Journal of Botany: British and Foreign, 3: 86-90 (Mar 1, 1865)
Vegetable Hybridity
Charles Naudin

M. C. Naudin has recently communicated to the Academy of Sciences of Paris some observations on the hybridity in plants, which place in a striking light the possible variations of which the impregnation of one plant by the pollen of another may be the cause. At the same time they furnish important data in the discussion of questions connected with the origin of species. Thus although hybrid plants, when endowed with sufficient fertility to propagate their kind beyond the second generation, frequently and speedily revert to the type of one or the other of the original species, nothing is more common than to find that certain individuals not only persist in their divergence from both the primary types, but actually depart still more widely from the original parents, and even in some cases present considerable differences from the first generation of hybrids. The elucidation of these curious points has been the object of M. Naudin's experiments, of which the results are contained in the memoir of which we propose to furnish our readers with an abstract.

In 1862, M. Naudin experimented with four species of Datura, namely, D. laevis, ferox, Stramonium, and quercifolia, all belonging to a subgeneric group which may be divided into two series, in one of which the plants have green stems and white flowers, whilst in the other the stems are more or less tinged with brown and the flowers violet. D. Stramonium, laevis, and ferox belong to the former, and D. quercifolia, with some other species, to the latter. The four species are perfectly distinct, and show no tendency to variation.

The intercrossings, made with all necessary precautions, were very successful. They were effected in both directions; that is to say, the pistils of each species were impregnated with the pollen of the other. From the species D. laevis and ferox thus doubly crossed, the author obtained in 1863 sixty young plants of D. laevi-ferox, and seventy of D. feroci-laevis. The whole of these 130 plants grew freely, and were so perfectly similar in appearance that the two sets could not have been distinguished, the entire collection of hybrids being as homogeneous and uniform as if they had been a group of individuals of a fixed species or a pure and distinctly marked race.

On the other hand, to M. Naudin's surprise, these hybrids presented no appearance of being intermediate between the two well-marked species from which they were derived, so that any one ignorant of their origin would not have hesitated to regard them as forming a distinct species: and, curiously enough, whilst both the parents belonged to the section with green stems and white flowers, the hybrids would be referred to the other group, their flowers being violet and their stems brown.

This result was so unexpected and paradoxical that M. Naudin resolved to repeat his experiments, and this year he made a new sowing both of the hybrids and of the parent species. He obtained thirty-six new plants of D. laevi-ferox and thirty-nine of D. feroci-laevis, which were identical with their predecessors of 1863, having the stems brown, the flowers violet, and the fruit spinose. But the sowing of D. ferox furnished an explanation of this curious fact, for the author found that, at the moment of germination, the stem is of a deep violet-purple tint from the root to the cotyledons, and that this coloration persists in its original place throughout the life of the plant, forming a coloured circle round the stem. Thus the tendency to coloration seems to reside in the D, ferox, although here it is reduced to a rudimentary state; in the hybrid it becomes enormously increased, pervading all parts of the plant, and especially influencing the flower.

The second generation presented variations of a different and still more remarkable kind. The seeds of the above-mentioned hybrids sown last spring furnished nineteen plants of D. feroci-laevis and twenty-six of D. laevi-ferox. But in spite of the great similarity of their parents, these plants presented a most astonishing diversity of forms, so that out of the forty-five plants composing the two sets, no two were exactly alike. They differed greatly in size (some being four times as large as others), in general aspect, in form of leaf, in the coloration of the stem and flowers, in fertility, and in the size and superiority of the fruits. One plant of the laevi-ferox series had completely reverted to the type of D. laevis, except that the base of its stem still bore a violet ring; a few showed faint traces of resemblance to D. ferox, but the majority were more like D. Stramonium and quercifolia, with which they had no relationship, than the species from which they were descended. "In fine," says the author, after describing some of their chief differences, "the forty-five plants of the two lots formed, so to speak, as many individual varieties as if, the connection which should have bound them to specific types having been broken, their vegetation had deviated in all directions. This may be called disordered variation in opposition to another and very different mode of varying which will be mentioned hereafter."

In 1863, M. Naudin obtained a plant and seed of Mirabilis longiflora-Jalapa of the first generation, procured by impregnating the common purple-flowered Marvel of Peru with pollen of M. longiflora. The seed was sown, and the two plants grew to a large size, perfectly similar in every respect and intermediate between the parent species. They were moderately fertile, and furnished some hundreds of perfect seeds.

From seeds of the first plants obtained in 1862, M. Naudin raised six other hybrids, of course of the second generation. These did not resemble the hybrids of the first generation either in size or appearance. Two of them were nearly alike: they were vigorous, and flowered abundantly, but were quite barren. A third had almost reverted to the M. Jalapa, differing chiefly in the longer tube of the corolla; this was fertile. The remaining three were stunted in their growth, very dissimilar in appearance, and barren,—at least they produced only a few fruits in which the seeds were imperfectly formed. Three new plants of the second generation grown in 1864 presented the same diversities; they resembled neither those of the preceding year nor the first hybrids. One of them, which approached M. Jalapa in its characters, was very fertile; the others flowered irregularly and were barren. This second experiment gives further evidence of the disordered variation of the products of a hybrid plant, when they do not revert towards one of the parent species.

It becomes a question whether this tendency of the hybrids to vary continues to the third and following generations. In 1863 and 1864 the author observed the sixth and seventh generations of a hybrid, Linaria purpureo-vulgaris, which he had preserved for several years. In each case he had several hundred individuals. A good many of the last generation partially or completely reverted to the L. vulgaris with yellow flowers, and a few to the L. purpurea. A greater number presented no tendency towards either of the parent species, but nevertheless did not resemble the hybrids of the first generation. They presented all the phenomena of disordered variation.

Similar facts occur daily in the practice of gardeners. The two cultivated species of Petunia (P. nyctaginiflora, with white flowers, and P. violacea, with purple flowers) may be intercrossed and produce fertile hybrids. Those of the first generation are all alike; in the second, they become remarkably diversified, and this variation increases until the plants are often monstrous, the changes being assisted by the artificial impregnation of one variety by another. The same conditions of individual variability are exhibited by a host of other cultivated flowers, of which M. Naudin cites especially the Primulae and Roses of our gardens. In like manner, as he indicates, the varieties of our fruit-trees are strictly individual in their nature, it being universally admitted that it is only by grafting or budding that any particular variety can be propagated; hence he concludes they also may be regarded as hybrids between several unknown specific types.

But if hybridity, doubtless often produced by natural causes, such as the visits of insects to flowers of different but nearly allied species, be the cause of so much variation in cultivated plants, it becomes an important question whether the same cause may not give rise to a similar effect in such as remain in the wild state. In some genera, such as the Salices, Potentillae, Rumices, etc., the intermediate forms between apparently well-marked species are so numerous and so well graduated, that on examination it becomes difficult to limit the species, and these genera have always furnished subjects of dispute among botanists. In these forms the supposition that their numerous varieties may be due to the influence of hybridization is rendered more probable by the fact that they present peculiar favourable conditions for intercrossing. Now if we suppose the crossing of two of these species to give rise to fertile hybrids which do not all revert to the parent types, disordered variability will come into play and produce, in a few generations, a perfect chaos of undecided forms.

The distinction between this disordered variability and the ordinary variability displayed by many species to a greater or less extent, is that the varieties produced by the latter either disappear with the individual in which they are manifested, or become transmitted without alteration to the following generations, thus, under favourable circumstances, giving rise to a marked race,—whilst in the former the form becomes broken up in successive generations into individual variations without fixity. "Homogeneity and fixity of character are the distinctive sign of true races, as are diversity and want of permanence of the agglomerations of mongrels and hybrids."

The concluding paragraph of M. Naudin's paper indicates the direction in which these researches may be brought to bear upon some of the most important problems of the present day, especially in connection with anthropology. "I am unaware," he says, "whether facts analogous to those which I have just described have been observed in the animal kingdom, but I should not be surprised if it should be some day found that in it also intercrossings between well-marked races are the cause of individual variability, and that they are incapable of creating new races, that is to say, uniform aggregations capable of indefinite duration. It would certainly not be uninteresting to ascertain whether, by alliance with one another, very distinct races fuse into a new mixed but homogeneous race, or whether, as in plants, the effect of intercrossing is to produce an indefinite diversity of physiognomy and temperament."—(Reader.)

[At an early opportunity we shall give an abstract of the important researches into hybridity, which Wichura has published in his recent work on Willows.—Editor.]