If, however, a flower's own pollen should
first be placed by insects or fall on the stigma, it by no means follows that
cross-fertilisation will be thus prevented. It is well known that if pollen from
a distinct species be placed on the stigma of a plant, and some hours afterwards
its own pollen be placed on it, the latter will be prepotent and will quite
obliterate any effect from the foreign pollen; and there can hardly be a doubt
that with heterostyled dimorphic plants, pollen from the other form will
obliterate the effects of pollen from the same form, even when this has been
placed on the stigma a considerable time before. To test this belief, I placed
on several stigmas of a long-styled cowslip plenty of pollen from the same
plant, and after twenty-four hours added some from a short-styled dark-red
Polyanthus, which is a variety of the cowslip. From the flowers thus treated 30
seedlings were raised, and all these, without exception, bore reddish flowers;
so that the effect of pollen from the same form, though placed on the stigmas
twenty-four hours previously, was quite destroyed by that of pollen from a plant
belonging to the other form.
Finally, I may remark that of the four kinds of unions, that of the short-styled
illegitimately fertilised with its own-form pollen seems to be the most sterile
of all, as judged by the average number of seeds, which the capsules contained.
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