VII 
ON THE INFERTILITY OF CROSSES 
157 
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. 
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). 
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 ils 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 versd. 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. 
