Question

What is the basis for “pin” and “thrum” flowers in SI plants?  You may use a carefully labeled diagram with your answer.?

Answer 1

Introduction : SI plants are those in which the plants do not inbreed within the same species and promote outcrossing. This phenomenon is genetically controlled by one or more multi-allelic loci rely on a series of complex cellular interactions between SI pollen and pistil.

Explanation : In the SI pin flowers, the stigma is positioned at the top of the tube with anthers positioned halfway down. But in the thrum flowers the stigma is positioned halfway down the tube with anthers at the top. Also one of the notable difference between these pin and thrum flowers are the size of the pollen, it is small in pin flowers compared to that of the thrum flowers. The difference in the arrangement of anthers and the stigma in the pin and thrum flowers help in the process of cross pollination by the visiting insects. For example when a insect visits a pin flower it inserts its feeding tube (proboscis) into the flowertube to feed on the nectar, the pollen from the anther sticks onto the feeding tubeabout halfway down. If the same insect then visits a thrum flower the pollen already on its feeding tube is at the right level to meet and stick to the stigma about halfway down the flower tube which results in cross pollination. Also the pollen from the anthers at the top of the thrum flower sticks to the top of the insects feeding tube which results in cross pollinating the next pin flower it visits. Thus the self incompatible pin and thrum flowers acheive cross pollination.