SEXUAL REPRODUCTION IN FLOWERING PLANTS
Female gametophyte:
- Out of four megaspores, one megaspore is functional and other three degenerates.
- The functional megaspore developed into the female gametophyte.
- Female gametophyte is known as the embryo sac.
- Development of embryo sac from a single megaspore is called as monosporic type of embryo sac.
- The nucleus of the functional megaspore divided by mitotic division to form two nuclei which move to the opposite pole, 2-nucleated embryo sac.
- Two successive mitotic division leads to formation of 4-nucleate and later 8-nucleate stages of the embryo sac.
- All mitotic divisions are free nuclear type; karyokinesis is not followed by cytokinesis.
- Six of the eight nuclei are surrounded by cell walls and organized into cells.
- Three cells are grouped together at the micropylar end, constitute the egg apparatus.
- The egg apparatus, in turn consists of two synergids and one egg cell.
- Synergids have special filiform apparatus, which play an important role in guiding the entry of pollen tube into the synergids.
- Three cells arranged towards chalazal end are called antipodal cells.
- The large central cell has two polar nuclei.
- A typical angiosperm embryo sac at maturity is 8- nucleated and 7-celled.
Pollination:
- Transfer of pollen grains from the anther to the stigma of a pistil is termed as pollination.
- Both male and female gametes are non-motile.
Kinds of pollination:
Autogamy:
- Pollination within same flower.
- In open and exposed anthers and stigma autogamy is rare.
- Viola, Oxalis and Commelina produce two types of flowers:
- Chasmogamous: exposed anther and stigma
- Cleistogamous: closed anther and stigma.
- Cleistogamous flower is invariably autogamous and assured seed set even in the absence of the pollinator.
Geitonogamy:
- Pollination between two flowers of the same plant.
- Pollination by pollinating agent.
- Genetically similar to the autogamy.
Xenogamy:
- Transfer of pollen grains from the anther to the stigma of different plant.
- It is commonly called as cross-pollination.
- It brings genetically different types of pollen grains to the stigma.
Agents of pollination:
- Plant use two abiotic agent i.e. wind and water for pollination.
- One biotic agent for pollination such as animals.
- Majority of plant use biotic agent for pollination.
- Few plant use abiotic pollinating agent.
Anemophily:
- Pollinating agent is wind.
- Plants produces enormous amount of pollen when compared to the number of ovules available for pollination to compensate the uncertainties of pollination.
- Flowers with well exposed stamens.
- Large feathery stigma to trap air-borne pollen grains.
- Most wind pollinated flower contains single ovule in one ovary and numerous flower packed into an inflorescence e.g. corn cob.
- Pollen grains are light and non-sticky.
Hydrophily:
- Pollination by abiotic agent like water.
- This type of pollination is very rare, about 30 genera, mostly monocot.
- Vallisneria, Hydrilla and Zostera are the common example for Hydrophily.
- All aquatic plants are not Hydrophily.
- Pollen grains released into the surface of water and carried to the stigma by air current as in Vallisneria.
- In sea grass the flowers remained submerged.
- Pollen grains are long, ribbon like and carried passively inside the water
- Pollen grains are protected from wetting by mucilaginous covering.
Pollination by biotic agent:
- Majority of flowering plants use a range of animals as pollinating agents.
- Among the animal, insect particularly bees are the dominant biotic agents for pollination.
- Insect pollinating flowers are very large, colorful, fragrant and rich in nectar.
- Small flowers present in cluster to make them conspicuous.
- Flower pollinated by flies and beetles secrete foul odours.
- Nectar and pollen grains are the usual floral rewards for insects.
- In some species floral rewards are in providing safe places to lay eggs: e.g. Amorphophallus.
- A species of moth and Yucca plant cannot complete their life cycle without each other. The moth deposits its eggs in the locule of the ovary and the flower in turn get pollinated by the moth.
- Many insects may consume pollen or nectar without bring about pollination. Such floral visitors are referred as pollen/nectar robbers.
