(5/14. This has been remarked by many experimentalists in
effecting crosses between distinct species; and in regard to illegitimate unions
I have given in the first chapter a striking illustration in the case of Primula
veris.) Both with hybrids and illegitimate plants the innate degree of sterility
is highly variable in plants raised from the same mother-plant. In both cases
the male organs are more plainly affected than the female; and we often find
contabescent anthers enclosing shrivelled and utterly powerless pollen-grains.
The more sterile hybrids, as Max Wichura has well shown, are sometimes much
dwarfed in stature, and have so weak a constitution that they are liable to
premature death (5/15. 'Die Bastardbefruchtung im Pflanzenreich' 1865.); and we
have seen exactly parallel cases with the illegitimate seedlings of Lythrum and
Primula. Many hybrids are the most persistent and profuse flowerers, as are some
illegitimate plants. When a hybrid is crossed by either pure parent-form, it is
notoriously much more fertile than when crossed inter se or by another hybrid;
so when an illegitimate plant is fertilised by a legitimate plant, it is more
fertile than when fertilised inter se or by another illegitimate plant. When two
species are crossed and they produce numerous seeds, we expect as a general rule
that their hybrid offspring will be moderately fertile; but if the parent
species produce extremely few seeds, we expect that the hybrids will be very
sterile.
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