Flowers are the reproductive structures of flowering plants. They help plants produce new plants through sexual reproduction. The process involves the formation of male and female gametes, pollination, fertilisation, and the development of seeds and fruits.
Reproductive Parts of a Flower
The flower contains two reproductive organs: the stamen, which represents the male reproductive part, and the pistil or carpel, the female reproductive part.
The stamen consists of the filament and anther, while the pistil consists of the stigma, style, and ovary. The anther produces pollen grains, and the ovary contains the ovules.
Formation of Male and Female Gametophytes
The process begins with the formation of the male and female gametophytes.
Inside the anther, specialised cells called pollen mother cells undergo meiosis to produce microspores. The structure of the microsporangium provides the site for pollen development, while the formation of microspores from pollen mother cells is called microsporogenesis.
A similar process occurs during the formation of the female gametophyte inside the ovule. The structure of the anatropous ovule provides the site for female gametophyte development. A megaspore mother cell undergoes meiosis and produces four megaspores. Usually, one megaspore remains functional and develops into the embryo sac. This process is called megasporogenesis.
Thus, at this stage:
Male pathway
Inside the anther, pollen mother cells undergo meiosis to produce microspores.
Female pathway
Inside the ovule, a megaspore mother cell undergoes meiosis; usually one megaspore remains functional and develops into the embryo sac.
Pollination: Transfer of Pollen
For sexual reproduction to continue, pollen grains must reach the stigma. The transfer of pollen grains from the anther to the stigma is called pollination. Pollination can occur within the same flower, between flowers of the same plant, or between flowers of different plants of the same species. These are known as different types of pollination.
Pollination may occur with the help of wind, water, insects, birds, and other animals. Flowering plants also possess various mechanisms that promote cross-pollination and prevent self-pollination.
Wind
One of the agents that may help move pollen.
Water
Can also act as a pollination agent.
Insects
Animals such as insects may carry pollen between flowers.
Birds & animals
Other animals may also assist pollination.
Fertilisation: The Two Fusion Events
When the pollen tube reaches the embryo sac, it releases two male gametes. Each gamete participates in a separate fusion event.
The first male gamete fuses with the egg cell to form a zygote. This fusion is called syngamy.
The second male gamete fuses with the two polar nuclei, or with the secondary nucleus formed by their fusion. This is called triple fusion and results in the formation of the primary endosperm nucleus. Since syngamy and triple fusion occur within the same embryo sac, the process is known as double fertilisation. This is a characteristic feature of flowering plants.
Changes After Fertilisation
The post-fertilisation events cause significant changes to the structures of the flower.
The zygote develops into an embryo, while the primary endosperm nucleus undergoes divisions and develops into the endosperm, which provides nourishment to the developing embryo.
The ovule develops into a seed, and the ovary develops into a fruit. The seed contains the embryo and helps protect it until conditions are suitable for germination. Thus, the sequence from gametophyte formation to seed and fruit development completes the sexual reproductive cycle in flowering plants.