Koelreuter, Joseph Gottlieb
Koelreuter, Joseph Gottlieb
(b. Sulz, Germany, 27 April 1733; d. Karlsruhe, Germany, 12 November 1806)
Koelreuter was the son of an apothecary. At fifteen he went to the nearby University of Tübingen to read medicine and graduated in 1755. He spent the next six years as keeper of the natural history collections belonging to the Imperial Academy of Sciences in St. Petersburg. Although his chief duties concerned the classification of fish, he began his study to flower and pollen structure, pollination, and fertilization. When the Academy offered a prize for an essay on the experimental demonstration of the sexuality of plants, Koelreuter set out to produce plant hybrids.
In 1761 Koelreuter returned to Germany. In Leipzig and in Calw (Swabia), as the guest of Achatius Gaertner, he continued his hybridization experiments until his appointment as professor of natural history and director of the gardens in Karlsruhe, which belonged to the margrave of Baden, Karl Friedrich. Caroline, wife of the margrave, was an enthusiast for botany and protected Koelreuter from the jeal– ousy of the gardeners, who resented the intrusion of so much experimental botany in the margrave’s fine gardens, On her death in 1786 Koelreuter was dismissed. In 1775 he had married the daughter of a local judge; she bore him six daughters and one son. The son was given a good education and sent to St. Peters– burg to read medicine, but the rest of the family lived in Karlsruter complained of lack of recognition and financial support. He died embittered.
Koelreuter’s strength lay in his brilliant experimentation, which was combined with great curiosity. His deep commitment to the concept of the harmony of nature and to the purposive character of all organic structures led him to inquire where others had merely described. His enthusiasm for the current interest in alchemical notions colored his interpretation of the facts of fertilization and led him to undertake his famous experiments in the “transmutation” of plant species.
Two theories of plant fertilization were current in the eighteenth century among those who accepted the concept of plant sexuality. Those who adhered to the doctrine of preformation and were spermists identified the germ of the new organism with the granules in the fluid which was expelled from pollen grains immersed in water. Ovists, on the other hand, denied a genetic role to the pollen. Those who denied preformatioin tended to think of the agents of fertilization as fluids. These male and female “seeds” had to mix in order to generate the offspring. Koelreuter thought this granular fluid was too crude to be the male “seed.” Instead, he imagined that it was perfected to yield an oil which passed through the system of excretion canals in the wall of the pollen grain in order to reach the stigma. These canals were not really canals, and the oil he observed did not come from the interior of the grains.
The then current conception of fertilization as a mixing of liquids was further supported by Koelreuter’s apparent demonstration that more than one pollen grain was required to fertilize one ovule. A certain minimum number fof pollen grains had to be supplied to the stigma before any seeds were formed. Only in the case of Mirabilis did Koelreuter find that one grain sufficed to fertilize its uniovulate flowers; but because of the large size of these grains he rejected this first evidence that fertilization is a unitary and discrete process;.
His study of the curious architecture of pollen grains led Koelreuter to inquire into their function. He soon perceived that pollen, stamens, and stigmas are designed to insure efficient pollination. He drew attention to the agency of wind and insects in this process and described the special sensitive stamens of Berberis and the sensitive stigmas of Martynia. He perceived the significant fact that many hermaphrodite flowers fail to be self-pollinated because the stamens and stigmas ripen at different times. These observations were extended by C. K. Sprengel and led to the overthrow of the old view that the rolde insects in visiting flowers was to remove the harmful waxy and sugary secretions which would otherwise prevent seed formation. They also raised a new question: why are many hermaphrodite species adapted for crosspollination?
R. Camerarius had described experiments in support of the sexuality of plants in 1691, but doubt on this subject continued long after koelreuter’s student days in Tübingen, when his teacher J. G. Gmelin reprinted Camerarius’ account. Koelreuter perceived that if he could produce plant hybridds and show analogies between them and animal hybrids, he would achieve powerful support for the theory of plant sexuality. His first success was with the cross Nicotiana rustica X N. paniculata. Because his approach to the study of this tobacco hybrid was thoroughly scientific, the account he published in 1761 constitutes the first of its kind in the nliterature. Over the next five years he published further reports on such experiments, in which he discovered the uniformity and almost complete sterility of these plant hybrids, the identity of reciprocal crosses, and the contrast between these hybrids and their progeny. The latter were not uniform but tended to return to one of the parental species.
Because he believed in the preestablished harmony of nature, Koelreuter was delighted to find how infertile these plant hybrids were, for he saw this as a mechanism for preventing the confusion which unbridled crossing would yield. Unfortunate for this view was his subsequent discovery of fertile hybrids, especially in the genus Dianthus. To overcome this counterevidence he made a distinction between the natural world as it came from the hand of God and the artificial world of mas’s making. The latter was to be seen in zoological and botanical gardens, where species had been brought together which in nature had been deliberately separated to prevent their cross-breeding. He saw a corresponding distinvction between “prefect” and “imperfect” hybrids. The former were alll exactly intermediate between the two originating species and were almost completely sterile—mule plants. Imperfect hybrids, if fertile, were the product of crossing garden varieties; and if nonintermediate, they must have arisen from a fertilization between two species in which a tincture of pollen from the mother plant had also been active. Such products he termed “half-hybrids.”.
Koelreuter drew an analogy between the pollen and the sulfur of the alchemist, on the one hand, and between the female seed material and the mercury of the alchemist, on the other. By successive pollinations of a hybrid and its progeny he saw a biological means of effecting a transmutation. His first success in 1763, when he “transmuted” Nicotiana rustica into N. paniculata, spurred him on to further efforts. Only a lack of facilities prevented him form attempting the transmutation of the canary into the goldfinch. These experiments furnished the champions of bisexual heredity with strong evidence in their favor, evidence which told against the preformation theory. At the same time Koelreuter saw these experiments as a demonstration of the impossibility of producing new species by hybridization. Hybrids tended to revert to one of the stem species. By hybridization one species could be changed to another, but no new combination of existing species could reproduce its kind indefinitely. Where such new and permanent forms did arise, they were not to be given the status of new species.
Koelreuter may be said to have initiated the scientific study of plant hybridization. Mendel, Sprengel, and Darwin owed him a debt. If the world view which underlay his modern approach to nature belonged to that of the seventeenth century, his experimentation was not surpassed until the time of Mended.
I. Original Works. Koelreuter’s doctoral thesis was Dissertatio inauguralis medica de insectis Coleopteris, necnon de plantis quibusdam rarioribus (Tübingen, 1755). His famous experiments on plant sexuality and hybridization are described in his Vorläufige Nachricht von einigen das Geschlecht der Pflanzen betrefenden und Beobachtungen, nebst Fortsetzungen 1, 2 und 3 (Leipzig, 1761–1766), reprinted by W. Pfefer in Ostwald’s Klassiker der exakten Wissenschaften, no. 41. His work on the cryptogams is Das entdeckte Geheimniss der Cryptogamie (Carlsruhe, 1777).
II. Secondary Literature. The best account of Koclreuter’s life and work is by J. Behrens, “Joseph Gottlieb Koelreuter. Ein Karlsruhe Botaniker des achtzehnten Jahrhunderts,” in the Verhandlungen des Naturwissenschaftlichen Vereins in karlsruhe, 11 (1895), 268–320. For more recent accounts see R. C. Olby, “Joseph Koelreuter, 1733–1806,” in Olby, ed., Late Eighteenth Century European Scientists (Oxford, 1966), pp. 33–65, and Origins of Mendelism (London, 1966), pp. 20–36.
See also H. F. Roberts, Plant Hybridization Before Mendel (1929; repr. New York, 1965).