The Garden 1: 480-481 (Apr 20, 1872)
ON PURE HYBRIDIZATION, OR CROSSING DISTINCT SPECIES OF PLANTS.
ISAAC ANDERSON-HENRY, ESQ, F.L.S.*
*A paper read before the Botanical Society of Edinburgh, and since revised and added to by the author.

THE following are the rules I observe and the means which I take to insure success in my experiments with reference to this subject:—

   1st. I long held it to be of vital importance to have the separate plants intended for the parents in the cross, even though both were hardy, put under glass, and I still recommend it; for, by doing so, you heighten the temperature—an important thing—and you can better secure against the interference of winds and insects; and though Darwin holds the former of small account, I have reason for differing from him there. But in the height of summer pollen may be taken from an outside plant to cross an inside one, and vice versa. If the cross is to be made on an outside plant which cannot be conveniently removed, I cover it with a hand-glass or cloche.

   2nd. I hold it not enough merely to emasculate the intended seed bearing flower; I take off every petal, for the petals attract the insects, which seem guided more by their optics than any sense of smell. This act of emasculation in some cases I perform long before the expansion of the bloom; for in many plants—e.g., in the Papilionaceae, some of the Rosaceae, and Compositae—self-fertilization may, and does, often take place in the unopened flower. This is not all. I sometimes put a gauze bag over it; if I do not, the mutilated bloom may not escape that most troublesome of all insect pests, the humble bee, which in his unwieldy flight, may come across it by pure accident. But for the most part now I make clean work of it, and remove all other expanded flowers on the seed-bearing plant, and allow no kindred one to be near.

   3rd. Do not be in a hurry to effect your cross; wait till you find that the stigma is fully developed. In many plants this is shown by a glutinous exudation on the summit, as in the Ericaceae, the Onagraceae, &c. In other orders, such as the Geraniaceae and Malvaceae, it is indicated by the feathery expansion and recurvature of its separate divisions.

   4th. The next thing is to obtain properly ripened pollen grains from the male plant. This is done by carefully watching when the anthers burst, otherwise the insects may be before you; and so active are they, especially on such favourite food as the pollen of the Rubus tribe, that, to get it at all, I have found it necessary to encase the opening blooms in muslin bags till the pollen was ripe, and ready for use. Do not use, as is generally recommended, the camel-hair pencil, which, applied often and indiscriminately, may, and often does, convey, with the foreign, some insidious grains of native pollen, which, however few, are prepotent, and wholly neutralise the former. Take, where that can be obtained and afforded, the entire bloom of the intended male, and give the slightest brush with all its anthers over the stigma, or all the stigmas, if more than one, of the intended female. I will give my reasons for this by-and-bye. You may use for experiment, in some cases the long, and in some the short, stamens. To those of the proper dimorphic form I have made some allusion elsewhere; they occur in the species of Primula and in some of the species of the Linum tribe (as to both of which, see Darwin’s most remarkable papers in the Proceedings of the Linnean Society). Such anthers, at least two long and two short ones, occur in the two orders of the Linnaean class Didynamia, on which I may have a suggestion to offer here after, for I think something interesting may be worked out of this form. In cases where the anthers are few, as in the Linnaean classes Diandria, Triandria, &.c., you may use small pincers—a bit of wire so twisted as to form that implement, to carry in the pocket, is by far the handiest. I have used such an instrument all along, and find it better than any other form. In some tribes, the better to secure against invasion by insects, such especially as in some of the Rosaceee having large discs, a muslin bag may be used, so as eflectually to exclude them; I use it constantly in the Rubus tribe immediately after emasculation, taking it off and replacing it after the cross, and keeping it on thereafter till the cross has set.

   5th. In some cases it is a matter of some difficulty to procure, and when procured of no less importance to preserve, pollen. In dioecious plants—say the Aucuba—a friend may have the male and you have, as we all have, the female in abundance. You would like to store that pollen till your female plant, generally later, comes into flower. Many hold that pollen cannot be preserved in a vital condition for more than one or two, or perhaps three, weeks. In a recent publication which refers to this matter, namely, Max Wichura's "Observations on Hybridization," of which a very lucid abstract, carefully digested and translated from the original German by the Rev. M. J. Berkeley, is given in the January number of the Journal of the Royal Horticultural Society, that eminent authority holds it as "a fact of great importance that the pollen of willows retains its potency for some time. In some cases pollen ten days old was efficient, while vitality was still further prolonged by steeping it in a solution of honey" (of which I have doubts). "Pollen," he adds, "of Salix Silesiaca eight days old seemed almost as potent as ever; in twenty-eight days the traces of vitality were very slight, while that of Salix cinerea had become weak in sixteen days." Now, I am not aware that there is less vitality in the pollen of willows than in that of any other family, and, as many experimentalists hold kindred views to those here enunciated by Wichura, I deem it a matter of some importance to give you one or two instances of my own experience. I have carried in my pocket the pollen of Rhododendron again and again from six weeks to two months and upwards, and still found it potent. Of the Japanese forms of the genus Lilium I have kept pollen effective in the same manner for equal periods. In fact, generally speaking, I have found the pollen of most plants to remain good for similar periods. Having last year get the new and beautiful Clematis Jackmanii to flower, and anxious to preserve its pollen as long as possible, I collected and stored it in its anthers in a simple pill-box. On the 4th of July 1866, I so gathered and put it into a drawer of a cabinet in my own sitting room, where it remained wholly away from damp. On the 5th of June 1867, having first carefully emasculated a flower of Clematis candida, I crossed it with the pollen, then eleven months old, and from this cross I have this autumn gathered and sown eight well. developed seeds. Now, both parents are hybrids, with a large infusion of alien blood in them, so that here the vitality was put to its severest test. Subsequent experiments satisfy me that the vitality of all pollen may not be so long preserved, for I have found that of the Aucuba inert after being stored about six weeks. But as some bits of stems had got mixed, these may, by inducing damp, have destroyed it. I would therefore recommend it to be brushed off pure and stored in silk paper. I notice this result here. (somewhat out of place) to suggest the propriety of storing, and, if needful, of importing, pollen, which, if wrapt up in silk paper, might even, enclosed in a letter, reach this country still potent, by the overland route from India, or, after two or three months’ voyage, from all parts of South and North America. Let collectors and friends in distant countries be instructed as to this, and we may soon have an improved progeny of the rarest things, even before such novelties from which they are derived have been obtained from their own seeds in this country.

   6th. There is another matter of much consequence to be attended to in the crossing of distant species; I mean the times and seasons for effecting the cross; yet not one of those most experienced in the art, from Darwin downward, has touched upon this point. It has been forced upon my attention for more than twenty years. I have found that I could, on some few propitious days which occur throughout the season, successfully effect crosses I could not effect with all my care at other times. I have adverted to this in the paper I formerly submitted to you, and I again refer to it. There are some crosses which I have effected at such times, and which I would have tried in vain to accomplish at times less favourable. If you have, say, two plants of Rhododendron, one a tiny thing, to cross with a large species, or if you wish to attempt a cross between an Indian azalea and a rhododendron, watch for a propitious time. Such times occur, often few and far between, when there is less of sun than of that latent form of heat, which frequently occurs before thunder, from the air being more than ordinarily charged with electricity. Or they may occur in the spring season, when there is much ozone present, whose influence I have often found to tell most favourably in promoting the germination of long-sown seeds. It was to the presence of ozone, or to some other form of electrical agency, I attributed the almost simultaneous germination of some New Zealand seeds of a shrub which I got from that country under the name of "Black Maupan," a species of Pittosporum, which sprang up together on the morning of the 16th March 1863, after they had lain dormant two years and eight months. Such atmospheric conditions, to whatever cause they may be due, I have found not unfrequently to occur with the east winds of March and April; at which times I have seen many other long-sown seeds spring quite suddenly and unexpectedly. Seize upon all such seasons for difficult crosses. As to the time of the day, you may operate best perhaps from ten a.m. till six p.m.

(To be continued.)

1: 506-507 (Apr 27, 1872)
(Continued from page 481.)

APPEARANCES IF THE CROSS HAS SUCCEEDED.

WE shall suppose the cross now performed. Your next anxiety will naturally be to find out whether it has taken. Almost all experimenters have noticed that soon—I would say from six to ten days—an alteration is observed on the stigma and style. You will find the viscid matter on the former dried up, while the latter has begun to shrivel. You will naturally conclude that it is all right, and that the fertilizing pollen has now passed down into the ovary, and in some cases you may be right. But these appearances are deceptive, especially if you find the style maintain an erect position. And singularly, as I now write, I find, on glancing at the Gardeners’ Chronicle of the 19th October, 1867, that this state of matters had been observed last summer by the learned editor of that publication and described in his leading article of that day. He there observes, "We have ourselves, in following some experiments on cross-breeding this season, noticed that the stigma becomes changed—withered, almost immediately after contact with the pollen, even if no perfect seeds be produced." Now that gentleman is quite right; but I did not note the withering effect to be just so immediate as he had observed it, though it might have been so in the Epilobium tribe, to which his experiments refer. Another effect I particularly noted last summer was that, in attempting to cross an Indian azalea with a rhododendron (which, however, in that instance failed), not only did the stigma and style decay, but the divisions of the calyx took on a purplish tint, and a honeyed secretion continued long to exude from the disc. Another still more misleading condition often arises, as is noticed in the same leading article of the Chronicle: "The ovary will swell, the fruit will set, in some cases without any contact with the pollen at all, though of course no embryo is produced." Wichura has noticed the like result, and the following degrees of failure noted by him have so often occurred in my own experience, that I cannot do better than cite them in his own words, from the Rev. Mr. Berkeley’s translation already alluded to, which I only alter according to my own experience:—1st. The organs submitted to hybridization (the stigma and style) soon wither, but do not in all cases soon fall off. 2nd. The ovaries swell and ripen, but do not contain a trace of seed. 3rd. The ovaries may seem filled (I say may seem partially filled), having in some instances the small protuberant swelling outside as if seeds were within, and yet no seed be there. 4th. Seeds are present, but small, languid, and incapable of germination. 5th. Seeds apparently perfectly developed which do not germinate. 6th. Seeds which germinate, but the young plants are weak, and wither in a short time, dying off often times after developing the seed-leaves. I have had all these conditions and results amply illustrated; and of the second of these results I had, last summer, mortifying proofs in a muling operation I tried, by fertilizing a flower of the new Arabis blepharophylla with my still newer Draba violacea. The cross, to all appearance, had taken; the seed-vessel swelled better than the others where no experiment was made, and while the valves of the silicules of these last opened and showed no trace of seed in them, the siliquas of the former remained closed, showing by outward development that two seeds were certainly within. But I found on opening the ripe seed vessels that there was no perfect sped in the interior, but only an abortive production. While Wichura s accuracy in the above degrees of failure is consistent With what I have myself had ample experience of, I cannot, from like experience, endorse the views he has formed on some of his successful results. At page 72 of the above article in the Journal of the Royal Horticultural Society, Mr. Berkeley, commenting on Wichura’s paper, observes:—"Gaertner, indeed, supposes that in genera which are rich in species, there are some which have a prepotent influence when hybridizing, so that in some hybrids the type either pf the male or female prevails. Amongst the various hybrid willows, though the genus is so rich in species and so prone to hybridizmg, Wichura has never seen a prepotent type, and doubts Gaertner’s statement, especially as he makes it in very qualified terms." Mr. Berkeley very judiciously remarks that it is not very easy to determine, "by examination of types, whether a hybrid is more like the mother or father—the perfect distinction is subject in many cases to great difficulties, since very much depends on the subjective view of the observation; for, in consequence of the frequent intermelting of both characters, the one observer finds in a hybrid the maternal type, while another thinks the paternal type prevalent. By which I regard Mr. Berkeley as very modestly dissenting from his author. And further on, at page 78 of the same Journal, Wichura speaks out still more absolutely. "When both parents," says he, "belong to the same species, we cannot tell what part the male and female parent take respectively in the formation of the progeny. But dissimilar factors are united in hybrids, and an intermediate form is the consequence. The products which arise from reciprocal crossing in plants, unlike those which are formed amongst animals, are perfectly alike." I regret to differ from so great an authority as Wichura, and must venture to demur to the doctrine in more decided terms than Mr. Berkeley does. I have had so many instances of hybrids taking sometimes to one side and sometimes to another—but most frequently to that of the mother —that to those who, like myself, have made experiments with many genera, it would be needless to give instances. The converse is the rarer case—i.e., where the paternal type comes out most marked. Yet I remember one eminent instance of a seedling Veronica, from the batch of seedlings from which I obtained V. Andersonii (V. salicifolia, V. speciosa), being so like the male parent V. speciosa, that I presented it to a friend in the belief it was purely and simply the latter species; but when it bloomed, it showed, by the longer spike and lighter and brighter colour of the flowers, and by their being a bright crimson instead of very deep purple, which is the colour of the flower of the V. speciosa, that the blood of the V. salicifolia was there. I can well understand that, as respects the family of willows, from their being so attractive to bees, and from their being naturally so prone to intermix (insomuch that few can tell what is a species and what is a hybrid), Wichura has not much overstated the fact, and that a distinct inter-mediate form may generally be reckoned on.

I must dissent still more strongly from what Wichura lays down in continuation of the above passage at page 78, as to reciprocal crossings. "The products," he says, "which arise from reciprocal crossing in plants, unlike those which are formed amongst animals, are perfectly alike. It is of no consequence which is the male and which the female parent. It is, therefore, a mathematical necessity that the pollen-cells must have just the same part in the act of generation as the ovules." And, based mainly on this doctrine, he follows up and amplifies it in a series of aphorisms which, he admits, are to be "considered conjectural, and require to be submitted to proof," an admission for which he is to be commended, and all the more if he submitted to the like test the dogma on which they mainly rest. It humbly appears to me that his statement had been suggested from his experience among the Salices—of all plants the most mongrel in a state of nature. Now, in all this, Wichura appears to me to imply that if a distinct intermediate may be formed, and is formed, by crossing A on B, so may an exactly similar intermediate be reciprocated by crossing B on A. And M. Naudin, in his experiments among the Daturas, enunciates the same belief, and holds "that there is not a sensible difference between reciprocal hybrids of two species." That distinguished observer, like Wichura, seems to have confined his experiments to herbaceous or soft-wooded plants. But, from a long and large experience among both hard and soft wooded plants, I demur, 1st, to the capability of the parents being in all cases made subject to such reciprocity; and, 2nd, to the statement where such reciprocity does hold, that the progeny are perfectly alike, whether A or B supply the pollen.

In my various crossings I have experimented on many hard as well as soft wooded genera—in particular, I would here instance among the former the species of rhododendron. In these I have again and again been baffled to reciprocate a cross which on one side was comparatively easy to be effected. When the lovely and fragrant Rhododendron Edgeworthii first bloomed in this country, all were eager to see its beauty and perfume transfused into dwarfer and hardier forms. Some tried the cross by making R. Edgeworthii the female or seed-bearer, others by making it the male. I tried it in both ways, but all my efforts failed where I attempted the cross on the R. Edgeworthii. But while it would not be brought to bear hybrid seed, I had no great difficulty in effecting a cross from its pollen on R. ciliatum, another of Dr. Hooker’s beautiful Sikkim species having all the desirable requisites of hardihood, dwarf habit, and free-flowering tendency; and, singularly, just as I had obtained and sent off blooms of this brood to lay before the committee of the Horticultural Society of London, Messrs. Veitch, of Chelsea, anticipated me in having a plant of this identical cross first exhibited before that committee, which is now well-known and generally cultivated under the name of "Rhodedendron Princess Alice." Now, neither I nor anyone who ever tried it, so far as I know, ever effected the inverse cross of R. ciliatum on R. Edgeworthii; and if they did, the progeny would long ere now have appeared in nursery catalogues. There is yet one other instance I may notice as an illustration of what I am now contending for. In my former paper I noticed, as an exception to a rule I had found almost general—viz., that European had great aversion to cross with Asiatic species—that I had, notwithstanding, effected such a hybrid by crossing R. eleagnoides (another of Dr. Hooker’s acquisitions, a tiny Sikkim species) on the European R. hirsutum, and of having sent the survivor of the two plants which came of it to Kew, of which, by the way, Dr. Hooker writes me, that it dwindled away and died after being a few years in their hands; but by no possible means could I invert that cross, or get that same very interesting tiny yellow-flowered species, R. eleagnoides (a form of R. lepidotum), to submit to a cross from any species whatever.

I shall now advert to the second point which Wichura lays down as a fact—viz., that the progeny of reciprocal crossing, whether it is A on B or B on A, are precisely alike. While my past experience goes with what I observed last summer, it may perhaps suffice to give the latest instance. Having, through the kindness of Dr. Hooker, obtained seeds of a beautiful new Californian Arabis (A. blepharophylla) with large fine rose-tinted flowers, I felt desirous to in fuse that colour into some of the other kinds I possessed. After trying it on several, especially on A. albida, in vain, I at last effected a cross—a reciprocal cross—between it and A. Soyeri, a white-flowered species from the Pyrenees, something like A. albida, but with glabrous foliage. Of the cross A. Soyeri on A. blepharophylla I have raised six plants, the product of two very largely developed seed-pods. These plants are alive and healthy, and promise an improved vigour over either parent. That the cross was sure, I had the best proof, from there being no seeds in the normal pods of the seed-bearer. Of the inverse cross from one weakly seed-pod I raised one plant, which, after maintaining a sickly existence for some two months or so, has died off. But while this last cross was equally certain as the others, like it, the plant had more of the mother than the father in it. In fact, I have oftener found the maternal type most marked in hybrid progeny. I have various crosses effected between distinct species of rhododendron, where, while the male manifests his presence, the female type prevails. I have it in R. Jenkensi crossed by R. Edgeworthii, R. caucasicum by R. cinnamomeum, and the hybrid from this latter cross crossed again with R. Edgeworthii, and especially the Sikkim species R. virgatum crossed with another of my hybrids, R. ciliatum by R. Edgeworthii—all having more the foliage and the aspect of the mother than the father.

I have another hybrid of the same R. virgatum, the female parent crossed, I believe, by Rhodothamnus chamaecistus, a tiny procumbent plant of three inches, but all set with flower buds—not, as in the male parent, at the tips of the shoots, but, as in the female, at the axils of the leaves. I have stated my belief that the Rhodothamnus is the male parent, but I cannot do so confidently, from the tallies having got into confusion—the specimens being planted out. But as some plants were obtained from that cross, and as this is the smallest, I regard it as likeliest to be the true progeny; and the cross being an extreme one—a mule, in fact, it is open to question. But as I have this season effected still more extreme—certainly more unlikely—crosses in that family, where there could be no miscarriage, you may, I think, take it as true in the meantime. I could overwhelm you with proof. Darwin, at page 333 of the last edition of his "Origin of Species," has observed the above tendency. "When two species," he says, "are crossed, one has sometimes a prepotent power of impressing its likeness on the hybrid; and so I believe it to be with varieties of plants."

Naturalists of the highest note—Gaertner, Kolreuter, Naudin, and Wichura—are far from being at one on the subject of variability, as Darwin has shown, especially as relates to crosses, lst, between species and species; 2nd, between species and varieties; and 3rd, between mongrel offspring. But this is a complex subject, and when such high authorities are not at one, and Darwin admits that he cannot reconcile them, it is manifest that the case is still open to further probation. In dealing with the views of Gaertner, to whose testimony he deservedly accords great value (page 331), Darwin says that Gaertner, whose strong wish "it was to draw a distinct line between species and varieties, could find very few, and, as it seems to me, quite unimportant, differences between the so-called hybrid offspring of species and the so-called mongrel offspring of varieties. And, on the other hand, they agree most closely in many important respects. The most important distinction is, that in the first generation mongrels are more variable than hybrids; but Gaertner admits that hybrids from species which have long been cultivated are often variable in the first generation; and I have myself seen striking instances of this fact. Gaertner further admits that hybrids between very closely allied species are more variable than those from very distinct species, and this shows that the difference in the degree of variability graduates away. When mongrels and the more fertile hybrids are propagated for several generations, an extreme amount of variability in their offspring is 507 notorious; but some few cases, both of hybrids and mongrels, long retaining uniformity of character could be given. The variability, however, in the successive generations of mongrels is, perhaps, greater than in hybrids." So reservedly does Darwin deal with a subject on which the opinions of others could be brought to bear; but as they are not all concurrent, and not unfrequently conflicting (which they may well be from the various subjects experimented on) he has said, with commendable moderation, all that can be said on the subject.

(To be continued.)

1: 521-522 (May 4, 1872)
(Continued from page 507.)

FROM my readers I respectfully claim the same kind indulgence which Darwin has shown to the testimony he has had to deal with, in judging of the views I have offered, and am now to offer, on the experiments which I mean to lay before you. But ere I enter upon them, it is necessary to premise, especially as regards that form of dimorphism which occurs among many plants in the Linnaean classes from Pentandria up to Decandria—in having very generally one if not two pairs of stamens shorter than the other stamens in the same flower. And the same dimorphic form often occurs in even a more marked degree in many plants of the class Tetrandria. It is also the distinctive character of the two orders of Didynamia to have two long and two short stamens.

As observed in my former paper, it is now seventeen years since my attention was drawn to the long and short stamens, but to the latter more particularly in some muling operations there alluded to, where, by using them, I crossed that large species of rhododendron, R. cinnamomeum, on the pigmy Rhodothamnus chamaecistus. I refer to these short stamens again, as the means by which I succeeded in effecting some extraordinary crosses which, I confidently believe, but for their use and my improving a propitious time, would have been utterly impracticable. As I have said, I at first worked only with short stamens. These I use in all cases where I wish to cross a large on a small species. I have now found that the converse holds, and use the long stamens where I wish to cross a small on a large species. In all extremes I use the longest or shortest pair of stamens as the case demands. The short pair is generally well distanced by the others—the longest pair is often not just so much in advance. There is often an intermediate pair of short stamens, which in cases less extreme are exceedingly serviceable, but there are seldom such intermediates among the long ones. My reason for the use of these short, intermediate, and long stamens is intelligible enough. If I wish to cross a large on a small species, the smallest grained pollen being in the short stamens, I take the pollen of these stamens of the large plant as best fitted to pass down the tubes through the stigma to fertilize the ovules of the smaller species, and so effect the cross on it; and so, caeteris paribus, with respect to the other forms.

I shall restrict the instances I am now to cite to the last few years, noticing first,—

CASES OF CROSSING WITH SHORT STAMENS.

The first cross I shall notice is one already alluded to, viz., Rhododendron virgatum with my own hybrid rhododendron B (R. ciliatum crossed on R. Edgeworthii); and as this cross is memorable and instructive in several points of view, it is proper to give you its history. On April 20, 1864, I find from my note-book that "I took off all expanded blooms of R. virgatum and removed the stamens from all unopened ones on the plant, there being none left for self-fertilization; done in fine sunshine—west wind—with three short anthers of B "— i.e., the hybrid male, being the identical cross which produced Veitch’s rhododendron, Princess Alice. Of this cross I ripened four capsules of seed, which I sowed on January 28, 1865, and, with some failures, got up by December that year seven nice healthy plants, all of which, however, save one, I lost by an accident. That one plant is new setting for bloom—not at the axils, as the female parent (R. virgatum) generally shows, but at the extremities of the sheets, as in the male (R. ciliatum crossed by R. Edgeworthii). But, as I have had occasion to observe already, the type in all else is more that of the female than of the male parent. By the mother’s side this plant is a hybrid, by the father’s it is a mongrel, and yet it has a fair share of vigour in it. As in its sexual aspect, so in its height, it is that of the mother. A few cilia are noticeable on its leaves, but it has none of the tomentose or dense hairiness of the male parent; and so in this also it partakes most of the glabrous foliage of the mother. Again, this doubly-crossed plant, and the crosses which produced it—all extreme—show how such crossing may hasten on the reproductive or flowering state. Never in all my experience have I seen or heard of rhododendrons offering bloom at two years of age. I have rhododendrons now fifteen years from seed which have never shown the slightest tendency that way, though ten and twelve years I would consider about the mean at which they attain their flowering condition. If by such crosses the like precocity can be generally secured, practical florists may turn them to some account in their profession. You will please observe that I am now dealing with hard-wooded shrubs, where there is in general more fixedness of structure and habit, than in those on which the physiologists I have cited have chiefly experimented, and which are less liable to be modified by the manifold influences which affect the more pliant and shorter lived herbaceous genera.

   2nd. The next cross in the rhododendron tribe effected by the short stamens to which I would direct attention is very recent, and one with which I took the utmost pains to prevent miscarriage. The beautiful R. jasminiflorum of Java, with its delicious perfume and its long tubular five-lobed, flowers, of snowy whiteness, so like Erica Aitonii, so like, too, in form and fragrance, the sweet-scented jasmine, and so unlike all its own congeners, is the subject of it; and as I regard this cross of some scientific as well as of some practical value, I shall offer no apology for giving you particulars. I made it the subject of many attempted crosses by many of its own tribe—all of which failed except two, which, by the way, afford a good illustration of what I alluded to in my former paper of the sympathies of plants, and perhaps, too, of natural selection, though whether it be in the mode which Darwin regards as leading to diversity of species I cannot positively assert, yet I think it is worthy of his consideration. While it rejected so many of its legitimate brethren of the rhododendron tribe pure and simple, I was somewhat surprised that it took kindly with my hybrid B already noticed—i.e., R. ciliatum crossed by R. Edgeworthii—a hybrid of the first degree, having large flowers of three inches diameter, perfumed, and also of snowy whiteness. After the bloom had been long emasculated, on April 17, 1867, I effected the cross with the short anthers of the hybrid B. The cross took admirably——the seed-pod swelled, and was pulled fully ripe. about 12th July last. On the 15th of that month I sowed the seeds. For the purpose of comparison, I sowed a pod of its own plain native seeds which I had gathered previously, and had, in fact, sown it some ten or twelve days before I sowed the cross. These are both now up. While the native seeds have produced a fair show of feeble plants, the crossed seeds have come up in more than double the number of plants, doubly vigorous in growth and habit, and with leaves so much larger than those of the normal form as to remove all doubt about the verity of the cross.

   3rd. The next illustration I have to give you is of a small-foliaged Indian azalea, eighteen inches high, which I crossed with the tall and robust shaggy-foliaged Rhododendron Edgeworthii. Two things more unlike in every feature from which to effect a union can hardly be imagined. Yet, with the short anthers—and it was with the very shortest I could find on R. Edgeworthii that I effected it—the cross, after careful emasculation, was done on 6th May last. The seed-pod swelled to its due dimensions, and, appearing to be ripe, I cut a slice off it, and sowed the seeds so early as the 13th, and the residue on 28th, September last, and I have now got up one or two plants. If I shall be so lucky as to bring it to maturity, the progeny of this cross (one never before accomplished perhaps) should be a sweet-scented azalea, having a rose variegation like the female parent, a novelty in its tribe; for though the Azalea sinensis has been crossed by rhododendrons, I am not aware of any authentic cross, or cross of any kind, between the rhododendrons and this proper Indian azalea.

   4th. I have still further a cross of the same nature, between another Indian azalea and Rhododendron jasminiflorum, the latter being again the seed-bearer; and I here refer to it mainly as showing another tendency of this rhododendron towards natural selection, or rather perhaps of sympathy between it and remote species, if not genera, for the azaleas have till lately been regarded as a separate tribe from the rhododendrons. The cross was effected in August last, when it again rejected its more natural allies, and formed a union with the Indian azalea, a late rose-coloured spotted variety, a seedling of my own raising. The seed-pod of this cross is now at maturity.

   5th. But I have now to call your attention to a cross in this same family bearing on Darwin’s doctrine of natural selection, or of sympathy, in a still more remarkable manner, which I effected last summer between that most gorgeous of all the rhododendron tribe— namely, the lovely white, large-flowering, sweet-scented R. Aucklandi of Dr. Hooker otherwise, R. Griffithii—and an Indian azalea, the latter being the seed-bearer. I made the cross on two separate days on two separate blooms, carefully emasculated some time before; and on the same azalea I tried other crosses with several of the rhododendron tribe, viz., with a fine form of R. arboreum, R. Edgeworthii pure, and the above hybrid seedling B (R. ciliatum x R. Edgeworthii). But while every one of these failed, the crosses by R. Aucklandi, which were effected respectively on the 30th April and 1st May, took most kindly. Both pods swelled; and the seed-pods, though green, appeared to be sufficiently ripe when I pulled them. I counted the seeds in one of these pods, and found them to be about 324, all finely formed, but, I fear, too green to vegetate freely, though some which I sowed appear to be coming up. I cannot vouch for this cross being effected with the shortest stamens, for the stamens with which I effected it were kindly sent to me from another source, as I did not myself possess the male plant; but as I invariably select the shortest for such crosses, my firm belief is that I had so selected these in this instance, and I had a plentiful supply of all lengths to choose from. In the above cases of crossing a small with a large species, I hold firmly by the opinion that but for the use of the short stamens I could not have succeeded. I have few recorded instances of having extended my experiments with them far into other families. I certainly tried the pelargonium in a plant I had of the beautiful white flowered Madame Vaucher. I fertilized a bloom with its two shortest stamens, which, however, were very little shorter than the remaining ones; and, from the three seeds which came of it I raised two fine plants, far more compact and somewhat dwarfer in habit than the parent, having the flowers equally fine, and elegantly thrown up above the plant. But the short stamens of this section of the Geraniaceae are very little shorter than the others, and I therefore cannot rely much on the results as establishing the hypothesis I contended for in my former paper—namely, that where all other things are equal, a cross or simple fertilization with the short stamens tends to dwarf the progeny—to my belief in which, however, I still adhere. The instances I have given support this other hypothesis, that by their use you my cross a large on a small kindred species, a result which, without them, you might not effect.

(To be continued.)

1: 574-575 May 18, 1872
(Concluded from p. 507.)

II. CROSSING WITH LONG STAMENS.

I HAVE made fewer experiments with the long stamens, but I have one before me now no less remarkable, perhaps, for its far-reaching result than any I have alluded to as done with the short stamens. It is a cross which I effected on the tall Rhododendron formosum, fertilized with a scarlet-flowered Indian Azalea, on the 11th June last. The seed-pod is finely developed, but I have taken care in this instance to avoid pulling it too early. And I may here notice, once for all, that to obtain the seeds of a cross—especially if it be extreme—sufficiently ripe, you must allow a longer time for it than for the ripening of the normal seeds on the same plant.

In all the foregoing crosses I had, perhaps, less an eye to accomplish a purely scientific experiment than to effect a beneficial result; for, after all, it is the quid sit utile which those for whom this paper is mainly intended will have most in view; and, in my estimation, science is best promoted when she is made to minister to some useful end.

The following experiment among the species of Clematis illustrates my view of sympathy as well as of antipathy, and I would add, of unnatural selection:—Having many years ago (long before the Messrs. Jackman, who have accomplished such wonderful results) been myself working on the members of this genus, I thought of making another experiment on it, with a view to infuse a richer colour into a new and larger-flowering progeny; and, as I have observed already, I managed successfully to cross with pollen, kept for eleven months, the beautiful four-petalled Clematis Jackmanni on a thirteen-petalled flower of the fine C. candida. But it is of a cross on Messrs. Jackman’s smaller, but no less beautiful, C. rubro violacea I am now to speak. Though, like its congener C. Jackmani, it sometimes comes with five or even six petals, it is in its general type a four-petalled flower. With a view to improve it in this feature, I crossed it also with pollen of the large-flowered Clematis candida, taken from a bloom having seventeen petals, though this clematis— a French hybrid, I believe, from C. lanuginosa—is in its normal state a six or eight-petalled flower. Though I crossed two flowers, after careful emasculation, I only gathered three seeds, but these all of unusually large dimensions. After the cross had taken, I left the normal blooms on the crossed plant to their fate; and though visited by insects innumerable, and though the native pollen was abundant, not one native seed, or any except the three produced by the cross, were ever formed on the plant; and the singular thing was that, with its own native pollen, abortive on itself, I successfully crossed the fine double white-flowered Chinese C. Fortunei; and a cross more prolific in the seeds it yielded I have not seen in the tribe before. I know not the parentage from whence this C. rubro-violacea was derived, though I believe it to be a mongrel with none of the Fortunei blood in it; yet mark how kindly the latter took with it—another instance of remarkable sympathy; although I have no record of it, I think I failed to get C. rubro-violacea to reciprocate this cross.

In all these instances of sympathy and antipathy, and especially in this section of the natural order Ranunculaceae, there is something apparently so inexplicable that I can only concur with what Darwin has observed in his paper on the existence of two forms in the genus Linum, where in summing up the good gained by the inevitable crossing of the dimorphic flowers, and numerous other analogous facts, he says, that these all lead to the conclusion that some "unknown law of nature is here dimly indicated to us." And this law, when discovered, may disclose more mysteries, tending, perhaps, to the wider divergence of species, with constitutions and habits better fitted for the climates and localities in which they may be cast, as well as for subserving the purposes they are intended to fulfil in the economy of nature. In looking at Ranunculaceae, with their innumerable male and female organs (and the same thing occurs in the Myrtaceae, most of the Rosaceae, some of the Hypericaceae, and in many other families and tribes), the idea was long ago suggested to me, that each separate row, from the enter to the inner circle of the stamens, might have some separate function, just as I believe that the long and short stamens have their separate functions; and with the view of testing the matter, I had last summer begun experiments with these outer and inner stamens; but other aims and objects interfering, I gave up the experiment after I had begun it on these Clematises.

But to make success certain, it is my custom, as I have already stated, in crossing any of these polyandrous flowers, to take the entire bloom of one kind, and lightly to brush over, with all its anthers, the stigmas of the flower to be crossed, and leave nature to make her own selection. In referring to the Rubus tribe and its species, I am reminded of an intention I expressed in my former paper of perhaps returning to them afterwards. I again experimented upon them last summer. But though I tried various crosses among them, and reciprocated the cross, I had no success in any, except between the R. biflorus and the R. Idaeus, and that only where I made the latter the seed-bearer. And to make sure of either event—success or failure—l had the R. Idaeus early potted and put under glass, emasculating every bloom I meant to cross; and for more security I stripped off all other flowers—nay, more, I put the emasculated flowers under fine gauze bags, to ward off the invasion of insects. When ripe for crossing I removed the bag, and, on effecting the cross, I replaced it. In this way I succeeded in ripening three berries of the cross R. Idaeus by R. biflorus, of which I sowed the seed between the 5th and 16th July, though as yet none have vegetated. But R. biflorus stubbornly rejected a reciprocal cross. Again I tried both of these on R. rupestris, and the latter on them; and though R. rupestris showed some sympathy with R. biflorus, in a slight tendency to form seeds, these came to nothing. In all these attempts I applied, as I have said, all the anthers of the male flower.

I cannot quit this part of the subject without offering some additional suggestions to those of you who wish to act on any hints I have it in my power to give:—

   1st. If your desire be to hasten the flowering condition of plants— I recommend you to cross violently—i.e., where the allies are not too near akin, and above all, in the case of mongrels; for nature, ere she gives up, ever makes a violent effort to reproduce.

   2nd. If you wish to make your hybrid flower more freely, as well as early, adopt the same advice.

   3rd. By following it, you will find that you have attained a further advantage. Your plant will remain longer in bloom, because most mongrels, especially those among herbaceous or soft-wooded plants, to which these suggestions apply, are impotent to produce seed, or nearly so, and in such cases the blooms remain long upon the plant. I have another idea, not sufficiently tested, however, in reference to the first point among hard-wooded as well as soft-wooded plants, that all such as ripen their seeds more quickly than others (some among the rhododendron tribe ripen seed in half the time that others take) will reach more quickly their flowering state.

Lastly, as to fruits, on which, however, I have only partially tried my hand, I entertain the belief that we are on the eve of a revolution, and that by judicious and persevering crossing we may not only transfer the delicious aroma of one to another, and communicate hardier and more abundant bearing habits to the hybrid progeny, but further, especially in stone fruits, such as peaches, plums, apricots, &c., we may, in addition to these advantages, increase the size of the fruits and diminish the size of the stones; and among vines, get rid of, or greatly diminish, the number of the seeds. And all this I hold to arise from that law of nature by which she not merely strains her efforts to reproduce (to which, however, she has assigned limit), but extends it when these have failed, to make provision for her creatures’ want. These views gather strength from what has been already done; and I may especially allude to what Mr. Standish of Ascot has achieved among grapes, of whose extraordinary results an interesting account is given at p. 135 of the Journal of the Royal Horticultural Society for July 1866.

In conclusion, permit me to observe that, while my aim has been, in all the experiments I have brought before you, rather to achieve something useful and practical than to test the theories which Mr. Darwin and others—especially the Continental savans—have been so much engrossed with, I cannot refrain from making some remark on the results and the conclusions which some of them have come to while prosecuting a series of crossing operations, namely, that such crosses do and must eventuate in sterility. M. Naudin seems, like Wichura, as already observed, to have limited his experiments chiefly to herbaceous or soft-wooded plants; and among such, especially among calceolarias, I too have often found myself brought to the terminus of bitter and hopeless sterility. I remember one instance where I had reached a perfect monster for size in that tribe, but except in that particular it had no other desirable property. Determined, however, to improve it by crossing, I found on trial I could make nothing of it, and on examination I found its stigma was a hollow tube, and that its anthers were hard masses, and contained not one particle of pollen. Man may run into such mistakes, but he cannot thence conclude that unviolated nature does so. Speaking from a general recollection, which does not admit of my specifying instances, I have often found among hybrid seedlings some of a vigour which, in that respect, were in advance of either parent. May not such often occur in nature? and, as a naturally selected parent becomes the progenitor of a hardier and more vigorous race (which having in it, according to Darwin’s views, a tendency to diverge), may it not culminate in the long lapse of time into a distinct species, and even annihilate the weaker one which gave it being? So that, in nature’s crossing, may not fertility and vigour take the place of sterility and weakness, into which she so generally dwindles when modified by man’s device?