The
degree to which distinct plants will thus be intercrossed depends on the
number of spikes in full flower at the same time on the same plant. With
proterogynous flowers and with depending racemes, the manner in which
insects visit the flowers ought to be reversed in order that distinct
plants should be intercrossed. But this whole subject requires further
investigation, as the great importance of crosses between distinct
individuals, instead of merely between distinct flowers, has hitherto
been hardly recognised.
In some few cases the special movements of certain organs almost ensure
pollen being carried from plant to plant. Thus with many orchids, the
pollen-masses after becoming attached to the head or proboscis of an
insect do not move into the proper position for striking the stigma,
until ample time has elapsed for the insect to fly to another plant.
With Spiranthes autumnalis, the pollen-masses cannot be applied to the
stigma until the labellum and rostellum have moved apart, and this
movement is very slow. (10/35. 'The Various Contrivances by which
British and Foreign Orchids are fertilised' first edition page 128.)
With Posoqueria fragrans (one of the Rubiaceae) the same end is gained
by the movement of a specially constructed stamen, as described by Fritz
Muller.
We now come to a far more general and therefore more important means by
which the mutual fertilisation of distinct plants is effected, namely,
the fertilising power of pollen from another variety or individual being
greater than that of a plant's own pollen.
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