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The sexual phases of the rose (the male and female gametophytes) are highly reduced
and hidden from view inside the sporophyte. Buried within the ovules and anthers of rose
flowers are diploid megaspore mother cells and microspore mother cells, respectively. These
spore- producing cells undergo meiotic (“reduction”) division, yielding haploid megaspores
and microspores containing only 7 chromosomes each. These two types of haploid spores
are the first stages of the female and male gametophyte generations. The spores then divide
mitotically to produce the mature gametophyte generations: the male pollen grains and the
female embryo sacs, which produce the gametes, sperm and egg.
Pollination delivers the sperm to the egg via the pollen tube. During fertilization, the
sperm nucleus fuses with the egg nucleus, thereby establishing the diploid number of chro-
mosomes (2N = 14 in the case of roses) in the zygote. As a result, all of the following devel-
opmental stages of the sporophyte, from embryo to seedling to mature plant, are diploid.
Strasburger was the first to demonstrate that morphological alternation of generations in
plants is accompanied by a parallel alternation between the haploid (1N) and diploid (2N)
generations. The haploid and diploid stages of the plant life cycle are indicated in Figure 18.6.
There is one more critical step, which, unlike alternations of generations, is unique to
flowering plants. In 1902, the Russian botanist Sergius Nawaschin discovered the phenom-
enon of double fertilization, which occurs only in angiosperms. As described earlier, the pol-
len tube produces two sperm cells during development, but only one of them fertilizes the
egg. The second sperm cell fuses with the central cell, a specialized binucleate cell (having
two nuclei) within the embryo sac. Because fusion of three haploid nuclei is involved, the
resulting cell has triple the number of chromosomes of the haploid nucleus and is therefore
called triploid (3N). The triploid cell then undergoes repeated mitotic divisions to form
the endosperm of the seed, the white tissue that provides nourishment to the developing
embryo.^17 Cereal grains are especially rich in endosperm, which accounts for their impor-
tance as the primary staple crop of humans.Darwin and Mendel: Solving the Ancient Riddles
of Outcrossing and Degeneration
By the middle of the nineteenth century, the two- sex model of plants was at last firmly estab-
lished. However, there remained several basic questions that would have to be answered
before sexual reproduction in plants could be fully comprehended. Two of these questions,
in particular, are of such long- standing and fundamental importance that they must be
included in any full account of the history of the sexual theory: the questions of the bio-
logical significance of outcrossing and the cause of so- called “degeneration” in fruit trees.
The scientific community could not even begin to address these two questions until two
tectonic events shook the scientific community and changed the scientific landscape for-
ever: the publication of Darwin’s Origin of Species in 1859 and the rediscovery of Mendel’s
forgotten papers on inheritance in peas in 1900.
The phenomenon of outcrossing was especially contentious. Sprengel’s comprehensive
studies on the relationship between flower structure and insect pollination had been coldly
received by Goethe and the nature philosophers, who believed that plants were by definition
self- sufficient and needed no outside agents to complete their life cycles. They also objected