And now we see that
this same conclusion is independently and strongly fortified by the
consideration of the illegitimate unions of trimorphic and dimorphic
heterostyled plants. In so complex and obscure a subject as hybridism it is no
slight gain to arrive at a definite conclusion, namely, that we must look
exclusively to functional differences in the sexual elements, as the cause of
the sterility of species when first crossed and of their hybrid offspring. It
was this consideration which led me to make the many observations recorded in
this chapter, and which in my opinion make them worthy of publication.
CHAPTER VI.
CONCLUDING REMARKS ON HETEROSTYLED PLANTS.
The essential character of heterostyled plants.
Summary of the differences in fertility between legitimately and illegitimately
fertilised plants.
Diameter of the pollen-grains, size of anthers and structure of stigma in the
different forms.
Affinities of the genera which include heterostyled species.
Nature of the advantages derived from heterostylism.
The means by which plants became heterostyled.
Transmission of form.
Equal-styled varieties of heterostyled plants.
Final remarks.
In the foregoing chapters all the heterostyled plants known to me have been more
or less fully described. Several other cases have been indicated, especially by
Professor Asa Gray and Kuhn, in which the individuals of the same species differ
in the length of their stamens and pistils (6/1.
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