Pollination can be made according to various modes: – Self pollination. The stigma of a flower receives the pollen of the same plant. This mode is frequent, but not compulsory, in cultivated Grasses. It is on the other hand compulsory for flowers that do not open (cleistogamous ) such as the Violet. – Crossed pollination. The stigma of a flower receives the pollen of another plant. Cross pollination can be promoted: – By dioecism: male flowers and female flowers are on separate plants (dioecious species), – By dichogamy: male and female organs mature at different times. The pollen is before released while the stigma is immature (protandry) or the stigma is receptive while stamens are still young (protogyny), – By hercogamy: some structures prevent pollen from being transferred on stigma of the same flower (rostellum of the Orchis), – By heterostyly: in Primula, flowers with high style and stamens situated on the base of the corolla must be pollinated by flowers with short style and stamens situated on the top of the corolla, – By self sterility: flowers can’t be self pollinated because of dimorphism in pollen grains and stigma surfaces. Means of pollination are the wind (anemophily) or insects (entomophily), less often water. In the first case, flowers generally have a well developed and coloured perianth. In the second case, there is no perianth or it is reduced and uncoloured.