Two forms of primrose (Primula vulgaris) flower 1/4
Cross pollination in Primrose (Primula vulgaris) - the series shows both ‘pin eyed’ and 'thrum eyed' forms of the flower and illustrate the mechanism that ensures cross pollination.
Primula vulgaris (Primrose) has two forms of flower, a situation known as heteromorphy. In the two forms, the styles are of different lengths. If the style reaches to the top of the flower tube, it is the 'pin eyed' form, and the anthers are halfway down the tube. In the other form (known as 'thrum eyed'), the style is halfway down the tube and the anthers reach to the top. Heteromorphy is one example of a mechanism that encourages self-pollination.
When an insect inserts its proboscis into the flower, pollen is deposited in the right place for the style of the other form of the flower - so pollen from a pin eyed flower reaches the stigma of a thrum eyed flower, and vice versa. The stigma of a thrum eyed flower has short papillae whereas the stigma of a pin eyed flower has long papillae.
Pollen grains from a pin eyed flower are small and more likely to stick to the short papillae on the stigma of a thrum eyed flower (than to the longer papillae of a pin eyed flower). The converse occurs with the large pollen grains from a thrum eyed flower.
Even if self-pollination does occur, it rarely leads to fertilisation so that heteromorphy in these plants appears to be associated with self-incompatibility.
This image shows a pin eyed Primrose flower - plan view
The flower is approximately 30 mm across and 30 mm tall.
Image by John Bebbington FRPS
Two forms of primrose (Primula vulgaris) flower 1/4
Cross pollination in Primrose (Primula vulgaris) - the series shows both ‘pin eyed’ and 'thrum eyed' forms of the flower and illustrate the mechanism that ensures cross pollination.
Primula vulgaris (Primrose) has two forms of flower, a situation known as heteromorphy. In the two forms, the styles are of different lengths. If the style reaches to the top of the flower tube, it is the 'pin eyed' form, and the anthers are halfway down the tube. In the other form (known as 'thrum eyed'), the style is halfway down the tube and the anthers reach to the top. Heteromorphy is one example of a mechanism that encourages self-pollination.
When an insect inserts its proboscis into the flower, pollen is deposited in the right place for the style of the other form of the flower - so pollen from a pin eyed flower reaches the stigma of a thrum eyed flower, and vice versa. The stigma of a thrum eyed flower has short papillae whereas the stigma of a pin eyed flower has long papillae.
Pollen grains from a pin eyed flower are small and more likely to stick to the short papillae on the stigma of a thrum eyed flower (than to the longer papillae of a pin eyed flower). The converse occurs with the large pollen grains from a thrum eyed flower.
Even if self-pollination does occur, it rarely leads to fertilisation so that heteromorphy in these plants appears to be associated with self-incompatibility.
This image shows a pin eyed Primrose flower - plan view
The flower is approximately 30 mm across and 30 mm tall.
Image by John Bebbington FRPS