Pollination in flower is a mechanism of transfer of pollen grains from the male reproductory part, i.e. anther to the stigma of the female reproductive part, i.e. pistil. It is a process before fertilization of flower. Passage of pollen grains from an anther to a stigma occurs via pollinating agents or pollinators. A pollinator can be the biotic or abiotic agents that bring out the process of pollination.
Once the pollen grains stick to the platform of stigma, it starts germinating. On germination of a pollen grain, a pollen tube forms from which it penetrates to the embryo sac. Once the pollen grain reaches the embryo sac, it fuses with the female gametes to carry out fertilization.
Content: Pollination in Flower
- Definition of Pollination
- Types of Pollination in Flower
- Pollination Process Steps
- Agents of Pollination
Definition of Pollination
Pollination can define as the process before the fertilization of flower, which involves the shedding of pollen grains from an anther to the stigma of the pistil. Pollination can occur within the plant of the same flower, different species of the same flower or the different flowers.
It is carried out via specific vectors refers to as “Pollinators”. Generally, the gametes of a flower are non-motile, that’s why a flower needs a pollinator to proceed with the growth process following pollination and fertilization. Pollination can occur by the means of two methods, namely self and cross-pollination.
Types of Pollination in Flower
There are two methods of pollination, usually found in flower:
Self Pollination in Flower
In this, pollination occurs within the same flower or the different species of the same flower. It occurs when the same flower contains both the male and female sexual parts. Self-pollination usually occurs in hermaphrodite or dioecious plants, where a flower can pollinate by itself or less dependent on the external factors.
It can occur in the given two ways:
Autogamy: It is a type of self-pollination, where pollination occurs within the same flower. It is sporadic, but to happen, it requires the anther and stamen to lie close to each other. During autogamy, there must be synchronisation between the pollen release from an anther and stigma receptivity.
Examples: Chasmogamous and Cleistogamous flowers.
Geitonogamy: It is the condition where pollination occurs between the different flowers of the same plant. Geitonogamy is considered as a type of self-pollination because genetically pollination occurs in the same flower. However, this method requires a vector or external agents.
Advantages of Self-pollination:
- Self-pollination gives rise to genetically similar genes in flower.
- Eliminates recessive characters from a plant.
- It does not strongly depend upon the pollinating agents.
- Self-pollination reduces the loss of pollen grains.
Disadvantages of Self-pollination:
- Self-pollination does not result in the evolution of new characters.
- A plant is less resistant against environmental stress and diseases.
Cross Pollination in Flower
In this, pollination occurs within the two different flowers of different origin, i.e. pollen of one flower disperses to the stigma of another flower. This pollination gives rise to genetically different flowers. Cross-pollination always requires a vector or an agent to carry out the pollination. It also refers as “Xenogamy”.
Advantages of Cross-pollination:
- Cross-pollination gives rise to the evolution of new characters to a flower.
- Provides resistance to the plant against environmental stress and diseases.
- Helps in the reproduction of a unisexual plant.
Disadvantages of Cross-pollination:
- Cross-pollination results in a loss of pollen grains.
- There may some chances that unwanted characteristics add to a flower.
- Strongly depends upon the external factors like wind, water and insects for the pollination.
Pollination Process Steps
The process of pollination includes the following steps:
- Shedding of pollen grains onto the platform of stigma.
- On favourable conditions, pollen grains germinate and forms a germ tube refers as “Pollen tube”.
- The pollen tube then moves downwards from the stigma to the pistil, along with the sperm cells.
- The pollen tube comes in contact with the wall of embryo sac and penetrates it.
- Finally, the sperm cell fuses with the egg cells to bring out the fertilization process.
Agents of Pollination
As we have discussed earlier that pollination can achieve by two agents namely
- Abiotic agent
- Biotic agent
The abiotic agent includes wind and water, whereas biotic agent includes insect vectors to carry out the pollination process.
There are certain conditions for the wind pollination to occur:
- The pollen grains should be light and non-sticky.
- An anther should be exposed outside so that the dispersal of pollen grains by the wind currents become easy.
- Stigma is usually feathery, that traps the airborne pollen grains.
Wind pollination gives rise to a flower with a single ovule in each ovary. A wind pollinated flower lacks nectar and produce pollen in large quantities. The pollen transfer by an insect vector refers as “Anemophilous pollen”. A flower pollinated by wind current has a dangled out anther and stigma. It commonly occurs in grasses.
Water pollination occurs very rarely. It is limited only in the 30 genera of monocotyledons. It occurs mostly in the aquatic plants like Vallisneria, Hydrilla and Zostera etc. In water pollination, the male anther sheds its pollen grains on the water surface. Some of the pollen grains reach the female’s stigma by the passive water current.
In seagrasses, both the male gametes (pollen grain) and the female flower remains inside the water. Thus, the pollen grain reaches to the stigma of the flower in the form of long, ribbon-like pollen grains by the passive water current.
A water pollinated flower are:
- The flowers are usually dull in colour.
- Water pollinated flower consists of pollen grains that are protected by the mucilaginous covering.
- A flower lacks nectar.
It is the most common method of pollination, where a majority of flowers uses a range of insects like bees, butterflies, and beetles etc. as vectors. Among insect vectors, bee dominates over the other insect vectors. The pollen transfer by an insect vector refers as “Entomophilous pollen”.
Insect pollination gives rise to a flower with the following properties like:
- Insect pollinated flower is large and colourful.
- Produces a sugary fluid or nectar that attracts the insects.
- The pollens and stigma forms as a result of insect pollination are sticky.
- The anther and the stigma of the flower are firmly attached to the thalamus.
In insect pollination, the insect comes in contact with the sticky pollen grains in search for nectar. The pollen grains stick to the insect, and when it sits on another flower, it rubs the pollen grains to the sticky stigma.
There, through these three agents, viz wind, water and insects, a process of pollination is achieved.