Trans. of the Botanical Society 9: 101-115 (March 1867)
On the Hybridisation or Crossing of Plants.
ISAAC ANDERSON-HENRY, Esq.
This may be truly characterised as the ago of inquiry and investigation. Into every department of natural science men, well qualified for the work, have of late years come forward, most of them honestly intent on the pursuit of truth, to stick by its revelations, uninfluenced by theories of others, or the natural bias of their own minds. But few men who have proceeded far and discovered much are wholly free from yielding to the latter tendency. None have made such progress in discovery in this field of practical botany, or by better-tested experiments, perhaps, than our great countryman; and if ever man was more enticed than his fellows—I should rather say, his followers—in that field to run ahead and be drawn into speculations, he is that man. Dissent as we may, and will, from the conclusions to which his speculative generalisations lead, all of us must, with pride, acknowledge that Darwin has thrown more light into this department of natural science than was ever done before by any, or by all, who preceded him. What Newton was in the starry spaces, he has been in the fields below. And if Newton enunciated no theory to which his fellowmen refused assent, the same could not be said of some other philosophers scarcely less distinguished. We have all heard how Kepler, having made his extraordinary discoveries in the motions of heavenly bodies, since known by the laws which bear his name, which, though he could not at first establish by proof (yet their truth has since been amply confirmed), gave loose to fancy so far as to believe that the planetary bodies, even this earth on which we live, and move, and have our being, were themselves living creatures; and have we not all heard of the theory of Laplace, who even led Sir John Herschel, and after him a train of master minds, into the faith (which it was utter heterodoxy even to question) of innumerable creations going on, on every hand, in the remoter heavens, till Lord Rosse, one fine night, with his powerful telescope, resolved the nebulae in Orion, and dissolved the nebular hypothesis for ever? In fact, it seems the besetting sin of great and original minds, just by seeing farther and discovering more than their less enlightened brethren, to run astray and get bogged in erratic ground; and that either as a sequence of such discoveries or as a prelude to them—some starting with the truth and steadfastly adhering to it so far as facts will carry them, and theorising for the rest; while others, to attain remote conclusions by a jump, boldly set forward in pursuit of the elixir vitae, or that stone by which all metals might be transmuted into gold. And these last, pursuing an ignis fatuus from the beginning, stumbled in the end upon discoveries, which, however, have since been turned to good account.
If running into error, either at the outset or in the end, give any just title to distinction, I too might put forward a claim, securely based upon these double grounds; for were I to contrast my successes with my failures, it would be as setting off units against thousands.
|*"Book of the Garden." vo1. ii. p. 319.|
I started in my experiments about or before the year 1840, and in firm reliance on the truth of the Lamarckian doctrine, as I subsequently wrote in an article I furnished, on Hybridisation, to M'Intosh's "Book of the Garden." I stated my belief that "nature in the beginning, as conjectured by Linnaeus (I should have said Lamarck), was occupied by but few original types of the innumerable vegetable forms which have been transmitted to us. How these few first types have become varied and multiplied from classes to tribes, from tribes to genera, and from genera to species and endless varieties, belongs to those mysteries of divine agency which set all inquiry at nought," &c.* Yet I could not discard the faith (natural enough), that by skilful manipulation, and studying the times and seasons favourable and unfavourable to such operations, I could make nature stretch a point so as to restore union, if not unity, between and among things related by cousinship some fifty times removed; and by this means transmit among flowering plants all that was beautiful in colour and elegant in habit, from any one vegetable form to any other at all akin to it. In like manner, among fruits, how easy did it appear to infuse the rich aroma of the strawberry into every cognate thing. When I look over my notanda of fifteen or twenty years back, I see veritably recorded in my experiment books such and such things to be done in this way—of which, perhaps, the simplest might be to cross certain species of the Rubus with the Fragaria, among which genera the raspberry in the former and the hautbois in the latter tribe looked promising. Again, having got a small trailing species I raised from Andean seeds (Rubus glabratus), I meant to cross it with R. arcticus, or with R. Chamaemorus—in short, I formed great designs of intercrossing the Rubus family, one with another, on a large scale, and felt certain of a successful issue. Though I failed in all of these, I never doubted my ability, and certainly it looked among the most feasible of things, to improve the bramble by crossing it with the raspberry. And although I have heard that such a union has been accomplished, I remain an unbeliever; for, among all the most intractable things I ever took in hand, I found the intercrossing of any one species of Rubus with another (and I have tried them in all possible ways, and under the most favourable circumstances) the most impracticable, and as yet, I have only to record universal failure. I may, however, return to this tribe hereafter.
Some of my earliest efforts, however, were among what have been since denominated "florists' flowers," such as the calceolaria, the dahlia, the fuchsia, &c.
At that time every colour had been brought out in the dahlia save blue; and some began to speculate upon such a colour being realised, though none, so far as I was aware of, ever suggested the means of accomplishing it. This, however, seemed to me no great matter to achieve. I looked over the tribes bearing the nearest affinity to it, among its natural family, the Compositae, having the desired colour, and I found many flowering plants, such as Aster, Agathaea, Kaulfussia, &c., having various tints of blue, sufficient, as I thought, for my purpose. With the pollen of these upon a white dahlia a blue might, I believed, be obtained; but it was calling spirits from the vasty deep, for neither blue nor even white, not even a ripened seed, ever came of it.
I need not allude here to similar efforts with similar results among other tribes; for, as an untaught experimentalist in botany, I felt fettered by none of its laws, and became a law unto myself, believing that failure now might be success again, and so I went forward. Unvarying failure, however, damped my zeal bit by bit, and I began to see that I could not transfer a colour alien to any one genus from another genus remotely akin to it, to which such colour was common. Yet ere I leave this subject I may observe a rather unusual freak of nature, which I set down as due to an experiment of a somewhat similar kind. At the period I refer to, now upwards of twenty years ago, we had few species of fuchsia save F. globosa and fulgens. It was the rage then, as it partly is still, to bring out large blooms; so, with that object in view, as well as to infuse some intermixture of colour, I crossed, or attempted the crossing, of F. globosa with a yellow-flowered Œnothera. I cannot vouch for the cross being true, for the progeny was a plant to all external appearance a fuchsia, like F. globosa, its female parent, having flowers of the ordinary globosa form and size, but with lightish green tips on the tetrapartite calyx. Now, although I could not regard this plant as a hybrid or mule, yet I could not reject the evidence of these light tips so far as to believe the Œnothera had not influenced them. The fuchsia and Œnothera, though so unlike, stand in no remote degree from each other in the tribe Onagrariae, a tribe, be it remembered, of but few genera.
Well, this was like the "glorious nibble” to the zealous angler, who fished all day on the faith of it. But it helped to keep me from further seeking to outrage the modesty of nature, as in my effort to change a white into a blue dahlia. Yet for that wild dream of fancy I was not wholly without warrant, or without a colourable pretext, for ere I gave into it I had, by crossing a red-flowered calceolaria on one with purely white flowers, produced a change in the purity of the untouched flowers of the latter plant, many of which soon thereafter became flushed with a roseate tint. I communicated the result to Dr Neill, who, I remember, felt great interest in it, instancing something of a like nature produced by grafting operations, communicated to him by Mr Brown, of Perth.
Ere I pass from the field of my dreamland, it is right to observe that any efforts I made at that time to transmit perfume from one plant to another led to no satisfactory results, just from having no plants then near enough related to effect it. That it may be done, however, and done effectually, I have since again and again proved, as others too have proved among rhododendrons, by crossing R. ciliatum with R. Edgworthii. I made this out long before these Sikkim species came to help us—viz., by crossing our common European alpine species, R. ferrugineum and R. hirsutum, with R. formosum (otherwise R. Gibsonii), a beautiful Indian species; and the like result I obtained by crossing the latter on R. atrovirens—of all which crosses I have still many plants in my garden, all quite hardy, and which flower abundantly every year. In these latter hybrids the perfume is faint compared with what it is in the Edgworthii cross; but of the Rhododendra hereafter.
To revert to my earlier operations. Finding that nature would not do as I bid her, I resolved, as far as possible, to find out her way, and do as she bid me. I felt she had a clue to unravel if I could only find the end of it; so I took in sail, lowered my expectations, and betook myself to merely muling.
To go no farther back with my experiments than 1842, I tried to accomplish breeds between several kinds of fuchsia and Epilobium, a genus certainly not far remote from the fuchsia, especially the F. excorticata of New Zealand. The F. corymbiflora, then new, I drafted into service likewise, crossing it with Œnothera serotina, &c. I wrought with Œ. rubra and also Œ. alba, to whose agency I had a notion the first pure white fuchsia (named Venus victrix) was in some way due. I also wrought with Epilobium angustifolium, and some time after this with Zauschneria californica, still more closely allied to the fuchsia. But I failed in all to effect an intermediate. In some cases where I inverted the cross, making the fuchsia the male parent, there appeared to be seeds partially ripe, borne by the Epilobium, but nothing ever came of them. I was reluctantly obliged to abandon this family altogether. One great object I had in view was to effect a change in the colour of the bloom, as well as in the habit and hardihood, of the fuchsia.
In the following year (1843) I changed my tactics. The fuchsia Venus victrix having a small flower, I entertained the hope, by crossing it with a fine large flowered variety, called F. splendida, to produce a larger variety, with all the purity of that still the purest of all the white varieties. The better to secure this, the thought struck me of shading both the pollen and seed-bearing plants. With this view I shaded the female plant (Venus victrix) by covering, with thin muslin bags, its emasculated blooms, and, at maturity, fertilised these with the unsunned pollen of F. splendida. I did get ripened seeds, and sowed them, but nothing came of them—at least I obtained no purer white flowered seedlings. I inverted the cross, making F. Venus victrix the female, and F. fulgens the male parent, but with no better success. I, however, got ripe seeds by this cross also.
I, again, that same year, to get better expanded flowers, tried a modified form of muling, and effected crosses between F. cordifolia (mater) and F. fulgens (pater), adding the pollen of the latter to that of the Œnothera serotina. I got ripe seeds, but of these, though sown, I have no record; and I have since been fully satisfied that in such cases foreign pollen added to native is impotent.
I hope, gentlemen, you will bear with me while I follow out some more of these operations, which, however discouraging to me hitherto, were yet, at least some of them, not wholly so disheartening as to preclude hope. There is no denying of the truth: I had pinned my faith, right or wrong, to a theory, to make the whole world (I mean the vegetable part of it) kin. I had drank too deep of the Lamarckian spring to lose heart without further trial.
So, persevering in muling, I had got mulish in the belief in my ability in the end to surmount all, at least many, of the difficulties I had met with. But I will not tax your forbearance with the innumerable and literally fruitless experiments I made in this field, and of most of which I kept no record; so I shall pass on to some more favourable results I accomplished some few years afterwards.
I find from my note-book for the year 1847, that I made some attempts among the Scrophularineae, a family most of whose extant tribes stand in much closer relationship to one another than the Onagrarieae. The Torenia asiatica, a most beautiful plant, introduced about that time from India, offered to me a very tempting opportunity of forming by it a union with a Californian plant of a nearly allied genus—namely, the Diplacus puniceus. And I may here notice a fact I have found of almost universal occurrence among my experiments, that when I had to cross an American with an Asiatic species, it took much more kindly than crossing either of these, especially the former, with European species; and lest I shall not have another opportunity of recurring to this subject, I may here observe also the decided preference of plants of the southern hemisphere to intercross among themselves, however remote their original homes may be—e.g., I found how much easier it was to cross Australian and New Zealand plants with their allies of South America, than with European or kindred things in the northern hemisphere. I have also observed that proper American species have greater aversion to cross with European than with Asiatic species, and that Asiatic species have no less aversion to intermix with European kinds.
There is only one instance, I remember, of effecting a successful cross between an Asiatic and a European species, and that was in crossing a small species of rhododendron with yellow Helianthemum-like flowers, being a form of Rhododendron lepidotum called R. elaeagnoides, of the Sikkim ranges, on R. ferrugineum, a European kind. Of this cross I raised two plants; one died, and I kept the other for years; it flowered with me, the blooms being dirty red, splashed with a pale yellow tint. It was an odd looking thing, and I afterwards sent it to Kew as a botanical curiosity. What became of it there I never heard.
In the early summer of 1847 I crossed the Diplacus puniceus with Torenia asiatica, and on 3d August I got, as I believed, a pod of ripened seed, but I found I had pulled it too early, and the seeds do not appear to have sprung, at least I have no note of it if they did. I may observe here that it was and is my custom always to remove the native anthers long before maturity, deferring the cross generally for three or four days, or even a week thereafter—in fact, till the stigma was fully matured—always marking it as so emasculated; and this was carefully done here. From another pod of the same cross, but not so marked, I obtained nine plants, but what became of these I have no record; very likely they were failures.
I further tried, and succeeded in crossing another Torenia—viz., T. intermedia—with the Mimulus, making the latter the seed-bearer. Of this cross the pod contained seventeen seeds, which I sowed on 17th September 1847, and at October 19th I had nine plants finely vegetated. But of these I have no further record. I perhaps lost them during the following winter; for in the few proper mules I have succeeded in raising I ever found them most difficult to rear. In this cross, as well as in the following, I had, I find, made sure against self-fertilisation by timeous removal of the male organs.
I further, in 1847, attempted a cross between the Digitalis purpurea, var. alba, and the Torenia asiatica, making the former the seed-bearer, and got something like immature seeds, which I sowed, but nothing came of them. In the same summer of 1847 I effected another small success by obtaining one pod of ripe seeds of Isotoma axillaris crossed with Lobelia ramosa. These I sowed on 18th September, and at 24th November I potted off nine small seedlings. On September 30th, I obtained of the same cross another ripened pod containing sixteen seeds, which I do not observe I had sown.
On December 4th, of same year, I obtained another pod of ripened seeds from the same Isotoma axillaris, crossed in this instance with Lobelia coronopifolia, from which I raised ten plants, potted off 12th January 1848. What came of them I have no record; but I have found out, much to my cost, that of few things sown as late as October, and before January, can much account be given in the following March. Yet those who, like myself, have only been able to gather such seeds so late, are placed on the horns of a double dilemma—they must either sow at once, at great risk, or put off till spring, when, with longer keeping, the vital principle, weak at first, may be gone.
Yet the fact of seeding and raising such things at all was encouraging, and tended to make me more observant of the ways and means by which success might be made more certain. That climate, and especially some peculiarly favourable states of the atmosphere, had much to do with it, I felt every year more and more certain. I had by this time found that crosses which I could easily effect at one time were utterly impracticable at another, and that this ill-conditioned state would sometimes extend over whole summers. But of all times and seasons, I found such weather as frequently preceded thunder, and, oddly enough, which sometimes followed it—when there was a genial balmy texture in the air, not so much from sun heat, as sometimes arises from the presence of a larger portion of electricity than is usual, when every living thing seems more than ordinarily alive and happy—to be the season of all others for a bold experiment, and I seldom failed to improve it. Such a season happened in the spring of 1850; but before I relate a small success I then achieved, I must relate another observation which had been forced upon me from some of my manifold efforts in this way, and I think it will be better I state it by giving it as set down in my note-book, under dates April and May 1850:—"Discovered that the short stamens of Rhododendron cinnamomeum, and particularly of R. catawbiense, crossed the small Rhododendron (Rhodothamnus) Chamaecistus." By the short stamens of R. cinnamomeum I had further crossed the same pigmy species of Rhodothamnus with a large white-flowered Indian species; the cross being performed in the beginning of March 1850, which I noted as then (19th May) well on towards ripening.
Off the above muling operations I got some pods of ripened seeds, especially where the Rhodothamnus was crossed by the large Indian rhododendron, than which none to all appearance could be more perfect—yet nature had been too far strained. I sowed the seeds, and though I preserved the seed-pot for years, not one seed ever vegetated. I was, however, more successful with another cross, which I felt impelled to try by the extraordinary mule raised by Mr Cunningham, of Comely Bank, and about which so much mystery was observed—and for one good reason, as I have since learned, that he himself, crossing at the time so many things, one with another, did not precisely know its parentage, further than that the seed was borne by a Menziesia crossed, as he might believe, by the Rhodothamnus Chamaecistus. I have now reason to believe, from having wrought much in this section of the Ericaceae, that the seeds of his mule called Bryanthus erectus were borne, not by the Menziesia coerulea, as was generally believed, but by M. empetriformis crossed by Rhodothamnus Chamaecistus; for I had myself been trying a similar cross, before I knew of Mr Cunningham's, between M. coerulea and the same Rhodothamnus, which, however, had failed. Having, thereafter, when in his nursery, been shown Mr Cunningham's mule by himself, though he declined to say what the parentage was, I felt assured that I was not far off the truth when I told him my belief that it was the very cross I had been attempting, which he, however, would not admit. But, not to be baffled, I set to work afresh, but now inverting the cross, making M. coerulea the seedbearer; for before it was the Rhodothamnus. It succeeded. I obtained ripened seeds in June 1850, and in September following I had four young plants, which, however, were unfortunately devoured one night by a snail. In this I had another instance of success being secured by inverting a cross.
From this time forward I dealt liberally with the short stamens, believing, as I still do, that with these—I mean the pair of shortest stamens which occur in numerous families of plants—the larger species may be made to intermix with the smaller species of its tribe, where otherwise no union could be effected.
But there is one singular result which I think I have fully established as ensuing from the use of these short stamens, especially where the kinds crossed are homogeneous, or not remotely allied, and where there is no great disparity in size between their sexual organs.
Pursuing the use of these diminutive organs, especially where I crossed a larger on a smaller species, I find that I had, in the spring of 1855, manifestly used the short stamens of Rhododendron Edgworthii in effecting a cross on R. ciliatum (both Sikkim species), with the object of warding off the tall reed-like growth of the former, and securing the dwarfer, bushier habit of latter species in the progeny. I gathered and sowed the seeds of this cross on 7th November 1855, and I have still three plants alive of this brood, the height of two of them being only 4, and that of the other 4 1/2 inches. These are now in their twelfth year, and have never shown the smallest tendency to bloom. [Two of them are now on the table before you.] I have no doubt of these being the produce of the short stamens.
Earlier in the same autumn I gathered and sowed seeds of the same cross, effected, I have no doubt, with pollen from the longer stamens, for the plants are taller than R. ciliatum, and shorter than R. Edgworthii, the male parent, of whose delicious perfume the flowers largely partake.
My attention was first called to the like effect of dwarfishness being produced from the use of the short stamens in the pelargonium tribes by an article written by Mr Beaton in 1861, in the "Journal of Horticulture;" and it is not unlikely that the same law holds in other races where the short stamens occur.
I was first induced to use these pigmy stamens of the larger species from the belief that their pollen must necessarily be smaller and finer in its granules than that of the larger anthers, and therefore more likely to pass down through the ducts of the female organs of the smaller kindred species. I cling to this idea still, and, if I am correct in it, may not the several pairs of anthers—i.e., the intermediate and longest styled anthers—have severally their separate functions? Much patience is needed for pursuing experiments here, but it is worth the trial. May not the colour and perfume of flowers, and the size, fecundity, and aroma of fruits, too, depend upon the proper selection here? In all my difficult crossing operations latterly I used the pollen of all the anthers, leaving nature to select for herself.
This suggests to me a matter I may as well treat of here as afterwards, and which is referred to by Mr Darwin at page 545 of the latest edition of his "Origin of Species," where he holds the sterility of hybrids to be a different case from that of a first cross, the reproductive organs of hybrids being more or less functionally impotent, and for which he shows apparently very valid reasons. Yet I may refer to a pretty well-known hybrid of my own as affording, in its case, no failure from either of its parents in this respect. The Veronica Andersonii, a hybrid between V. Lindleyana (mater) and V. speciosa (pater), yields seeds, perhaps, in larger abundance and of equal fertility with those of either of its parents. Another brood which I raised, between Veronica decussata (V. elliptica, Hook.), of the Falkland Islands, and V. speciosa, of New Zealand, bears seeds in equal abundance with its parents, and likewise of equal fertility. These were, of course, seeds of original hybrids. But I found in another hybrid, which I effected many years ago, between our own alpine veronicas—viz., V. saxatilis, having rich bright blue flowers, and V. fruticulosa, having white flowers, striated with pink—which I honoured with the name of our honorary secretary, Veronica Balfouriana, having blue flowers striated with red, that the seeds of the second generation from it, at least of one whose flowers diverged from the colour of either of its parents in becoming red, no seeds of that red descendant, though tried over and over again in successive years, ever vegetated.
I may now, ere I quit this subject, observe a very singular phenomenon in increased fecundity in seed-bearing of a first-cross. When the beautiful and fragrant Rhododendron Nuttallii was introduced from Assam into our gardens, I shared, in common with others, a desire to try what could be made of it by crossing with other species; and of its progeny in this way I have two or three broods, some of them now of 2 and 3 feet high. These operations were performed in the spring of 1862. On the 12th January 1863 I pulled from Rhododendron Dalhousiae a seed-pod so crossed with R. Nuttallii, the size of which I carefully measured, and found it was 1 5/8 inch in length, by 2 inches in girth; whereas the largest normal seed-pod gathered, equally ripened from the same R. Dalhousiae at the same time, I found to be only 1 1/4 inch long, by 1 1/2 in girth. But the most remarkable thing of all about this cross was that, though the seeds were fully ripened, and in such abundance as I never before saw equalled in the family, and though, when sown, the seedlings came up thick as chickweed, yet every one of them died off in the seed leaf or second pair. Though they came up in thousands I could not preserve one of them. Yet, singularly enough, I have raised no end of another brood, obtained by crossing R. formosum with R. Nuttallii with another hybrid of my own, obtained by crossing with R. Dalhousiae. The fertility here was due, I have no doubt, to the infusion of the "blood" of R. formosum. As a proof of this, I at the same time sewed the seeds of a cross between R. formosum (pure) and R. Nuttallii; the seedlings came up in double profusion. Are these results due to a nearer or remoter vicinage of original habitat? for all are Himalayan species, the R. Dalhousiae being from the Sikkim, the R. formosum from the Khosia, and the R. Nuttallii from the Assam or Bhotan ranges.
But I am digressing sadly. My object in the experiments above noted has mainly been to show that nature abhors all alliances in relationship beyond the closest affinities. Members of many genera, besides those so close as the well-known instances of the apple and pear, the gooseberry and currant, obstinately resist all intermixture by crossing. I have already noticed, in addition, the bramble and raspberry, and I could add many others equally closely allied, but equally intractable. I may return to this question of unaccountable antipathies in a subsequent paper, where I may notice some equally unexpected sympathies between unlikely species, of which I may here note only a single instance.
Having two very distinct species of Browallia in my garden—one a shrub, growing from 4 to 6 feet in height, viz., Browallia Jamesonii, an orange-flowered species, and another, a tiny blue-flowered herbaceous annual, from 6 to 9 inches high, both from the Andes—it occurred to me to try a cross between them. I made the cross on the B. Jamesonii, having previously most carefully emasculated the blooms. The cross was made on the 17th June 1865. I gathered the seed on 5th July that year, but the seeds, though well formed, being immature, never vegetated. This cross I mean to try again; for the seeds, though not ripe, were no mere embryos, which often occur in such extreme crosses.
|*Since writing the account, I have just read of the Cytisus purpurascens or Cytisus Adami, and stating, as I have done, on the authority of a notice given of it in Lindley and Moore'e "Treasury of Botany," of its being a hybrid, I have this morning read another account of its origin in The Farmer of yesterday, where, reporting the proceedings of the last meeting of the Royal Horticultural Society, it is stated, "Mr Lee, Cliveden, Bristol, sent most remarkably dissimilar examples of apples from the same branch of a tree of orange Pearmain, which was a fertile subject of comment at the meeting. The tree was the true variety, and the other samples were of a russetty cast, instead of the bright crimson colouring common to the original. Rev. Mr Berkeley instanced Cytisus Adami as a sport of a similar character, which is believed to have been produced by grafting Cytisus purpureus on the laburnum, and by some accident one cell of the stock and one of the graft having each become divided, and then united together, the result had been a plant partaking of the nature of both. Mr Berkeley suggested that it would be most interesting to know the stock upon which the orange Pearmain had been worked." Whatever be its origin, the facts I have stated, and which probably many of us have seen with our own eyes, of the same tree producing three kinds of flowers, and two, if not three, different kinds of leaves, there can be no doubt of these having resulted from the operation of grafting. The two kinds of fruit, too, of the Pearmain seem to have arisen from the same cause. And it would seem, also, that many of the sports we see and hear of in roses, in changing colour, and betaking themselves to a climbing habit, are due to the same cause.|
I cannot close this part of my paper without suggesting to others an experiment I intend to attempt myself, if spared, on a small scale this ensuing summer, and that with a view to bridging over some of those gaps over which nature will not of herself be made to leap. I have been again and again surprised how near, and even afar off, allied things will incorporate, by the simple act of grafting—an act by which the sap and every vital principle which sustains the one must now animate both, and yet the two living things so made one will not, in their separate state, unite by any act of crossing yet resorted to. Hence it occurred to me that a union might be tried between the two separate subjects by fertilising the flowers of the ingrafted plant with the pollen of that species which forms its stock. For example, I have several pear trees ingrafted on the common white thorn, which flower and bear abundantly every year. I mean to try how far the pollen of the latter (Crataegus Oxyacantha) may not fertilise the other, and produce an intermediate. I wish much that others would try what could be done in similar cases. The rationale of this experiment is strongly supported by a circumstance noticed in Loudon's "Gardener's Magazine," vol. xiv. p. 430, of a male plant of the Carica Papaya, a dioecious plant, having borne female flowers at the extremity of the racemes of a male plant, which was accounted for by the fact of the plant having been, two years before, inarched on a female plant of the same species, that part which bore such female flowers having grown subsequent to the act of grafting. The familiar case, too, of the Cytisus purpurascens or Adami, a hybrid between the common laburnum and Cytisus purpureus, affords another striking instance of the influence of the stock on the scion. For when grafted, as it generally is, on the more vigorous laburnum, shoots sometimes of a mixed character, partaking of both stock and scion, and sometimes of the laburnum, pure and simple, with its proper foliage and flowers, spring out from the branches of the C. purpurascens.* Other instances might be given, but for the purpose in view those cited may suffice.
I am yet but on the threshold of a vast subject, with what relates to pure hybridization—i.e., the crossing of one species with another distinct species of the same genus—and the crossing of varieties scarcely touched upon. With some of my experiments in these still ample fields, I may tax your patience in some other paper at a future time.