Darwin Day Different Forms

I didn't know that today is Darwin Day until a mate told me. It originated in England but is now more widely celebrated in America. Darwin wrote The Different Forms Of Flowers On Plants Of The Same Species in 1877, after he had written The Origin Of The Species. He was a keen botanist and became fascinated by primroses and heterostyly.

I remember being shown the difference between pin-eyed and thrum-eyed primroses when I was quite a young child. I have just learnt that the word thrum means tassle.

The tassle of anthers in a thrum-eyed flower sits at the eye of the bloom. In a pin-eyed flower they are out of sight under the stigma. The thrum-eyed flower in my pic is the violet one which is on the right.

This is a mechanism which prevents self fertilisation. Crosses between pin and thrum morphs succeed, self-fertilisation and crosses between plants with the same flower type fail. Darwin said, "I do not think anything in my scientific life has given me so much satisfaction as making out the meaning of the structure of these plants."