YII 



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, Avhile 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). 



Long-styled form. Short-styled form. 



Fig. 17. — Priinula veris (Cowslip). 



The meaning and use of these diff'erent 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 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. 



