Sex and the Single Cryptogam j 473473 473
was not new with Schleiden. Several contemporary asexualists, including Schleiden’s uncle,
Johann Horkel,^23 were looking for parallels between the development of cryptogam spores
and pollen grains. They believed that the main difference between the spores of cryptogams
and the pollen grains of flowering plants was that pollen grains germinated on the stigma
instead the soil. For example, in 1837, William Valentine, a British naturalist, stated that
“it is a well- established fact that the embryo, or essential part of the seed, is derived from
the pol len.”^24 What set Schleiden apart from the other asexualists was his claim to have
witnessed the process under the microscope. A. G. Morton has suggested that Schleiden
simply confused the tip of the pollen tube with another type of cell in the embryo sac— yet
another example of “believing is seeing.”^25
Whatever the source of the error, Schleiden’s supreme confidence in the truth of his arti-
fact led him to propose a radical revision of floral biology and sexual identity. Based on the
Aristotelian doctrine that the female parent by definition provides the material substance
of the embryo, Schleiden concluded that the pollen tube must therefore be a female struc-
ture that reproduces vegetatively. Since the ovule played only a nutritive role in housing
the embryo, Schleiden inferred that it was a purely asexual structure.^26 In a bizarre twist of
logic, Schleiden made the case for a unisexual, plants- as- female model based on his identifi-
cation of the pollen tube as the female parent.
Because of his high standing in the field, Schleiden’s pollen- parthenogenesis theory was
accepted by many microscopists. In 1846, Amici presented compelling evidence from his
studies with orchids that the role of the pollen tube in embryo formation was to activate a
pre- existing egg cell within the ovule. Amici’s theory also acquired adherents among micros-
copists, and a full- blown controversy ensued. To settle the matter, the Dutch Academy of
Sciences offered yet another prize, which was awarded in 1850 to Hermann Schact. In fact,
Schact’s essay supported Schleiden’s asexual pollination theory! Thus, the prize only suc-
ceeded in muddying the waters.
Even before its publication, the work was refuted by several eminent botanists, includ-
ing Wilhelm Hofmeister. Hofmeister’s paper, published in 1849, described observations on
thirty- eight species belonging to nineteen genera and demonstrated that, in all cases, the
egg cell that ultimately developed into the embryo was already present in the embryo sac
prior to pollination. Indeed, one of Schleiden’s own students confirmed the observations
of Amici and Hofmeister,^27 and, six years later, Schleiden quietly retracted his asexual pol-
lination theory.^28
By 1856, sexual fusion of sperm and egg had been demonstrated in three different algae,
Fucus, Vaucheria, and Oedigonium. Based on these and similar findings in animals, the
German zoologist Oscar Hertwig formulated the general definition of fertilization as the
union of the nuclei of the male and female gametes: sperm and egg.^29 Although evidence
was still lacking in flowering plants, it was widely assumed that the fusion of egg and sperm
was the basis of fertilization in angiosperm embryo sacs as well.A Kingdom Divided Cannot Stand
The discoveries of sex in the cryptogams and Schleiden’s retraction of his theory of par-
thenogenic pollen tubes had removed the last barriers to the universal acceptance of sex in
plants. But although sexuality was now deemed to be a feature of all plant life cycles, it was