These instances
clearly prove that the flowers on the same plant have often varied
independently of one another in many important respects, such variations
having been fixed, like those on distinct plants during the development
of species.
It was therefore necessary to ascertain by experiment what would be the
effect of intercrossing flowers on the same plant, in comparison with
fertilising them with their own pollen or crossing them with pollen from
a distinct plant. Trials were carefully made on five genera belonging to
four families; and in only one case, namely, Digitalis, did the
offspring from a cross between the flowers on the same plant receive any
benefit, and the benefit here was small compared with that derived from
a cross between distinct plants. In the chapter on Fertility, when we
consider the effects of cross-fertilisation and self-fertilisation on
the productiveness of the parent-plants we shall arrive at nearly the
same result, namely, that a cross between the flowers on the same plant
does not at all increase the number of the seeds, or only occasionally
and to a slight degree. I will now give an abstract of the results of
the five trials which were made.
1. Digitalis purpurea.
Seedlings raised from intercrossed flowers on the same plant, and others
from flowers fertilised with their own pollen, were grown in the usual
manner in competition with one another on the opposite sides of ten
pots.
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