Our plant would then make a close
approach in structure to a heterostyled dimorphic species; or to a trimorphic
species, if the stamens were reduced to two lengths in the same flower in
correspondence with that of the pistils in the other two forms. But we have not
as yet even touched on the chief difficulty in understanding how heterostyled
species could have originated. A completely self-sterile plant or a dichogamous
one can fertilise and be fertilised by any other individual of the same species;
whereas the essential character of a heterostyled plant is that an individual of
one form cannot fully fertilise or be fertilised by an individual of the same
form, but only by one belonging to another form.
H. Muller has suggested that ordinary or homostyled plants may have been
rendered heterostyled merely through the effects of habit. (6/5. 'Die
Befruchtung der Blumen' page 352.) Whenever pollen from one set of anthers is
habitually applied to a pistil of particular length in a varying species, he
believes that at last the possibility of fertilisation in any other manner will
be nearly or completely lost. He was led to this view by observing that Diptera
frequently carried pollen from the long-styled flowers of Hottonia to the stigma
of the same form, and that this illegitimate union was not nearly so sterile as
the corresponding union in other heterostyled species.
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