PAUL D STEWART / SCIENCE PHOTO LIBRARY PAUL D STEWART / SCIENCE PHOTO LIBRARY
In 1862 Darwin reported how some plants have two forms of flowers which differ in the height and structure of male and female parts. He coined the term heterostyly. He showed it was a mechanisms for the plants to avoid self fertilisation. The positioning of the stigma high in a particular plant (thrum eyed) facilitates getting pollen from a butterfly probosis that has picked up pollen from the anthers positioned high on different plants (pin eyed). They will not so easily pick up pollen from their own flowers whose anthers are further down - but those can pass to other plants. In his 1887 autobiography he wrote "no little discovery of mine ever gave me so much pleasure as making out the meaning of heterostyled flowers". Recent research (Gilmartin New Phytologist August 2015) has shown that such flowers were known since the 16th century, but none before Darwin understood why.
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