HYBRIDISATION AND ITS
By the Rev. Professor G. HENSLOW, M.A., F.L.S., V.M.H., &c.
|*Introduction to the Handbook of the British Flora, 1865, p. xxxvii.|
INTRODUCTION; DEFINITION OF A SPECIES.—In endeavouring to find some clue to the interpretation of hybrids, as to why some species when crossed fail, while others succeed and bear fertile offspring, it is desirable to consider what is the present idea of a species. Two considerations were formerly maintained, viz. morphological structures and a presumable physiological affinity. Thus Bentham defined a species as follows: "A species comprises all the individual plants which resemble each other sufficiently to make us conclude that they are all, or may have been all, descended from a common parent."*
|†Amaryilidaceae, p. 336.|
This definition may be sufficient as long as no physiological question is raised as to the capabilities of different species of the same genus intercrossing. Dean Herbert, however, soon found that another element must be considered, and that was interbreeding. Since the practice of hybridising plants has been extensively pursued ever since he wrote, the idea has been maintained that if two species would cross and produce fertile offspring, then they must be regarded as of common parentage, and as being only varieties of one and the same species. Thus Dean Herbert writes, referring to experiments of Knight: "The President adopted in his writings a principle or dogma, which seemed to be then much relied upon by botanists, that the production of a fertile cross was proof direct that the two parents were of the same species, and he assumed as a consequence that a sterile offspring was nearly conclusive evidence that they were of different species." He then further adds: "I held also . . . that the production of any intermixture amongst vegetables, whether fertile or not, gave reason to suspect that the parents were descended from one common stock and showed that they were referable to one genus; but that there was no substantial and natural difference between what botanists had called species and what they had termed varieties ... If two species are to be united in a scientific arrangement on account of a fertile issue, the botanist must give up his specific distinctions generally and entrench himself within the genera."† Testing the question as to the more or less agreement in external features between so-called closely allied species being correlated with fertility in their hybrids, we now know that the general rule maybe formulated that such is the case; yet there are so many exceptions that the suggestion of Herbert for systematists to follow must be disregarded, and that they must continue to describe new species and genera solely by the morphological characters they present.
This is practically what is always done; so that for purely systematic purposes it would seem that physiological affinity must be neglected altogether, as, e.g., when masses of dried plants are sent to Kew from some newly explored country.
What then is a definition of a species? The following may perhaps answer the question. A species is known by a collection of, presumably, relatively constant characters; which may be taken from any or all parts of the plant. But how many features are required to distinguish a species from a sub-species or variety is a matter of opinion, and will always remain debatable. Indeed, the difference between an "artificial" and "natural" system of classification depends greatly on this point: for any group in the former is based on one, two, or very few points of agreement; in the latter it is generally on as many as possible. Though, in many cases, a single character may coincide with the strictest affinity, such as the tetradynamous stamens of the Cruciferae, the papilionaceous corolla of a great section of Leguminosae, &c.; and when we come to other large groups with irregular corollas, we find that systematists professing to classify plants on a natural system do not hesitate to drop into an artificial one when it suits their purpose. For example, Liliaceae are separated from Amaryllidaceae solely by having a superior ovary. Yet elsewhere we can find both inferior and superior ovaries in genera of the same order, as in Samolus and Primula of Primulaceae; or, again, in species of the same genus, as Saxifraga tridactylites, S. umbrosa, and half-superior in S. granulata.
But although the two orders mentioned above are separated on account of this single character alone; yet, testing it by crossing, no known attempt to unite two members of these orders has ever yet succeeded, as far as I can hear from experimenters. It would seem, therefore, that they have been differentiated at so remote a period that they have lost all physiological connection.
So too with genera; the corolla of Snapdragon only differs from Toadflax in having a small pouch at the base, which elongates into a spur in the latter. I can hear of no cross raised between them. Now Rhododendron, Rhodora, and Azalea are as much entitled to be called genera respectively, if systematists may separate genera by such slight differences as the above; and there is no reason why they should be merged into one, solely because they will interbreed: for if interbreeding is to be a test, then those polymorphic forms of one and the same species that cannot be intercrossed with complete fertility ought to be separated, as of Lythrum; to say nothing of Linum perenne and some Orchids which cannot bear seed with their own pollen.
Rhododendron jasminiflorum has a corolla as unlike that of a typical Rhododendron as can well be imagined—indeed, Mr. Burbidge likens it to Erica Aitonii—and might be regarded, therefore, with justice as a different genus; since systematists separate the genera of plants with irregular corollas entirely by that organ in many cases—as in the Scrophularineae. Now it will cross readily with R. Javanicum, which has the typically formed corolla; but not with the American, or species of other countries. On the other hand, Mr. Burbidge crossed R. jasminiflorum with an Indian Azalea as the male parent.
|*See observations by Mr. C. C. Hurst relative to this matter, Journ. R.H.S. 1898, p. 475.|
Let us take as another instance, the "genera" Laelia and Cattleya. Species of these two have yielded many so-called "bi-geners"; but are they worthy of the name? Now the variations in the forms of the flowers of different species of each of these two genera do not differ more among themselves than between different species of these genera. In other words, they would form one genus if the perianth alone were the basis of classification. But this is not the feature relied upon, but the number of pollen masses—i.e. a single feature, and therefore, so far, an artificial character—just as Linnaeus would unite the Ash tree, Veronica, a Grass, and the Duckweed because they have two stamens. Turning to the "Genera Plantarum" we find that Cattleya has four pollinia. Then follow three genera with eight in two series; those of the upper series very often much smaller than the lower. Then comes Laelia, also with eight pollinia in slightly unequal series.*
Here, then, is obviously a closely graduated series based on a single character, and a purely artificial one. The classification is therefore not strictly natural, though the series of so-called "genera" may be. Consequently, though we may call the cross-products "bi-geners," it is only so from Bentham and Hooker's classificatory point of view.
These observations apply to Epidendrum and Sophronitis as well.
Similarly with the so-called bi-gener between Lapageria and Philesia. Those genera stand together in the "Gen. Pl.," being No. 10 and No. 11 in Liliaceae: both are mono-specific, and both live in Chili. The distinguishing features are recorded as being, in Lapageria, "Leaves 3-5-nerved," and " the segments of the perianth sub-equal." In Philesia, "Leaves 1-nerved," and "the exterior segments of the perianth are much shorter than the interior." But much greater differences in nervature occur in species of Plantago; and also between the outer and inner whorls of the perianth of species of Iris.
Consequently, to be true to principles of natural classification, it would seem that the above two genera should be regarded simply as two species of the same genus.
|†On Hybridisation amongst Vegetables.|
Once more, in the "Gen. Pl." Gloxinia (gen. 6) and Achimenes (gen. 7) belong to the sub-tribe Gloxineae; while the genera Gesnera (gen. 18) and Sinningia (gen. 19) are in the sub-tribe Eugesnereae. There is, however, no special feature to separate them—a fact which Dean Herbert perceived and discussed at length some seventy years ago.†
He mentions also that Sinningia and Gloxinia produced fertile hybrids.
|‡For successful Bi-geners
and Hybrids in Gesneriaceae,
see Burbidge's Prop. And Improv. of Cult. Plants, p. 331 seqq.
Gloxinera—i.e. Gloxinia x Gesnera—was raised in 1894.‡
Selenipedium and Cypripedium are genera which Bentham and Hooker admit to be scarcely distinguishable except by the ovary being one-celled in the former, from a want of cohesion of the placentas, and its habitat, viz. South America, the nearest home of the latter being Mexico.
|§ Gard. Chron. Oct. 10, 1896, p. 435.|
Though it has been found difficult to cross these, yet Mr. Swinburne, of Winchcombe, near Cheltenham,§ raised small plants from S. Schlinii x C. Spicerianum, male patent; also between S. Dominianum x C. Chamberlaini. The plants were raised in 1896, but have not yet flowered.
BI-GENERIC FAILURES.—If species of the same genus, but natives of widely distinct countries, often refuse to cross; a fortiori, would it be anticipated that genera of the same order would fail? The genus Hippeastrum has been used for attempted crosses with other genera from warm countries, of the order Amaryllidaceae. Thus, it has failed with Sprekelia, the former being of tropical and South America; the latter—a monotypic form—of Mexico.
Hippeastrum has failed to produce healthy progeny with Clivias of South Africa. Mr. Wright observes: "This attempted cross was successful so far as the actual cross went; but the progeny were so weak that the seedlings only lived about a year. This proved to be the case with three distinct lots of seeds."
|*Gard. Chron. Jan. 5, 1895, p. 16.|
M. Rodigas, of Ghent, makes a suggestive observation: "The decay in the persistent leaves of many plants of Clivia may be attributable to the employment of pollen from Amaryllis and Hippeastrum, the leaves of which are deciduous."*
Hippeastrum has also failed with Vallota and Haemanthus, both of South Africa. Attempts have been made to unite Hippeastrum with Urceolina (Andes) and Pancratium (Mediterranean regions) without success; but as these genera belong to a different sub-tribe, as well as widely different countries, the probability of their having any physiological or constitutional affinity was proportionally diminished. Similarly, attempts to cross Griffinia, allied to Hippeastrum, with Eucharis and Urceolaria failed.
Of other genera of the Amaryllidaceae of widely different countries that have failed are Amaryllis Belladonna (South Africa) with Lycoris (Japan, China, &c.).
Of two genera of the same sub-tribe—Cyathiferae—Pancratium canariense failed with Eucharis grandiflora (Andes).
But genera from the same country may fail, as Cyrtanthus with Vallota, genera closely allied and both of South Africa. Similarly, Zephyranthes brachyandrum has failed to cross with Hippeastrum stylosum, H. sub-barbatum, H. equestre, and H. vittatum, though these two genera are very closely allied; but while Zephyranthes are natives of tropical and sub-tropical America, Hippeastrum belongs to South America.
|†Other failures among genera of AMARYLLIDACEAE are Elisena longipetala x Hymenocallis calathina; of IRIDEAE, Cypella plumbea x Herbertia pulchella, Iris Robinsoniana x Marica caerulea; of AROIDEAE, Alocasia x Caladium.|
Crinum and Amaryllis (gen. Nos. 26 and 27 in " Gen. Pl.") have failed to cross as far as the following species are concerned: C. Moorei, C. fimbriatulum, and C. zeylanicum.†
A large number of bi-geners have been attempted at the Utrecht Botanic Gardens, but without results. The following is a selection: Helleborus x Caltha, Caltha x Eranthis, Caltha x Nymphaea, Caltha x Paeonia, Fuchsia x Oenothera, Bellis x Cineraria, Hemerocallis x Lilium, Pancraticum x Crinum, Phalaenopsis x Vanda, &c.
Of other genera in which the morphological characteristics would warrant an a priori probability of success in crossing, but failed on practice, is Streptocarpus x Didymocarpus; but while the former genus is found in South Africa and Madagascar, species of the latter are natives of the Malay Archipelago, and E. Asia; so that in this case constitutional affinity does not correspond with morphological resemblances.
That Streptocarpus should fail to be crossed by Gesnera was more likely, as, besides being natives of different hemispheres, they are distantly located in the "Genera Plantarum."
Of sub-genera that failed to cross, Mimulus x Diplacus may be mentioned.
Though Epidendrum radicans has been successfully crossed as male with Sophronitis grandiflora female, yet S. violacea has been crossed with E. radicans and E. Obrienianum without result.
CONSTITUTIONAL AFFINITY AND STERILITY.—Admitting the fact that the closer agreement there may be between the forms of two species, the more likely is it that they will cross, yet it is not universally true; so that, leaving "morphological affinity" out of the question, we have to depend on, for want of a better expression, what one may call "constitutional affinity," cautioning the reader that this is a phrase which covers our profound ignorance of the true nature of physiological affinity!
|*Amaryllidaceae, p. 340.|
Dean Herbert observes: "Some crosses are sterile and some quite fertile, without any apparent reason, except the greater or less approximation of constitution in the parents; and that the cross-bred plant which has seemed for a long course of years to be absolutely sterile becomes under some circumstances productive."*
|†Origin of Floral Structures, p. 209.
‡Ibid. p. 320.
The last sentence is important, for it introduces another fact, that sterility and fertility are not absolute features, but vary in the same plant according to circumstances; and it applies to self-fertilisation as well as crosses and hybrids. Thus, Darwin found that the dimorphic forms of Linum perenne were self-sterile; but Mr. T. Meehan, of Germantown, Philadelphia, had one form only in his garden, which never set seed for fourteen years; yet, then, one branch bore flowers which became homomorphic and immediately fruited. Under cultivation Primulas of various kinds, as P. sinensis, can become self-fertile in a similar way.† Eschscholtzia californica was self-sterile in Brazil, but acquired great self-fertility in England in three years—nearly 87 per cent.‡
|§ Amaryllidaceae, p. 342.|
Dean Herbert says elsewhere: "Experiments have confirmed the view to such a degree as to make it almost certain that the fertilisation of the hybrid or mixed offspring depends more upon the constitution than the closer botanical affinity of the parents."§
He illustrates this by the genus Crinum, showing that while certain nearly—i.e. morphologically—allied forms are difficult to produce fertile hybrids, others so distinct as to have been placed in different genera do so, the interpretation being that the latter were aquatic plants, and therefore presumably of the same constitution; whereas in the former case the ineffectual cross was made between an aquatic and a terrestrial form frequenting dry localities.
This hybrid between C. capense (aquatic) and C. scabrum continued for sixteen years to be sterile, but "produced one good seed in 1834, and again in 1835." He mentions another instance of a sterile hybrid, called C. submersum, growing near Rio Janeiro in company with a small variety of C. erubescens. It was exactly intermediate between this species, which is aquatic, and C. scabrum of high ground.
|*Amaryllidaceae, pp. 343 and 345.|
Of dissimilar species readily crossing, Dean Herbert* alludes to "the prickly, angular Cereus speciosissimus, the flexible C. flagelliformis or Whip-plant, and the unarmed C. phyllanthocides, are nearly the most dissimilar; yet they have produced mixed offspring, which readily bears edible fruit of intermediate appearance and flavour."
As illustrations of failures through constitutional differences, Mr. Buffham could obtain no success between perennial and annual species of Sunflower. With Rhododendrons, Mr. Veitch could get no hybrids between the East Indian hybrids and the Himalayan section, nor with the R. arboreum section.
With Primroses, all British species fail to cross with Primula sinensis either way; and a significant fact is that all kinds of cultivated P. sinensis fail now to cross satisfactorily with the original wild form, according to Mr. Sutton's experience; for though he was successful in obtaining ten plants from two crosses between P. sinensis "Chiswick Red," the female, and the original P. sinensis as male parent, all ten plants were very weak, and all died while in the first stage of flowering. Messrs. Sutton, however, have never been able to obtain any seed from the original P. sinensis when pollinated by any other variety. It is self-fertile. P. sin. "Stellata," apparently the same as "The Lady," and representing an early stage of cultivation, can cross (either way) with the normal cultivated forms.
P. obconica failed to be crossed with P. sinensis by Messrs. Sutton; but with Mr. Wright, of Chiswick, it so far succeeded that while the progeny resembled the mother; that of the second generation indicated the effect by bearing flowers with four, five, six, or seven petals, and once eight; no such disorganisation occurred in the first generation.
It would therefore seem to be a common, if not a general, rule that species of different countries present greater difficulties in crossing than those of the same country, which probably grow under similar conditions. Thus, Fuchsia procumbens of New Zealand refuses to unite with the South American species, which readily intercross. Mr. W. G. Smith tells me that the outline of the pollen-grains of the former species is spindle-shaped, while of the latter it is a spherical triangle.
Indian Azaleas are difficult to unite with the deciduous species of Japan.
Begonia "semperflorens" section will not be successfully crossed with the tuberous section.
Begonia ricinifolia x B. tuberosa (hyb.) produced seeds but no plants at the Utrecht Botanic Gardens.
As instances of failures between species with marked morphological differences, yet residing in the same country, may be mentioned the "Fancy" Pelargonium with the scarlets. Krelage failed also in attempting Pelargonium gibbosum with P. zonale. Other failures between species crossed by this experimenter were Aristolochia elegans x A. braziliensis; Stanhopea eburna x S. tigrina, which bore fruit, but it decayed before ripening.
With regard to Cypripediums, Mr. Veitch sends the following observations as the result of his experience. No progeny has yet been raised from crossing the species of Selenipedium with Cypripedium (Sect. Coriaceae, Benth., Paphiopedium, Pfitzer), or vice versa, or between species of either of these sections and the hardy cypripedia (foliosae, Benth.).
The species chiefly used in the experiments were, of Selen., well-nigh all in cultivation; of Cyp. (coriaceae), the group of species known among horticulturists as the barbatum section, distinguished by their one-, rarely two-, flowered scapes, their tessellated foliage, and their semi-lunar staminode; and the group called the Stonei section, distinguished by their many-flowered scapes, their pendent, narrow petals and shieldshaped staminode; and in Cypripedium (foliosae), our native species, C. Calceolus, and the American species C. spectabile, C. pubescens, &c., have formed capsules in abundance, but they were invariably barren.
Interesting experiments upon the capability of pollen have been made by Professor E. Strasburger, which show that very similar effects of imperfect fertilisation can be produced where it cannot be said that there is any affinity at all. Thus, he found that Lathyrus montanus would put out pollen-tubes, which will enter the ovary of Convallaria latifolia; those of Agapanthus umbellatus will penetrate deep into the style of Achimenes grandiflora. Those of Fritillaria persica will not only enter the ovary of species of Orchis, but will even excite the development of the ovules and will cause them to begin to swell. The pollen-grains of Achimenes grandiflora will not, on the other hand, penetrate the stigma of Agapanthus.
The possibility of the pollen-grains of one species or genus developing tubes on the stigma of another species or genus does not depend upon the possibility of hybridisation between them. As a rule, the pollen-tubes penetrate the style or ovary to a depth proportional to the relationship of the species; though Lathyrus montanus and Convallaria, as mentioned above, are exceptions.
That varieties of the same species exhibit greater capacity for exciting the development of pollen-tubes than species of the same genus, depends simply on a greater resemblance in the composition of the nutrient material, furnished to the pollen grains and tubes by the stigma and style.
Hybridisation is an evidence of sexual affinity, but its non-occurrence is no evidence of the absence of affinity.
EXCESSIVE PREPOTENCY, OR FALSE HYBRIDS.—The question as to the influence of the male or female parent respectively has often engaged the attention of hybridisers. In some features one parent has seemed to predominate, in others the other parent; while perhaps as a general rule neither does so, but the progeny are strictly intermediate between them.
Experience, however, leads one to the conclusion that, starting from the intermediate condition, either parent may predominate in every degree, up to an apparently exact imitation of itself in the hybrid offspring. In other words, its influence has been so prepotent as to arrest all trace of the other parent in the offspring.
croisement, ou Fausse Hybridation (1894).
See Gard. Chron. 1894, Nov. 10, p. 568.
M. Millardet, who studied the hybrids between Alpine and American Strawberries, called these extreme results "False Hybrids."*
This peculiarity was early observed, for Gaertner records the fact that Datura Stramonium x D. ceratocaulis bore two fertile plants which resembled the female except in height. Their seeds produced D. Str. normal.
|†Op. cit. p. 537.
‡A section of Passiflora (Gen. Pl. vol. i. p. 811).
D. Laevis x D. Str. bore forty plants resembling the male parent. Mr. Burbage records somewhat similar results as obtained by Mr. Anderson-Henry with Veronicas,† observing on a particular instance: "I have seldom seen two hybrids with so much of one parent and so little of the other." Mr. T. Meehan, of Germantown, Philadelphia, has experienced the same thing; for example, he says that "Disemma aurantia x Passiflora coerulea as the male parent, gave rise to a progeny which was simply Disemma,‡ with no trace of the Passion-flower." Again, Fuchsia arborescens x garden hybrids "bore seedlings which, both in foliage and flowers, were F. arb., and nothing more." Lastly, Quercus palustris x Q. imbricaria resembles the female parent entirely, except that it has numerous entire leaves as well, which are like those of Q. imb., but in venation and all other characters it is wholly Q. palustris.
In speaking of Fuchsia longiflora x F. fulgens, Mr. Meehan observes that "several dozen plants were raised, all being from one berry; but no two of the many seedlings were alike. Some nearly approached the female, others the male parent. None could fairly be said to be intermediate."
Herr Max. Leichtlin found from his experiments that "the female parent gives to the offspring form and shape of the flowers; while the male parent gives more or less the colouring of the flowers; and if it is richer and freer-flowering than the female, this property is transferred to the offspring." To whatever degree it may be true for certain plants, no absolute law appears capable of being formulated. Thus, Dr. Denny remarks on Pelargonia: "The result of my experience, derived from experiments as regard the relative influence of the parents, certainly tends in the reverse direction to my previous ideas, which were derived from books, from which I gleaned that the form of the flower and constitution and habit of the plant were inherited from its mother; while the colour of the flower only was supposed to be conveyed by the father. The recorded results of my crossings indicate an immense preponderance of influence over the progeny on the part of the father in all respects in colour and in form, in the quality, in size and substance of the flower, as well as in the production of variegation of the foliage, and in the habit and constitution of the plant also, provided the plants employed were of equal strength."
|*On the Relative Influence of Parentage," Journ. R.H.S. New Series, vol. iv. pp. 18, 19, and 23.|
Dr. Denny "fertilised without much difficulty a variety (Peltatum elegans) of the Ivy-leaved section by the pollen of the zonal ...The foliage [of the cross] resembled almost entirely that of the mother; which is the reverse of my experience of the results produced between varieties."*
|†"Notes on Some Curiosities of Orchid Breeding," Journ. R.H.S. 1898, p. 442.|
Mr. C. C. Hurst experienced the same thing in Orchids. Thus he writes: "In May, 1891, Mr. R. Young, of Sefton Park, Liverpool, crossed Cypripedium barbatum with pollen of C. niveum. Fourteen hybrids were raised . . . . Every one of nine which flowered was C. barbatum, without a trace of the father parent, C. niveum."†
|‡ "Cross-fertilisation of Florists' Flowers," Journ. R.H.S. 1897, p. 205.|
Mr. James Douglas has also shown how the same phenomenon occurs in the case of Carnations.‡
Mr. Moore, of the Royal Botanic Gardens, Glasnevin, informs me that he has never succeeded in crossing Lachenalia pendula with any other species or variety. On one occasion he thought he had succeeded in doing so with L. aurea, but the seedlings were only aurea. He also failed to raise a hybrid between Helleborus niger and any other species. When crossed with H. orientalis the progeny proved to be H. niger.
|§ Journ. Lin. Soc. xv. p. 164, where other instances are mentioned.|
NON-RECIPROCITY IN HYBRIDS.—As a general rule, it may be stated that hybrids are not only intermediate in character between their parents, but that they are alike when either parent is the male or female. But it is not always so; and the peculiar difference may occur of a species being readily crossed with others, yet refusing to cross them in return, or vice versa. Thus Mr. J. Scott says: "I inserted pollinia of Oncidium microchilum into the stigmatic chamber of eight flowers of O. ornithorhyncum; of these, three produced capsules containing about 21 per cent. of good seed. I also tried the converse experiment, and applied pollinia from O. ornithorhyncum to the stigmatic chambers of twelve flowers of O. microchilum, but in this case I failed in causing a single capsule to swell."§
||| Op. cit. p. 299.|
Mr. Burbidge|| records a similar fact of Rhododendron Edgworthii as narrated by Mr. J. Anderson-Henry, who writes: "While it has been repeatedly made the male, it has never submitted to become the female parent . . . . R. Nuttalli behaved in the same manner." He further adds: "This remarkable circumstance of non-reciprocity has perplexed and defied me in innumerable instances throughout my long experience in these pursuits."
Mr. E. Scaplehorn, of Mayford, Woking, writes me with regard to Clematis coccinea: "I understand, from experiments made here respecting the new C. coccinea hybrids, that C. coccinea when used as the female parent did not produce any material results; but only when the various varieties of C. Jackmanni were crossed with the pollen from C. coccinea, was the production of these hybrids possible."
Again, Mirabilis longiflora x M. Jalapa proved a failure, though the reciprocal hybrid was a success.
|¶ Gard. Chron. Jan. 5, 1878, p. 19.|
Professor F. Parkman records his experience of a like kind with Lilies. ¶
|* Communicated by Mr. Krelage, from Mr. J. K. Budde, the curator.|
Oncidium Papilio x Phalaenopsis grandiflora was crossed, but failed at the Utrecht University Botanic Gardens; but the reverse cross has succeeded, the seeds of which have been saved and sown, but the result is at present unknown.*
|† See also paper by Mr. C. C. Hurst, Gard. Chron. Jan. 7, 1899, p. 14 seqq.|
Non-reciprocity also occurred in Epidendrum. Thus Mr. Veitch found that E. radicans could be successfully used as the male parent with E. evectum, Cattleya Bowringiana, Laelia purpurata, and Sophronitis grandiflora; but when itself has been used as the seed-bearer the cross invariably failed. Trials have been made with the pollen of its own progeny, as Epiphronitis Veitchii (=E. radicans, male; Sophronitis grandiflora, female), and of E. Obrienianum (E. radicans, male; E. evectum, female), but with no better result. Yet when pollinated with its own pollen it seeds freely. †
INFERTILE "CROSSES" BETWEEN VARIETIES.—As different species of one and the same genus may or may not yield successful hybrids—thus, while the "Fancy" Pelargonia and the zonals mostly breed together, respectively, yet these two sections will not unite—so, too, is it the case with varieties. Mr. C. E. Pearson writes me on crossing Pelargonium vars.: "The French seem to have differentiated three strains of zonal Pelargoniums, two of which are quite sterile with our own. The first failure was with a variety called, I think, Dame Blanche (some twenty years ago), which refused to cross, either as male or female, with our English varieties, but was fertile with its French contemporaries. The second was with the 'Bruant' race of zonals, a strain with huge trusses, but very irregular pips with large windmill-sail petals. I have not tried all these, but those I did were all sterile, both ways.
"The most recent French strain, originating in Jules Chrétien, crosses freely with ours.
"I may also mention that there is a close relation between colour and fertility in some zonals, the very dark crimsons being so nearly sterile as to make seed-raising difficult, the sterility being in proportion to the depth of the colour."
|‡See Darwin's Cross and Self Fertilisation of Plants, pp. 142 note and 352.|
This last observation refers to self-fertilisation, and agrees with Darwin's. It is because the flowers are strongly proterandrous. In the paler varieties, as "Christine," they are more nearly homogamous, and consequently are very self-fertile.‡
|§ Op. cit. p. 22.|
Dr. Denny found similar inconsistencies among varieties of Pelargonia in 1873. He writes: "I have alluded to the antipathies and affinities we find to exist, without any explicable cause; for instance, I have found it impossible to fertilise three or four varieties of the scarlet Pelargonium (viz. the Duke of Cornwall, Dr. Muret, Beauté de Suresnes, and all that section of the doubles which sprang from Beaute de Suresnes), which to all appearance are mere varieties of the zonal section—save with one another."§
A florist informed me (in 1880) that he found the "rough-leaved" fancy Pelargonia, the flowers of which have a blotch on all five petals would not cross with the smoother-leaved forms, the flowers of which have blotches—at least distinct—on two petals only. I do not know how far this has been corroborated, if at all.
PARTIAL HYBBIDISATION.—A feature not infrequently observed by hybridisers is that a result of foreign pollen applied to the stigma of a different species is that it may take effect in varying degrees without securing any fertilisation of the ovules at all.
One of the first botanists to note this fact was Wichura in the case of Willows, in which the following degrees of failure were observed
|* Abstract in Journ. R.H.S. New Series, vol. i. p. 63.|
"The gradation in the number of seeds was very various. Sometimes the seeds were few, but fertile and active; sometimes, on the contrary, numerous, but with only a few fertile mixed with a number of abortive seeds; sometimes tolerably numerous, without any such admixture; but, in general, hybrids yielded on the whole a smaller number of seeds than plants impregnated with their own pollen."*
|† On Hybridisation amongst Vegetables.|
The above quotation was published in 1866; but Dean Herbert experienced the same results with Alstroemerias, and records Mr. Bidwell's crossing Passiflora coerulea with P. onychina, which bore a fine orange fruit, but devoid of seeds.†
|‡The reader is referred to Origin of Floral Structures, p. 165 seqq., for a discussion on the influence of the pollen-tube upon the ovary.|
Similar results have occurred repeatedly since then with other plants. Thus, just as Wichura found that silky hairs were the sole result of pollination in Willows, so is it in Orchids; as, e.g., in an attempt at hybridisation by Mr. Veitch in the case of Phalaenopsis Luddemanniana x P. Schilleriana, amabilis, and grandiflora. There was no seed, but hypertrophy of the placental hairs, giving the appearance of a pod full of cotton wool.‡ This has also occurred with many other Orchids.
|§ Communicated by Mr. Krelage.|
Phalaenopsis grandiflora x Stanhopea tigrina, as well as Stanhopea eburna x S. tigrina, developed fruits; but no seed could be obtained which would germinate in the Utrecht Botanic Gardens.§
That the pollen may only affect the fruit is a well-known phenomenon. Thus, for example, Dr. Bonavia crossed the native Pumpkin of India, called Koomra, with the American Squash pollen. The ovary enlarged and came to maturity, but did not contain a single seed. When it is fertilised with its own pollen, it is full of good seed.
|*Gard. Chron. May 16, 1885, p. 630.|
Mr. Veitch observes that "neither the Cattleyas nor the Brazilian Laelias will cross freely with the Mexican Laelia albida, autumnalis, majalis, rubescens (better known in gardens as acuminata), &c. Numerous crosses have been effected both ways, and capsules have been produced, but the seed has always proved barren."*
Similarly, Mr. Salter, of Woodhatch, writes me: "Dendrobium Phalaenopsis, var. Schroederianum, was crossed with D. Ainsworthii, D. formosum, D. Cassiope, and D. heterocarpum, and the fertilisation seemed perfectly successful; and the pods swelled in the ordinary manner until the ripening season. When, however, they were split open, there was no seed, only a kind of fluff [hypertrophied hairs] upon the internal ridges of the pods."
Sophronitis violacea and S. grandiflora have both been crossed, reciprocally, by Mr. Veitch; but although capsules were formed no fertilisation had taken place.
Empty capsules also resulted from crossing Vanda tricolor with species of Angraecum and Phalaenopsis. So too between species of Phalaenopsis. P. Luddemanniana, the male parent of some of the finest hybrids of this genus raised, has been crossed with the pollen of well-nigh every species of P. in cultivation; but nothing has yet resulted therefrom. The application of the pollen of species of P. to the stigma of P. Ludd. stimulates the ovary into growth, but seems to be impotent to fertilise the ovules. Similar results have attended P. violacea when used as the female parent, with the single exception of the hybrid P. Ludd. violacea raised from it, and P. Ludd., male. The above result, which is Mr. Veitch's experience, was repeated in the cases of Cymbidium male x C. Lowianum, C. giganteum, and C. Traceyanum; and several other genera gave similar results.
Lastly, Campanula Van Houttei, believed to be a hybrid, as it is generally barren, was crossed by Mr. E. Scaplehorn with C. mirabilis. The ovary enlarged and contained ovules, but the latter were deformed, yet much larger than in the case of those not crossed, proving that a partial impregnation had taken place. A similar result followed on crossing the first named with C. medium.
A general result is seen in a hybrid, or bi-gener, vegetating freely and healthily, but failing in various degrees in its reproductive system. It may show in its leaves &c. every intermediate character, or none at all, being exactly, to all appearances, like one of the parents. Results of this kind have followed attempts to cross Geraniums with Pelargoniums, Abutilon with Hibiscus, Raspberry with Strawberry, and Gooseberry with Black Currant. With regard to the last two, the former of them has flowered but borne no fruit, while the latter has borne fruit as well.
FALSE INFERENCES FROM FAILURES.—That a cross may fail in one season and not in another shows that no absolute rule can be laid down, as in the following case in the experience of Dr. Bonavia, who writes me as follows: "When in India I imported a Dutch Hippeastrum. It flowered. At the same time I had a large number of Lucknow Hippeastrums in flower. I endeavoured to cross the Dutch H. with the Indian ones; but no result occurred either with the Dutch pollen or the reverse. I tried the same experiment in the next year, again with no result whatever. But in the third year I tried again, both ways, and I got a number of seeds in each case, which germinated. It struck me that possibly either the Dutch bulb was too young or that it was not sufficiently acclimatised to afford results either with its pollen or its pistil; for the climate, or the season, or the cultivation &c. may interfere with a successful result."
Mr. C. C. Hurst has given us the following important conclusion from his own experiences: "The nigro-hirsute section of Dendrobiums are well known to be bad setters. Mr. R. Eichel, of Bradford, tells me that for eight years he has failed to cross D. formosum with pollen of the deciduous section; but that he has now seedlings two and a half months old of D. formosum x D. nobile (male) from four seed-pods. This circumstance proves to us once more how misleading and unsatisfactory purely negative results are, and in this there is much hope for the future. However many times a cross has failed to set, we can never be sure that it may not be accomplished by some one. Very trifling conditions seem to affect the delicate and susceptible organs of reproduction, causing apparent sterility. For instance, it is said that Epidendrum ciliare can only be fertilised with success in the evening, when the flowers begin to emit their fragrant perfume; though this did not prove to be the case with Mr. Veitch."
|*Gard. Chron. 1896, Jan. 11, p. 50.|
One of the most unlikely results occurred with Poppies. M. Henry L. de Vilmorin succeeded in raising a hybrid between Papaver bractecetum and one of the double-flowered varieties of P. somniferum. The seedlings were annuals, and bore single carmine-coloured flowers. At first the plants were nearly all sterile; but subsequently seed was freely produced.*
|† Wochenschrift, Berlin, 1859.
‡ Bull, Bot. Cong., Amsterdam, 1866.
With regard to Ferns, Stelzner† describes the hybridisation of two species of Gymnogramma, viz. G. chrysophylla and G. lanata. One plant so raised was so different from the parents that Koch named it G. Stelzneriana. It proved to be entirely barren. But in 1864‡ he repeated the experiment, and had a number of plants of the hybrid, all of which were wholly fertile.
|§ "On Hybridism considered as a Cause of Variability in Vegetables," Journ. R.H.S. New Series, vol. i. p. 1.|
M. Naudin experienced a similar result with Daturas: thus, "D. laevis, ferox, Stramonium, and quercifolia, four species perfectly distinct, between which there are no known intermediates, and moreover they do not appear susceptible of variation. Nevertheless, though very distinct, these species are sufficiently closely related to admit of reciprocal impregnation, and to give rise to hybrids; which, though sterile at first, become very fertile at a more advanced period."§
||| Essay on Hybridisation amongst Vegetables.|
One cannot do better than conclude with the hopeful advice of Dean Herbert, in alluding to the artificial separation of genera by botanists: "Let the cultivator not be discouraged by every nominal generic separation; but let him take his own view of apparent affinities, and bring the accuracy of those separations to the test."||
THE VEN. ARCHDEACON MEREDITH: You said, Professor Henslow, that the French Pelargonium would not hybridise with the English. Are there any special characteristics by which the French Pelargonium is known; or would any other Pelargonium hybridise with the English?
PROFESSOR HENSLOW: The report was sent to me by one of the numerous friends I have to thank for a variety of information. There was no other remark on the subject, merely the statement of the fact.
ARCHDEACON MEREDITH: I have tried it myself, but have generally failed.
PROFESSOR HENSLOW: Then that corroborates my informant.
ARCHDEACON MEREDITH: Are there any differences between Pelargoniums grown in other countries, such as Germany or America?
PROFESSOR HENSLOW: That I cannot tell you. I am passing on my information just as I received it.
ARCHDEACON MEREDITH: Is it possible for plants to get naturalised to the particular country where they are reared? After a time do they lose their foreign characteristics?
PROFESSOR HENSLOW: I think that is so. As I have said the Primula, according to Mr. Sutton, will not now cross with the original form from which it is derived—the Chinese form. I do not know of any other instance; perhaps other cultivators may know.
MR. GEORGE BUNYARD, V.M.H.: I would like to say that a gentleman in our neighbourhood has made many curious crosses, among which is one of the Gooseberry and the Blackcurrant. He succeeded in producing fruit on it, and I tasted it; but it was of no use whatever from a commercial point of view, and I did not entertain his offer to sell the plant to me. He also had a cross between the Blackcurrant and Ribes sanguinea, and between the Ribes sanguinea and the Gooseberry (p. 168).
PROFESSOR HENSLOW: Do you know whether the seed was retained?
MR. BUNYARD: Yes.
PROFESSOR HENSLOW: That is an advance on the information that I possessed.