Benefits of Cross-Pollination
Because of the specific benefits of cross-pollination, flowering plants have evolved many devices to prevent self-pollination and to encourage cross-pollination. The most common ones are described as follow:
i. Dichogamy: In many species the anthers and the stigma come to maturity at different times. That is, the dehiscence of anthers and the receptivity of the stigma of a flower do not coincide. In the sunflower plant, the anther dehisces before the stigma becomes receptive and thus self-pollination cannot occur. This condition is termed as protandry. In Mirabilis, and Magnolia the stigma becomes receptive before the anthers dehisce. This condition is called protogyny.
ii. Herkogamy: Some species show structural adaptations to prevent pollen grains from coming into contact with the stigma of the same flower. In many herkogamy species the relative position of the anthers and the stigma is such that self- pollination cannot occur. For example, the stigma in many plants projects beyond the level of anthers and as a result the pollen of the same flower cannot land on the stigma. Similarly the pollinia (pollen in sacs) of orchids and Calotropis cannot reach the stigma of the same flower.
iii. Self-sterility: In many species, self-pollination does not result in fertilization. This is because pollen germination on the stigma or the growth of pollen tubes in the stigma or style is inhibited. For effective fertilization, pollen has to come from another plant. Self-sterility is widespread in flowering plants. It is estimated that about half the total number of species of flowering plants exhibit this phenomenon. It is genetically controlled and is considered a primitive character. It seems to have evolved very early in the evolution of flowering plants as an effective mechanism for out breeding.
iv. Dicliny: In these species flowers are unisexual. Male and female flowers are borne either on the same plant (e.g., many cucurbits). This condition is referred to as monoecious. When male and female flowers are borne on different plants (e.g., date palm, mulberry, cannabis) the condition is called dioceous. Since pollination in these, including the monoecious plants, involves two different flowers, it is considered as cross-pollination.