There is no great difficulty in understanding how an anemophilous plant
might have been rendered entomophilous. Pollen is a nutritious
substance, and would soon have been discovered and devoured by insects;
and if any adhered to their bodies it would have been carried from the
anthers to the stigma of the same flower, or from one flower to another.
One of the chief characteristics of the pollen of anemophilous plants is
its incoherence; but pollen in this state can adhere to the hairy bodies
of insects, as we see with some Leguminosae, Ericaceae, and
Melastomaceae. We have, however, better evidence of the possibility of a
transition of the above kind in certain plants being now fertilised
partly by the wind and partly by insects. The common rhubarb (Rheum
rhaponticum) is so far in an intermediate condition, that I have seen
many Diptera sucking the flowers, with much pollen adhering to their
bodies; and yet the pollen is so incoherent, that clouds of it are
emitted if the plant be gently shaken on a sunny day, some of which
could hardly fail to fall on the large stigmas of the neighbouring
flowers. According to Delpino and Hermann Muller, some species of
Plantago are in a similar intermediate condition. (10/44. 'Die
Befruchtung' etc. page 342.)
Although it is probable that pollen was aboriginally the sole attraction
to insects, and although many plants now exist whose flowers are
frequented exclusively by pollen-devouring insects, yet the great
majority secrete nectar as the chief attraction.
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