Reproduction is an important phenomenon for the progeny of any particular species. Plants reproduce by both asexual and sexual methods. Angiosperms or the flowering plants make use of the sexual methods of reproduction to reproduce. In most of the flowers, both male and the female gametes are present in the flowers. All the parts of a flower are involved, and each part has a unique role to play in the flowering plant reproduction. Sexually produced flowers are genetically different from their parents individually. In the asexual reproduction of plants, the fusion of gametes doesn’t take place and the offsprings are a look alike of the parent plants. In the flowering plants, although the parts are functional and active, some parts may even be sterile. To have a better understanding of the phenomenon of flowering plant reproduction, we must study the different parts of the plant involved in the process of reproduction.
There are four basic steps in the flowering plant reproduction. They are:
- Pollination
- Gametogenesis
- Fertilisation
- Embryo development
Angiosperms
Angiosperms, the highest in the classification of the Kingdom Plantae, have their seeds enclosed within the ovary which changes into a flower. Flowers of angiosperms have both female and male reproductive parts. This helps in the process of fertilisation and facilitates the production of seeds and flowers. A flower consists of two parts – essential and non-essential.
Looking at the flowering plant reproduction diagram, we can easily get an idea about the parts being essential or non-essential.
Here are the essential parts of the flowers –
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Androecium
The male reproductive organ houses the anther and filament. The anther and the filament collectively are called stamen. Filament supports the anther where the grains for pollination are produced. Pollen sacs are present in the stamen.
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Gynoecium
The female reproductive organ which includes stigma, style and ovary. The pollen enters pistil with the help of stigma. It reaches the egg and fertilises it in the ovary to form seed and flower.
The non-essential parts of the flowers which do not participate in the flowering plant reproduction process are –
Perianth, sepals and petals. They don’t involve themselves in the process directly but protect the reproductive organs of the flower.
Pollination
It is a process of deposition of pollen grains from the anther on the stigma region. Pollination is an important initial step in the flowering plant reproduction. Following this, we can easily understand how flowering plants reproduce. It is of two types –
· Self-pollination
It is the kind of pollination which takes place within the radius of the same plant. In such a pollination, the pollen is transferred to the stigma of the same flower of any flower in that same plant. It is possible only when both the male and the female reproductive parts are present together. Although the genetic material doesn’t get wasted and gets preserved, it often leads to the carrying forward of unnecessary and unwanted traits in the next generation of that plant. Example of autogamy is rose and example of geitonogamy is corn.
· Cross-pollination
In cross-pollination, the pollen is transferred from one plant to the stigma of the other plant. This occurs in the plants which are unisexual. Unisexual plants are the ones which have the female and the male reproductive organs in different plants. This process is also known as xenogamy. Cross-pollination occurs with the help of pollinators such as wind, insects, water, etc. For example, jasmine, calotropis, etc. Healthy progenies with variations are obtained. Depending on the pollinators, the rate of pollination varies. Adaptability is enhanced due to this. The crossing-over can cause non-fertile offspring and, thus, reduces the rate of reproducibility.
Gametogenesis
The process of production of haploid male gametes and diploid female gametes is called gametogenesis. They are the ones which participate in fertilisation and pollination. The female gametes are produced from the megaspore mother cell through a process called megasporogenesis. Gametogenesis which leads to the formation of male gametes occurs in two stages (in the case of angiosperms) –
Microsporogenesis and Microgametogenesis.
Gametogenesis which takes place to produce haploid female gametes occurs in two respective stages. –
Megasporogenesis and Megagametogenesis.
Inside the ovary, the ovules are present in multiple lobules. The megaspore mother cell is diploid and undergoes meiosis to form four megaspore which are haploid. Three of them disintegrate and only one is left in that ovule. This is known as megasporogenesis.
The megaspore nucleus divides to form eight nuclei mitotically.
Fertilisation
The formation of diploid zygote after the fusion of haploid male gamete and the diploid female gamete is called fertilisation. This gives rise to the embryo. In case of angiosperms, the fertilisation process takes place in the ovary. It gives rise to the seed and the fruit. Following the process of megagametogenesis, the generative cells form two male gametes. The egg fuses with the nuclei to form zygote and other sperm fertilises the polar nuclei to form the endosperm. This is known as double fertilisation. The polar nuclei are fertilised by another sperm following a process of triple fusion. The ovule forms seed and integuments form the seed coat.
Embryo development.
By looking at the flowering plant reproduction diagram, one can get a clear image of each and every step that takes place during the flowering plant reproduction. The development of the zygote which is formed by the fusion of the male and the female gametes into an embryo is called embryogenesis. The embryo then grows up and matures. It grows the respective parts which a plant has.
Conclusion
Reproduction in plants is a vital process as it keeps the balance of the ecosystem stable and in equilibrium. The zygote formed grows into an embryo. An Endosperm is a result of triple fusion in the embryo sac of the ovule. The ovules which are matured change into seeds.