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Cross-fertilized blossoms
defeat self-fertilized flowers in the struggle for existence.
No wonder Sprengel's theory was disproved by his scornful
contemporaries in the very case of his wild geranium, which sheds
its pollen before it has developed a stigma to receive any;
therefore no insect that had not brought pollen from an earlier
bloom could possibly fertilize this flower. How amazing that he
did not see this! Our common wild crane's-bill, which also has
lost the power to fertilize itself, not only ripens first the
outer, then the inner, row of anthers, but actually drops them
off after their pollen has been removed, to overcome the barest
chance of self-fertilization as the stigmas become receptive.
This is the geranium's and many other flowers' method to compel
cross-fertilization by insects. In cold, stormy, cloudy weather a
geranium blossom may remain in the male stage several days before
becoming female; while on a warm, sunny day, when plenty of
insects are flying, the change sometimes takes place in a few
hours.


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