Among genera we find some--such as Hippeastrum, Crinum,
Calceolaria, Dianthus--almost all the species of which will fertilise
other species and produce hybrid offspring; while other allied genera,
as Zephyranthes and Silene, notwithstanding the most persevering
efforts, have not produced a single hybrid even between the most closely
allied species.
_Dimorphism and Trimorphism._
Peculiarities in the reproductive system affecting individuals of the
same species reach their maximum in what are called heterostyled, or
dimorphic and trimorphic flowers, the phenomena presented by which form
one of the most remarkable of Mr. Darwin's many discoveries. Our common
cowslip and primrose, as well as many other species of the genus
Primula, have two kinds of flowers in about equal proportions. In one
kind the stamens are short, being situated about the middle of the tube
of the corolla, while the style is long, the globular stigma appearing
just in the centre of the open flower. In the other kind the stamens are
long, appearing in the centre or throat of the flower, while the style
is short, the stigma being situated halfway down the tube at the same
level as the stamens in the other form.
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