418 i Flora Unveiled
Koelreuter cited a second barrier to hybridization: the species- specificity of pollen fer-
tilization, a phenomenon he discovered serendipitously during a futile attempt to produce
hybrid plants fertilized by a mixture of pollen grains from different species. The design
of the experiment was inspired by the canard, widely accepted during the eighteenth and
nineteenth centuries, that a child could have multiple biological fathers.^28 Applying this
principle to plants, Koelreuter assumed that it was possible to generate hybrids of multiple
species by applying a mixture of pollen grains to the stigma. It was on this basis that he had
declared Linnaeus’s Tragopogon hybrid “only half a hybrid” because of its fertility and, in his
opinion, its marginally intermediate appearance. He speculated that the hybrid had been
generated by accidentally applying a mixture of pollen grains from both of the parental spe-
cies to the stigma of the female parent, so that it more closely resembled the female parent.
However, Koelreuter was unable to demonstrate multiple paternity in plants when he tested
his theory experimentally. Every time he dabbed a pollen mixture onto the stigma of a flower,
only the pollen from the conspecific (same species) plant fertilized the flower. Similarly, if he
placed a mixture of all- alien pollen grains on the stigma, only the pollen from the most closely
related species successfully fertilized the flower. Koelreuter therefore concluded that God had
arranged things so that flowers always favored conspecific pollen or, failing that, closely related
pollen, thus preventing the random mixing of species. Based on his own findings, Koelreuter
dismissed Linnaeus’s claims regarding the abundance of intergeneric crosses in nature as “the
premature births of an over- excited imagination,” as indeed they probably were.
Unlike Nehemiah Grew and Sebastien Vaillant, who described the fertilizing effect of
pollen in immaterial terms as a “prolifick breath” or “volatile spirit,” Koelreuter believed
that fertilization occurred by a mixing of male and female semen, which induced a chemi-
cal reaction:
Since it was produced from those two simple forces, it has also an intermediate, com-
posite, force. Just as from the mixture of an acid and alkaline salts a third, namely an
intermediate salt arises.He was, however, aware that the process of sexual reproduction was far more complex
than a simple chemical reaction. All of the information about an organism and its compo-
nent parts had to be passed on to the next generation via the semen. Knowing nothing about
genes or DNA, Koelreuter postulated that male and female semen were each homogenous
liquids that nevertheless encoded the instructions for making another individual of the
same species. In plants, the oily substance inside pollen grains represented the male semen
and the sticky fluid on the surfaces of stigmas represented the female semen. Koelreuter
hypothesized that after the male semen from the pollen grain mixed with the female semen
on the surface of the stigma, the mixture was absorbed by the style and transported to the
ovary where it produced the seed.
Based on his theory of the mixing of fluids, it was logical for Koelreuter to predict that
during hybridization the uniform essences of the two parents would be blended to form an
organism that was precisely intermediate in type. As noted earlier, Koelreuter’s first artifi-
cial hybrid between Nicotiana paniculata and Nicotiana rustica approximated this criterion,
but some of his later hybrids did not, which puzzled him greatly.
To explain why some of the traits of the hybrids resembled those of one parent more
than the other, Koelreuter postulated incomplete blending of semen, resulting in pockets