The transfer of pollen grains from the anther of one flower to the stigma of the same or different flower is called as pollination. Pollination can be of two types:
- Self-Pollination– The transfer of pollen grains from the anther of one plant to the stigma of the same plant.
- Cross- Pollination– The transfer of pollen grains from the anther of one plant to the stigma of a different plant.
Self-pollination is also called as inbreeding whereas cross-pollination is known as outbreeding.
Reasons for Outbreeding
Continued inbreeding or self- fertilization leads to a condition called as inbreeding depression. This condition is characterized by homozygous genes which are not as vital as they need to be leading to unhealthy offsprings.
In self- pollination, since both the male and female gametes share the same genes, there is no genetic variation seen which is necessary for a better, more productive offspring. So, most plants have many mechanisms that they employ to avoid self -pollination and promote cross-pollination.
Plants have many mechanisms and devices that they employ to promote cross-pollination. Let us look at a few of them in the following segments:
- Unisexuality: In this case, the plant bears either male or female flowers and is not hermaphrodite. It is also called as Dioecism.
- Dichogamy: In this mechanism, the stigma and anther mature at different times. Depending on who matures first, dichogamy can be further divided into two:
- Protandry: In this type, the androecium matures earlier than the gynoecium. Ex: maize plant
- Protogyny: In this type, the gynoecium matures earlier than the androecium.
- Herkogamy: This is a name given to a condition where there is a natural physical barrier that prevents the pollen of the same flower from entering the ovary.
- Self- Sterility: In this condition, there is a gene that recognizes the similar gene and does not allow the pollen grain to germinate. This is due to the self-sterile gene present in the ovule and the grain.
- Heterostyly: In some plants like the oxalis, the stigma and the anthers are placed at different levels. This prevents the pollen from reaching the stigma and pollinating it.
- Pollen prepotency: In this mechanism, the pollen of a different flower germinates faster than of the same flower thus preventing autogamy.
Pollination described above is just the beginning or rather the first step of pollen-pistil interaction. The pistil is the female reproductive part of a flowering plant comprising of the ovary, style, and stigma. The pollen-pistil interaction begins with pollination, followed by pollen adhesion to the stigma. After it adheres, it imbibes water and gets hydrated which initiates pollen tube germination.
This pollen tube penetrates through the stigma and the tube grows through the style and reaches the ovary. Once it reaches the ovary, the tube penetrates it and reaches the micropyle of the ovule and enters into the embryo sac. Here, the two male nuclei fuse with the megaspore and the vegetative nucleus to form the diploid zygote and the triploid endosperm respectively. This fusion of the male and female gametes is known as fertilization.
Solved Example for You
Q1: Which is the word given for describing the condition where the stigma and stamens are at different levels to avoid self- pollination?
Sol. The correct answer is the option ”a”. The condition where the stamens and the stigma are placed at different levels to avoid inbreeding is called as heterostyly.