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William Smellie published his Encyclopedia Britannica article debunking the sexual the-
ory, definitive proof of its validity had finally been provided by Joseph Gottlieb Koelreuter.
Indeed, Koelreuter is the unsung hero of the sexual theory, even though his work had sur-
prisingly little impact on the contemporary debate.Koelr euter’s Productive Jour ney
from Tübingen to Karlsruhe
Part of the confusion surrounding the role of sex in plants during the eighteenth century
was a consequence of the limited circulation of Camerarius’s Epistola. Even those botanists
who were aware of Camerarius’s work seemed to have read only abstracts or summaries
that overemphasized the exceptions in his experiments rather than the main findings. It
was the German physician and botanist Joseph Gottlieb Koelreuter, a fellow Tübingener,
who set the record straight. In his 1761 paper, Preliminary Report of Some Experiments and
Observations Concerning Sex in Plants, he gave Camerarius full credit for placing the sexual
theory of plants on a scientific footing:Rudolph Jacob Camerarius is indisputably the first who proved the sex of plants
through his own experiments instituted from this point of view. He, my fellow coun-
tryman, it is whom the learned world has principally to thank for this great truth,
which is so general, and of such great influence upon the physical and economic
sciences.As persuasive as Camerarius’s results were, however, they did not constitute “indisput-
able” proof of sex in plants. One could argue, for example, that removal of the maize tas-
sel (male inflorescence) injured the plant and prevented normal seed development. On the
other hand, Camerarius’s results with dioecious species such as spinach, in which female
plants were isolated from male plants, could not be explained away as the result of damage
to the female plant. In these cases, asexualists focused exclusively on the few exceptions
reported by Camerarius and ignored his more general findings.
The most compelling argument in favor of the sexual theory was plant hybridization.
As we saw in Chapter 13, preliminary evidence for plant hybridization was reported in the
early eighteenth century by Cotton Mather and Thomas Fairchild, but these observations
were never followed- up. Of the four plant hybrids claimed by Linnaeus, three were puta-
tive intergeneric hybrids that were never verified experimentally, whereas the fourth appar-
ent interspecific hybrid was never repeated. Since neither Mather, Fairchild, nor Linnaeus
provided unequivocal proof of plant hybridization, the skepticism of the asexualists could
still be justified on scientific grounds. It was left to Joseph Koelreuter to eliminate this last
potential objection to the sexual theory by providing the first rigorous demonstration of
plant hybridization.
Joseph Gottlieb Koelreuter was born on April 27, 1733, in Sulz, a little town on the Neckar
River in southern Germany, about twenty miles southwest of Tübingen. At the age of fif-
teen he entered the University of Tübingen to study medicine, where he came in contact
with the naturalist and explorer J. G. Gmelin, who had just returned from St. Petersburg.