Hybrids and Constitution
It was my opinion that fertility depended much upon circumstances of climate, soil and situation, and that there did not exist any decided line of absolute sterility in hybrid vegetables, though from reasons which I did not pretend to be able to develop, but undoubtedly depending upon certain affinities either of structure or constitution, there was a greater disposition to fertility in some than in others. Subsequent experiments have confirmed this view to such a degree as to make it almost certain—that the fertility of the hybrid or mixed offspring depends more upon the constitutional than the closer botanical affinities of the parents. The most striking and unanswerable proof of this fact was afforded by the genus Crinum, which is spread round the whole belt of the globe, within the tropics and within a certain distance from them, under a great variety of circumstances affecting the constitution of individuals, which nevertheless readily intermix, when brought together by human agency. The plant called Crinum Capense (formerly Amaryllis longifolia), impregnated by either Crinum Zeylanicum or scabrum, both at that time also called Amaryllis, produced offspring, which during sixteen years proved sterile, probably because, notwithstanding their botanical affinity, the first is an extra-tropical aquatic plant, and the two latter tropical plants which affect drier habitations and readily rot, at least in this climate, in a wet situation. The same C. Capense, impregnated by Crinum pedunculatum, canaliculatum, or defixum, produces a fertile cross, though they are so dissimilar as to have been placed in different genera, and the author was formerly reproached by botanists as having committed an absurdity when he insisted upon uniting them. The reason of the fertility of their joint produce seems to be, that they are all aquatic or swamp plants; and it may be further observed that the crosses with the two former, the plants being all extra-tropical, are much more fertile than that between C. Capense and defixum, because the latter is a tropical plant. The mules between Scabrum and Capense having continued so many years with every appearance of absolute sterility, without any change of situation or treatment, at last produced one good seed in 1834 and another in 1835. These facts were of such an overbearing nature, that it became impossible for those, who had charged the author with absurdity for uniting the parents under the genus Crinum (to which even certain other plants were then asserted to be more nearly allied than the species at that time called Amaryllis), to contend any longer that they, producing a fertile offspring, were of different genera, and they will probably be never again disunited in any botanical work; but the facts furnish much ground for the serious consideration of men of science. It happens (as if expressly designed to overthrow the theory, that the identity of species is proved by fertility or sterility in the mixed issue), that, while C. Capense, Zeylanicum, and scabrum, are very similar in their general appearance, and yield an offspring which has been found quite sterile except in the case of the two seeds above mentioned, C. Capense and pedunculatum are as unlike as perhaps any two species of any known genus; and if it were asserted that C. Capense and pedunculatum are one species, and C. Capense and scabrum two species, the assertion would appear, to any person looking at the plants, too preposterous to require a serious answer.
In further confirmation of the fact that the sterility depends on constitutional discrepancy, or difference of what medical men call idiosyncrasy, may be adduced the curious plant figured in the Botanical Magazine under the name of Crinum submersum, which was found by my collector in a pond or flooded spot not far from Rio Janeiro, in company with a small variety of C. erubescens, and appeared to be exactly intermediate between that aquatic plant and C. scabrum, which grows on high ground amongst the woods. It is absolutely sterile, the anthers being always shrivelled and the pollen dry, and it is not materially different from the mules raised in our stoves between C. scabrum and a larger variety of C. erubescens, the latter being of course a finer mule, but with exactly the same barrenness of the anthers. C. submersum is certainly a natural cross, in consequence of the pollen of C. scabrum having been brought to the lake by some humming-bird or insect which touched the stigma of the aquatic species. The same sterility has been found in C. amabile and C. angustum, which are undoubtedly mules accidentally produced between dry-land and swamp-species, the former probably between Zeylanicum and procerum, the latter between C. Zeylanicum and bracteatum; as also C. longiflorum (Amaryllis longiflora of the Botanical Register), which is an accidental cross between C. Capense and erubescens, one variety of it having been produced at Demerara, the other in Jamaica. The fact being established with respect to one genus, that the species which have most botanical affinity and general likeness, if they delight in a different state of soil or of atmosphere, produce a barren cross, while the most dissimilar, if they possess the same constitutional predilections, give birth to a fertile plant, cannot remain as an isolated circumstance, but must be considered by every unprejudiced and philosophical mind with reference to the whole vegetable creation. I have lately heard it admitted in conversation by an eminent botanist, that he had almost arrived at the conviction that there was but one rose, meaning that there seemed to be no natural impediment to the fertile intercourse of the great variety of plants which constitute the known species of that extensive genus. Let it be observed, if the fact is so, the reason is apparent enough; that, although some roses will endure a little more cold than others, there is a sameness of constitution throughout the genus, which affects a dry soil and a temperate atmosphere. The genus Calceolaria embraces plants very dissimilar to the eye of the botanist, as well as of the unlearned observer, of which some are absolutely stemless, and bear only leaves and flower-stalks, while others are shrubby, and acquire a strong woody stem some feet in height; yet there appears to be no limit whatsoever to their intermixture, and their produce may be crossed again indefinitely. Are we, then, to come to the result that there is but one Calceolaria, oversetting not only the nicer distinctions of botanical science, but the difference between herb and shrub? The African Gladioli, excepting those which, like the European, present their flowers in front of the stalk, have been intermixed by me without any difficulty occurring, and the crosses of the most dissimilar have proved abundantly fertile, and four or five sorts have been blended in successive generations. Some of the complicated crosses have produced seed less freely, and one treble cross (Hirsuto-Cardinali-blandus) has as yet produced none that has vegetated, probably because the last mule, G. hirsutus, is of a constitution much less suited to our climate than the other two. Are we then to come to the result, that these dissimilar species are all one natural Gladiolus? There is no outward sign of barrenness in G. hirsuto-Cardinali-blandus, which will probably bear seed under favourable circumstances; that there is no insurmountable natural impediment may be proved thus; the offspring of G. versicolor by hirsutus, of blandus by versicolor, and of Cardinali-blandus by tristis, have all borne seed, shewing that G. hirsutus is not of a separate race, and that the triple cross is not an impediment. I have crosses raised by me between the yellow Linaria genistifolia and the purple purpurea, and also between Penstemon angustifolium and pulchellum, both perfectly fertile and sowing themselves about the garden, and, from my having given them many years ago to more than one nurseryman, become common. It is scarcely possible to assert that these very unlike plants are respectively one, and at the same time to distinguish them from the rest of their own genera, especially the former. That whole portion of Amaryllideae which constitutes the genus Hippeastrum, and was confounded by botanists with a portion of the genus Crinum, not only interbreed freely, but produce offspring invariably fertile, because they are all of like constitution, and impatient of excessive moisture, though some will bear more cold than others. Amongst the Pelargoniums a similar convertibility has been found to exist within certain limits, which, if duly observed, will be sure guides to ascertain the genera, into which they ought to be subdivided, and by which the botanist, who is desirous that his labours should not be overturned hereafter, must be in a great measure ruled in classing them. Amongst the Cacti or Cerei the prickly angular speciosissimus, the flexible flagelliformis or whip-plant, and the flat unarmed phyllanthocides, are nearly the most dissimilar, yet they have produced mixed offspring, which readily bears eatable fruit of intermediate appearance, colour, and flavour. The fruit of the speciosissimus is large, green, and well-flavoured, round oblong; that of phyllanthocides small, purple, and very inferior; the mule from the former has purple fruit of a medium size and taste. The cross from the former by flagelliformis is now ripening here a short angular fruit, quite unlike that of the mother plant. The fertility of these crosses, and readiness to vary the appearance and taste of the fruit, though derived from such very dissimilar parents, is one of the most striking results of our experiments. I have had no opportunity of attempting to cross them with the plants called echino-cacti, but I do not see a single point in the generic character given of those plants which can uphold it, and I believe them to be of one genus with Cereus, and capable of intermixing; but I have had no opportunity of examining the flower of any of the plants called Echinocactus myself. Amongst melons I have had the Cucumis osmocarpus from Mexico, bearing a small egg-shaped white fruit and a small flower and leaf, very different from the Cucumis melo, fertilized accidentally by its pollen, thus occasionally producing fruit of twice the natural size with red flesh. Lobelia speciosa is a cross between L. siphylitica and fulgens, yet it reproduces itself abundantly.
The more these facts are considered, and the more they are multiplied, as they will be by the daily experiments of cultivators in other genera, the more strongly will my original suggestions impress themselves upon every botanist, who will look on the subject without prejudice, that the genera of plants are the real natural divisions; that no plants which interbreed can belong to separate genera; that any arrangement, which shall have parted such plants, must be revised; that any discrimination between species and permanent varieties of plants is artificial, capricious, and insignificant; that the question which is perpetually agitated, whether such a wild plant is a new species or a variety of a known species, is waste of intellect on a point which is capable of no precise definition, and that the only thing to be decided by the botanist in such cases is whether the plant is other than an accidental seedling, and whether there are features of sufficient dissimilarity to warrant a belief that they will be reproduced, and to make the plant deserve on that account to be distinguished by name amongst its fellows. The effect, therefore, of the system of crossing, as pursued by the cultivator, instead of confusing the labours of the botanist, will be to force him to study the truth, and take care that his arrangement and subdivisions are conformable to the secret laws of nature; and will only confound him when his views shall appear to have been superficial and inaccurate; while on the other hand it will furnish him an irrefragable confirmation when they are based upon reality. To the cultivators of ornamental plants the facility of raising hybrid varieties affords an endless source of interest and amusement. He sees in the several species of each genus that he possesses the materials with which he must work, and he considers in what manner he can blend them to the best advantage, looking to the several gifts in which each excels, whether of hardiness to endure our seasons, of brilliancy in its colours, of delicacy in its markings, of fragrance, or stature, or profusion of blossom, and he may anticipate with tolerable accuracy the probable aspect of the intermediate plant which he is permitted to create; for that term may be figuratively applied to the introduction into the world of a natural form which has probably never before existed in it. In constitution the mixed offspring appears to partake of the habits of both parents; that is to say, it will be less hardy than the one of its parents which bears the greatest exposure, and not so delicate as the other; but if one of the parents is quite hardy and the other not quite able to support our winters, the probability is that the offspring will support them, though it may suffer from a very unusual depression of the thermometer or excess of moisture, which would not destroy its hardier parent. Such is the case with the beautiful mule Rhododendron Altaclarae, of which the mother was a cross between Ponticum and Catawbiense, and the father the Nepal scarlet arboreum. We now possess a further cross by the impregnation of Altaclarae by arboreum, which will probably come so near the father in its colour, that if, as expected, it should be able to endure our winters, we shall have nearly attained the result, which would be otherwise most likely impracticable, of acclimating the magnificent Nepal plant; for it does not appear that in reality any plant becomes acclimated under our observation, except by crossing with a hardier variety, or by the accidental alteration of constitution in some particular seedling; nor that any period of time does in fact work an alteration in the constitution of an individual plant, so as to make it endure a climate which it was originally unable to bear; and, although we are told that laurels were at first kept in hothouses in this country, it was not that they were less capable of supporting our seasons than at present, but that the cultivators had not made full trial of their powers of endurance. The notion of Mr. Sweet that the roots produced by cuttings are hardier than those of seedling plants is probably fanciful, if he meant permanently so, which alone would be of importance. They may be tougher at the first period of propagation, while the seedling is in its infancy, but that, if not permanent, could have no effect in acclimating a plant. In truth it is not the root that is tougher, but the nucleus or base of the cutting from which the roots issue, and in which the life resides, which is tougher than in. a young seedling at the first. All his other experiments only tended to shew that some half-hardy plants would live through an English winter in very dry and sheltered situations, or during two or three years, till a more inclement season cut them off, but not that by any process of his they had become hardier; the word acclimating seems, therefore, to have been misapplied in his paper in the Transactions of the Horticultural Society. For the purpose of obtaining a large or a brilliant corolla, it will be probably found in the long run best to use the pollen of the species which excels in those points, because the corolla, in truth, belongs to the male portion of the flower, the anthers being usually either borne upon it, or in some manner connected with it by a membrane; but upon the whole an intermediate appearance may be generally expected, but with a great disposition to sport, especially in the seminal produce of the fertile crosses, as in plants which are apt to break into cultivated varieties.