These two forms have long been
known to florists as the "pin-eyed" and the "thrum-eyed," but they are
called by Darwin the long-styled and short-styled forms (see woodcut).
[Illustration: FIG. 17.--Primula veris (Cowslip).]
The meaning and use of these different forms was quite unknown till
Darwin discovered, first, that cowslips and primroses are absolutely
barren if insects are prevented from visiting them, and then, what is
still more extraordinary, that each form is almost sterile when
fertilised by its own pollen, and comparatively infertile when crossed
with any other plant of its own form, but is perfectly fertile when the
pollen of a long-styled is carried to the stigma of a short-styled
plant, or _vice versa_. It will be seen, by the figures, that the
arrangement is such that a bee visiting the flowers will carry the
pollen from the long anthers of the short-styled form to the stigma of
the long-styled form, while it would never reach the stigma of another
plant of the short-styled form. But an insect visiting, first, a
long-styled plant, would deposit the pollen on the stigma of another
plant of the same kind if it were next visited; and this is probably the
reason why the wild short-styled plants were found to be almost always
most productive of seed, since they must be all fertilised by the other
form, whereas the long-styled plants might often be fertilised by their
own form.
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