THE MEANS WHICH FAVOUR OR ENSURE FLOWERS BEING FERTILISED WITH POLLEN
FROM A DISTINCT PLANT.
We have seen in four cases that seedlings raised from a cross between
flowers on the same plant, even on plants appearing distinct from having
been propagated by stolons or cuttings, were not superior to seedlings
from self-fertilised flowers; and in a fifth case (Digitalis) superior
only in a slight degree. Therefore we might expect that with plants
growing in a state of nature a cross between the flowers on distinct
individuals, and not merely between the flowers on the same plant, would
generally or often be effected by some means. The fact of bees and of
some Diptera visiting the flowers of the same species as long as they
can, instead of promiscuously visiting various species, favours the
intercrossing of distinct plants. On the other hand, insects usually
search a large number of flowers on the same plant before they fly to
another, and this is opposed to cross-fertilisation. The extraordinary
number of flowers which bees are able to search within a very short
space of time, as will be shown in a future chapter, increases the
chance of cross-fertilisation; as does the fact that they are not able
to perceive without entering a flower whether other bees have exhausted
the nectar. For instance, Hermann Muller found that four-fifths of the
flowers of Lamium album which a humble-bee visited had been already
exhausted of their nectar.
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