) It is curious for how long a time the
flowers of the heartsease and of some other plants may be watched
without an insect being seen to visit them. During the summer of 1841, I
observed many times daily for more than a fortnight some large clumps of
heartsease growing in my garden, before I saw a single humble-bee at
work. During another summer I did the same, but at last saw some
dark-coloured humble-bees visiting on three successive days almost every
flower in several clumps; and almost all these flowers quickly withered
and produced fine capsules. I presume that a certain state of the
atmosphere is necessary for the secretion of nectar, and that as soon as
this occurs the insects discover the fact by the odour emitted, and
immediately frequent the flowers.
As the flowers require the aid of insects for their complete
fertilisation, and as they are not visited by insects nearly so often as
most other nectar-secreting flowers, we can understand the remarkable
fact discovered by H. Muller and described by him in 'Nature,' namely,
that this species exists under two forms. One of these bears conspicuous
flowers, which, as we have seen, require the aid of insects, and are
adapted to be cross-fertilised by them; whilst the other form has much
smaller and less conspicuously coloured flowers, which are constructed
on a slightly different plan, favouring self-fertilisation, and are thus
adapted to ensure the propagation of the species.
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