But there are marked exceptions, as shown by Gartner, to these rules.
So it is with illegitimate unions and illegitimate offspring. Thus the mid-
styled form of Lythrum salicaria, when illegitimately fertilised with pollen
from the longest stamens of the short-styled form, produced an unusual number of
seeds; and their illegitimate offspring were not at all, or hardly at all,
sterile. On the other hand, the illegitimate offspring from the long-styled
form, fertilised with pollen from the shortest stamens of the same form, yielded
few seeds, and the illegitimate offspring thus produced were very sterile; but
they were more sterile than might have been expected relatively to the
difficulty of effecting the union of the parent sexual elements. No point is
more remarkable in regard to the crossing of species than their unequal
reciprocity. Thus species A will fertilise B with the greatest ease; but B will
not fertilise A after hundreds of trials. We have exactly the same case with
illegitimate unions; for the mid-styled Lythrum salicaria was easily fertilised
by pollen from the longest stamens of the short-styled form, and yielded many
seeds; but the latter form did not yield a single seed when fertilised by the
longest stamens of the mid-styled form.
Another important point is prepotency. Gartner has shown that when a species is
fertilised with pollen from another species, if it be afterwards fertilised with
its own pollen, or with that of the same species, this is so prepotent over the
foreign pollen that the effect of the latter, though placed on the stigma some
time previously, is entirely destroyed.
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