20-07-2016, 04:09 PM
The flower-heads of the sunflower are well-suited for insect pollination as the crowding of the flowers ensures conspicuousness and the pollination of a maximum number of flowers by a single insect visit. The honey, secreted at the base of the style, is protected by the corolla tube from visits of short-tongued insects. When the flower open the receptive surfaces of the two stigmas are pressed together and occupy a position at the base of the tube formed by the united anthers; the latter split on the inside and the liberated pollen fills the cavity of the tube and exposes it to contat with visiting insects. Fianll, the style protrudes right throught the anther tube and the stigmas spread apart and expose their formerly hidden receptive surfaces. Thus, the life history of the flower falls into two stages, the first male and the second female. This favours cross-pollination as compared to self-pollination. The sun-flower is therefore practically self-sterile, though self-pollination may take place at a late stage, when cross-pollination has failed.