The Problem of Plant Sexuality
The competition on the question of plant sexuality proposed by the St. Petersburg Academy in 1759 provides an obvious point of entry into the study of plant hybrids and hybridization in the late 18th century. By the mid-18th century the question whether plants have sexual organs and reproduce sexually would seem to have been
decided by the many proofs offered by European naturalists. This was by no means the case, and many botanists continued to contest the idea. To end the conflict the Academy of Sciences in St. Petersburg proposed a prize question: "Either to confirm or to deny, by means of new proofs and experiments, as well as those already known, the doctrine of the sex of plants, preceded by a history and an account of all the parts of the plant that play some role in fructification and the formation of seed and fruit." The competition summed up received opinion on the subject of hybridization and initiated new research that marks the beginning of serious investigation of hereditary characters in plants. The Academy received three entries. Two were considered unsatisfactory; the third, however, was judged "praemio omnino dignus"; the prize was awarded to Carl von Linnæus, at the session of September 6, 1760.
Linnæus' paper offered nothing not already familiar to his readers; he simply restated positions on the subject of plant sexuality that he had defended throughout his career. In plants there is no fruit without flower; since the flower is a necessary antecedent to fruit, it follows that sexual organs must exist in the flower. In fact, Linnæus wrote, "the flower consists of nothing but sexual organs." Those parts with the rudiments of fruit, by analogy with the animal kingdom, must be female. Linnæus separated and combined male and female floral parts to show that seed developed only when pollen contacted the stigma, and argued that the possibility of fertilization of a female plant by a male of another species exhibited clearly the sexual duality of plants.
Linnæus considered the production of hybrids the decisive proof of the sexuality of plants. He cited four hybrids, but offered little by way of proof that they were the offspring of two separate species. One (Tragopogon hybridum ) had been hand-pollinated and marked
with a thread. (Koelreuter did not consider this cross above suspicion.) The other three plants (Veronica spuria, Delphinium hybridum, and Hieracium hybridum ), which had been found in the wild or in gardens, exhibited characters intermediate between two known species. Linnæus assumed without demonstration that these plants would reproduce through seed to form "constant varieties." This would constitute a new kind of metamorphosis in plants. "It cannot be doubted that we have here new species brought forth through hybridization." The bastard plant, although resembling the father outwardly, is the image of the mother with respect to the inner medullar substance and fructification. The numerous African Gerania , for example, led Linnæus to conclude that there are as many forms in one genus in the plant kingdom as have emerged from one species through the crossing of flowers. A genus is thus only the epitome of those plant forms that stem from a single mother and various fathers.
These speculations contradicted the few established facts concerning plant crosses. However plausible Linnæus' hypothesis, especially for genera in parts of the world where nature produces a copious variety of species, crosses between species are difficult to achieve. They are, moreover, impossible to perpetuate: because of their absolute or relative infertility, or because of the regressive degeneration to which their issue was subject, the aid of one of the parental stocks is required for fertilization. Insurmountable barriers of sterility, degeneration, and distribution limit the effects of crossing far more stringently than Linnæus imagined. Thus, for each species known to be distinct and constant, most naturalists assumed a common origin and epoch.
The key word is, of course, "assumed." The constancy of species characters and the absolute or relative sterility of crosses between species were subjects about which most naturalists of the 18th century found themselves obliged to make assumptions, just as they assumed that the development of a new individual was simply the
gradual distension of a preformed being. Trained in the identification and analysis of external conformation, they were at a loss to appreciate properties and novelties that breeding experiments alone might reveal. They classified so-called bastards as a special kind of anomaly, outside the rules ordinarily followed in reproduction. Each separate species is designed to function independently, yet to contribute "all those perfections towards the ends for which it had been determined." Intermediate forms lack this teleological justification, and naturalists reckoned among the wiser provisions of nature both the rarity of crosses between species in their free natural state and the infertility of bastards. Crosses between parents with very different organizations were seen as flatly impossible, and conjecture about crosses between genera and orders was idle speculation.
An adjunct of the St. Petersburg Academy, Joseph Gottlieb Koelreuter (1735–1806) had been a proponent of the sexual theory of plant reproduction since his early years as a student at Tübingen. One of his professors, J.G. Gmelin, had been among the first to recognize the significance of Linnæus' work on hybrids, and in 1749 had adopted the subject for his own inaugural lecture. In it, Gmelin called for experiments in hybridization. Koelreuter completed his
degree at Tübingen in 1755 and became an adjunct in natural history at St. Petersburg. When the Academy announced its competition on plant sexuality he recalled Gmelin's recommendation and set to work to produce plant hybrids.
Like Linnæus, Koelreuter considered the production of hybrids a decisive proof of the sexual duality of plants; unlike Linnæus, Koelreuter believed that nature limits this kind of anomaly, thereby preserving the order and harmony that had reigned in Eden. Two different species of animals living in a state of nature do not produce bastards; nature avoids disorder by means of natural instinct. She has equally certain methods for avoiding comparable disorder in plants.
Perhaps it has also been one of her intentions, in order to avoid just such a disturbing disorder, that she disposes one plant in Africa, and gives another its place in America. Perhaps it is partly on this account that she has confined within the limits of a certain region only those plants which in respect to structure have the least likeness with one another, and are consequently least likely to bring about disorder among themselves.
Bastards are products of the artifice of man, as exercised in botanical and zoological gardens.
Here at any rate man gives plants of a certain kind the opportunity that he gives his animals, often assembled from widely separated parts of the world, which he keeps penned in a zoological garden, or in an even narrower space.
Koelreuter was convinced that nature limits the potential for disorder, even under such unnatural conditions, and he set out to discover those limits.
Koelreuter produced his first plant bastards during the fall of 1760, after Linnæus won the Academy competition. The offspring of a cross between two tobacco species, Nicotiana paniculata and N. rustica , flowered the following March, and in the fall of 1761 Koelreuter published a brief account entitled Vorläufige Nachricht von einigen das Geschlecht der Pflanzen betreffenden Versuchen und Beobachtungen . He reported the results of continued experiments in
three Fortsetzungen (1763, 1764, and 1766). The Vorläufige Nachricht and the three Fortsetzungen offer a coherent account, not only of experiments in hybridization, but of the processes of pollination and fertilization. Koelreuter also published a number of individual papers on these subjects in the Commentarii of the St. Petersburg Academy.
The Vorläufige Nachricht continues the discussion occasioned by the prize competition. Indeed, the format of the piece follows closely the order prescribed in the prize question. For Koelreuter the production of plant bastards constitutes a decisive argument for sexual duality. Here, and only here, his analysis parallels that of Linnæus; elsewhere Koelreuter has taken pains to distance himself from Linnæus' wild claims and speculative flights.