid putting a choice female to an inferior male on account of the injury to her subsequent progeny which may be expected to follow.
Some physiologists have attempted to account for these remarkable results from a first impregnation by the close attachment and freely intercommunicating blood-vessels between the modified embryo and the mother. But it is a most improbable hypothesis that the mere blood of one individual should affect the reproductive organs of another individual in such a manner as to modify the subsequent offspring. The analogy from the direct action of foreign pollen on the ovarium and seed-coats of the mother-plant strongly supports the belief that the male element acts directly on the reproductive organs of the female, wonderful as is this action, and not through the intervention of the crossed embryo. With birds there is no such close connection between the embryo and mother as in the case of mammals: yet a careful observer, Dr. Chapuis, states[951] that with pigeons the influence of a first male sometimes makes itself perceived in the succeeding broods; but this statement, before it can be fully trusted, requires confirmation.
Conclusion and Summary of the Chapter.—The facts given in the latter half of this chapter are well worthy of consideration, as they show us in how many extraordinary modes one organic form may lead to the modification of another, and often without the intervention of seminal reproduction. There is ample evidence, as we have just seen, that the male element may either directly affect the structure of the female, or in the case of animals lead to the modification of her offspring. There is a considerable but insufficient body of evidence showing that the tissues of two plants may unite and form a bud having a blended character; or again, that buds inserted into a stock may affect all the buds subsequently produced by this stock. Two embryos, differing from each other and contained in the same seed, may cohere and form a single plant. Offspring from a cross between two species or varieties may in the first or in a succeeding generation revert in various degrees by bud-variation to their parent-forms; and this reversion or segregation of character may affect the whole flower, fruit, or leaf-bud, or only the half or smaller segment, or a single organ. In some cases this segregation of character apparently depends on some incapacity of union rather than on reversion, for the flowers or fruit which are first produced display by segments the characters of both parents. In the Cytisus adami and the Bizzarria orange, whatever their origin may have been, the two parent species occur blended together under the form of a sterile hybrid, or reappear with their characters perfect and their reproductive organs effective; and these trees, retaining the same sportive character, can be propagated by buds. These various facts ought to be well considered by any one who wishes to embrace under a single point of view the various modes of reproduction by gemmation, division, and sexual union, the reparation of lost parts, variation, inheritance, reversion, and other such phenomena. In a chapter towards the close of the following volume I shall attempt to connect these facts together by a provisional hypothesis.
In the early half of this chapter I have given a long list of plants in which through bud-variation, that is, independently of reproduction by seed, the fruit has suddenly become modified in size, colour, flavour, hairiness, shape, and time of maturity; flowers have similarly changed in shape, colour, and doubleness, and greatly in the character of the calyx; young branches or shoots have changed in colour, in bearing spines, and in habit of growth, as in climbing and weeping; leaves have changed in colour, variegation, shape, period of unfolding, and in their arrangement on the axis. Buds of all kinds, whether produced on ordinary branches or on subterranean stems, whether simple or, as in tubers and bulbs, much modified and supplied with a stock of nutriment, are all liable to sudden variations of the same general nature.
In the list, many of the cases are certainly due to reversion to characters not acquired from a cross, but which were formerly present, and have been lost for a longer or shorter period of time;—as when a bud on a variegated plant produces plain leaves, or when variously-coloured flowers on the Chrysanthemum revert to the aboriginal yellow tint. Many other cases included in the list are probably due to the plants being of crossed parentage, and to the buds reverting to one of the two parent-forms. In illustration of the origin of Cytisus adami, several cases were given of partial or complete reversion, both with hybrid and mongrel plants; hence we may suspect that the strong tendency in the Chrysanthemum, for instance, to produce by bud-variation differently-coloured flowers, results from the varieties formerly having been intentionally or accidentally crossed; and that their descendants at the present day still occasionally revert by buds to the colours of the more persistent parent-varieties. This is almost certainly the case with Rollisson's Unique Pelargonium; and so it may be to a large extent with the bud-varieties of the Dahlia and with the "broken colours" of Tulips.
Many cases of bud-variation, however, cannot be attributed to reversion, but to spontaneous variability, such as so commonly occurs with cultivated plants when raised from seed. As a single variety of the Chrysanthemum has produced by buds six other varieties, and as one variety of the gooseberry has borne at the same time four distinct varieties of fruit, it is scarcely possible to believe that all these variations are reversions to former parents. We can hardly believe, as remarked in a previous chapter, that all the many peaches which have yielded nectarine-buds are of crossed parentage. Lastly, in such cases as that of the moss-rose with its peculiar calyx, and of the rose which bears opposite leaves, in that of the Imatophyllum, &c., there is no known natural species or seedling variety, from which the characters in question could have been derived by crossing. We must attribute all such cases to actual variability in the buds. The varieties which have thus arisen cannot be distinguished by any external character from seedlings; this is notoriously the case with the varieties of the Rose, Azalea, and many other plants. It deserves notice that all the plants which have yielded bud-variations have likewise varied greatly by seed.
These plants belong to so many orders that we may infer that almost every plant would be liable to bud-variation if placed under the proper exciting conditions. These conditions, as far as we can judge, mainly depend on long-continued and high cultivation; for almost all the plants in the foregoing lists are perennials, and have been largely propagated in many soils and under different climates, by cuttings, offsets, bulbs, tubers, and especially by budding or grafting. The instances of annuals varying by buds, or producing on the same plant differently coloured flowers, are comparatively rare: Hopkirk[952] has seen this with Convolvulus tricolor; and it is not rare with the Balsam and annual Delphinium. According to Sir R. Schomburgk, plants from the warmer temperate regions, when cultivated under the hot climate of St. Domingo, are eminently liable to bud-variation; but change of climate is by no means a necessary contingent, as we see with the gooseberry, currant, and some others. Plants living under their natural conditions are very rarely subject to bud-variation: variegated and coloured leaves have, however, been occasionally observed; and I have given an instance of the variation of buds on an ash-tree; but it is doubtful whether any tree planted in ornamental grounds can be considered as living under strictly natural conditions. Gärtner has seen white and dark-red flowers produced from the same root of the wild Achillea millefolium; and Prof. Caspary has seen Viola lutea, in a completely wild condition, bearing flowers of different colours and sizes.[953]
As wild plants are so rarely liable to bud-variation, whilst highly cultivated plants long propagated by artificial means have yielded by this form of reproduction many varieties, we are led through a series such as the following,—namely, all the eyes in the same tuber of the potato varying in the same manner,—all the fruit on a purple plum-tree suddenly becoming yellow,—all the fruit on a double-flowered almond suddenly becoming peach-like,—all the buds on grafted trees being in some very slight degree affected by the stock on which they have been worked,—all the flowers on a transplanted heartsease changing for a time in colour, size, and shape,—we are led through such facts to look at every case of bud-variation as the direct result of the particular conditions of life to which the plant has been exposed. But if we turn to the other end of the series, namely, to such cases as that of a peach-tree which, after having been cultivated by tens of thousands during many years in many countries, and after having annually produced thousands of buds, all of which have apparently been exposed to precisely the same conditions, yet at last suddenly produces a single bud with its whole character greatly transformed, we are driven to an opposite conclusion. In such cases as the latter it would appear that the transformation stands in no direct relation to the conditions of life.
We have seen that varieties produced from seeds and from buds resemble each other so closely in general appearance, that they cannot possibly be distinguished. Just as certain species and groups of species, when propagated by seed, are more variable than other species or genera, so it is in the case of certain bud-varieties. Thus the Queen of England Chrysanthemum has produced by this latter process no less than six, and Rollisson's Unique Pelargonium four distinct varieties; moss-roses have also produced several other moss-roses. The Rosaceæ have varied by buds more than any other group of plants; but this may be in large part due to so many members having been long cultivated; but within this one group, the peach has often varied by buds, whilst the apple and pear, both grafted trees extensively cultivated, have afforded, as far as I can ascertain, extremely few instances of bud-variation.
The law of analogous variation holds good with varieties produced by buds, as with those produced from seed: more than one kind of rose has sported into a moss-rose; more than one kind of camellia has assumed an hexagonal form; and at least seven or eight varieties of the peach have produced nectarines.
The laws of inheritance seem to be nearly the same with seminal and bud-varieties. We know how commonly reversion comes into play with both, and it may affect the whole, or only segments, of a leaf, flower, or fruit. When the tendency to reversion affects many buds on the same tree, it becomes covered with different kinds of leaves, flowers, or fruit; but there is reason to believe that such fluctuating varieties have generally arisen from seed. It is well known that, out of a number of seedling varieties, some transmit their character much more truly by seed than others; so with bud-varieties some retain their character by successive buds more truly than others; of which instances have been given with two kinds of variegated Euonymus and with certain kinds of tulips. Notwithstanding the sudden production of bud-varieties, the characters thus acquired are sometimes capable of transmission by seminal reproduction: Mr. Rivers has found that moss-roses generally reproduce themselves by seed; and the mossy character has been transferred by crossing, from one species of rose to another. The Boston nectarine, which appeared as a bud-variation, produced by seed a closely allied nectarine. We have however seen, on the authority of Mr. Salter, that seed taken from a branch with leaves variegated through bud-variation, transmits this character very feebly; whilst many plants, which became variegated as seedlings, transmit variegation to a large proportion of their progeny.
Although I have been able to collect a good many cases of bud-variation, as shown in the previous lists, and might probably, by searching foreign horticultural works, have collected more cases, yet their total number is as nothing in comparison with that of seminal varieties. With seedlings raised from the more variable cultivated plants, the variations are almost infinitely numerous, but their differences are generally slight: only at long intervals of time a strongly marked modification appears. On the other hand, it is a singular and inexplicable fact that, when plants vary by buds, the variations, though they occur with comparative rarity, are often, or even generally, strongly pronounced. It struck me that this might perhaps be a delusion, and that slight changes often occurred in buds, but from being of no value were overlooked or not recorded. Accordingly I applied to two great authorities on this subject, namely, to Mr. Rivers with respect to fruit-trees, and to Mr. Salter with respect to flowers. Mr. Rivers is doubtful, but does not remember having noticed very slight variations in fruit-buds. Mr. Salter informs me that with flowers such do occur, but, if propagated, they generally lose their new character in the following year; yet he concurs with me that bud-variations usually at once assume a decided and permanent character. We can hardly doubt that this is the rule, when we reflect on such cases as that of the peach, which has been so carefully observed and of which such trifling seminal varieties have been propagated, yet this tree has repeatedly produced by bud-variation nectarines, and only twice (as far as I can learn) any other variety, namely, the Early and Late Grosse Mignonne peaches; and these differ from the parent-tree in hardly any character except the period of maturity.
To my surprise I hear from Mr. Salter that he brings the great principle of selection to bear on variegated plants propagated by buds, and has thus greatly improved and fixed several varieties. He informs me that at first a branch often produces variegated leaves on one side alone, and that the leaves are marked only with an irregular edging or with a few lines of white and yellow. To improve and fix such varieties, he finds it necessary to encourage the buds at the bases of the most distinctly marked leaves, and to propagate from them alone. By following with perseverance this plan during three or four successive seasons, a distinct and fixed variety can generally be secured.
Finally, the facts given in this chapter prove in how close and remarkable a manner the germ of a fertilised seed and the small cellular mass forming a bud resemble each other in function,—in their powers of inheritance with occasional reversion,—and in their capacity for variation of the same general nature, in obedience to the same laws. This resemblance, or rather identity, is rendered far more striking if the facts can be trusted which apparently render it probable that the cellular tissue of one species or variety, when budded or grafted on another, may give rise to a bud having an intermediate character. In this chapter we clearly see that variability is not necessarily contingent on sexual generation, though much more frequently its concomitant than on bud-reproduction. We see that bud-variability is not solely dependent on reversion or atavism to long-lost characters, or to those formerly acquired from a cross, but that it is often spontaneous. But when we ask ourselves what is the cause of any particular bud-variation, we are lost in doubt, being driven in some cases to look to the direct action of the external conditions of life as sufficient, and in other cases to feel a profound conviction that these have played a quite subordinate part, of not more importance than the nature of the spark which ignites a mass of combustible matter.
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