Flora’s Secret Gardens j 493493 493
in plants and why insect- and wind- mediated pollination became the norm. Without the
existence of two sexes in plants there could be no outcrossing and thus no way to suppress
the expression of mutated, deleterious alleles.
In addition, Darwin would no doubt have understood that an allele that is deleterious
under one set of conditions may be advantageous under another set of conditions. Sexual
reproduction is, in fact, a two- edged sword. On the one hand, it can suppress deleterious
mutations that arise spontaneously during an organism’s development. On the other hand,
by creating new combinations of alleles, it can undo whatever advantages have been gained
over many generations through the process of natural selection.^25 However, if the environ-
mental conditions were to change suddenly (due, for example, to the presence of pathogens
or shifts in sunlight, temperature, water availability, or soil chemistry), some of the so- called
“deleterious” alleles might prove to be advantageous, thus allowing the species to adapt to
the new conditions.
These two factors— the suppression of deleterious alleles and the increase in genetic
diversity— provide us with a plausible explanation for the existence of two sexes in plants,
although it is by no means the complete story. Although beyond the scope of this book, mul-
tiple factors have led to the evolution of sex, and different types of organisms have evolved
sexual cycles for different reasons.^26 It is likely that additional evolutionary factors favoring
the plant sexual cycle and alternation of generations will be discovered in the future.So Who Won the Debate, Sexualists or Asexualists?
Hofmeister’s brilliant synthesis demonstrating that the alternation of generations is a uni-
versal feature of plant life cycles did not just unify the plant kingdom. It can also be viewed
as the final resolution of the age- old quandary over plant sex.
Contrary to the usual triumphalist narratives of the history of science, both the sexualists
and asexualists turned out to be correct, although for very different reasons. Aristotle and
Theophrastus, along with their legions of asexualist disciples throughout the ages, arrived
at their opinion via culturally biased intuition about the inner nature of plants, which
led them to conclude, a priori, that plants were incapable of sex. On the other hand, the
metaphors and analogies used by the asexualists to describe plant reproduction were often
based on the female reproductive system, including eggs, uterus, and menstruation. Thus.
the asexualists were in fact proposing that plants were functionally parthenogenic females.
Nehemiah Grew inaugurated the sexual theory by comparing stamens to penises, anthers
to testicles, and pollen grains to sperm, while Camerarius provided the first experimental
proof of the sexual theory in dioecious and monoecious species. However, unlike the tes-
ticles of male animals, anthers do not give rise to gametes (sperm cells) directly— they pro-
duce spores by meiosis instead. Since spores are not gametes— that is, they do not fuse with
other spores to form a zygote— spore production is an asexual reproductive process. Thus
the stamen itself is actually a spore- producing asexual structure. The microspores inside the
anther divide mitotically to form a new haploid individual— the mature pollen grain with
its elongated pollen tube containing two sperm cells.
Similarly, the ovary of the flower, unlike the ovaries of female mammals, does not give
rise directly to the female gamete (egg) but rather gives rise by meiosis to the female spore,