Reproduction is one of the most important event help in the survival and growth of individuals. Each organism vary in terms of the growth, the method they reproduce and the lifecycle they manage. The initial phase of all the organisms is termed as the growth phase or the juvenile phase (In plants it is called vegetative phase). Individuals grow physically, physiologically and psychologically during the juvenile phase. The next stage is the beginning of the reproductive phase. During this phase, many changes occur within the plant. On the other hand, the age at which organisms reproduce and the longevity differ. For example, a rice plant lives for a few months whereas a banyan tree lives very long making both of them very unique in many interests.
Prerequisites for sexual reproduction
Sexual reproduction is the most common mode of reproduction in the majority of the eukaryotes. Sexual reproduction requires certain conditions to be met, such as age, sexual maturation, hormonal levels and health status of the 2 individual parents. But there is no wonder, even plants can reproduce sexually. This section will discuss the process involved through various phases of sexual maturity and sexual reproduction in flowering plants. This article is intended to explain the phases of reproduction in a flowering plant.
Sexual reproduction in flowering plants
Sexual reproduction is the process of producing one or more identical or non-identical offsprings as a result of the sexual contact between the male and female parts of the flowering plant. Most of the plant population reproduce through asexual reproduction, but flowering plants undergo sexual reproduction. Irrespective of a plant or animal, sexual reproduction produces a diverse range of features as a result of genetic variation occurring at the cell level at the time of the exchange of gametes. In a flowering plant, sexual reproduction can happen in 2 scenarios. The first scenario is that an individual flower has both male and female parts that come together to form a fused product. The second scenario is when a plant has either male or female; where 2 plants should come in contact for reproduction to occur. In order to understand the sexual reproduction in a plant, one should understand the parts of the flower.
Parts of a flower assisting in Plant Reproduction
In the majority of the plants, both male and female reproductive organs are present in an individual; they are known as bisexual flowers. Such flowers make male and female gametes fuse easy and it is due to the direct contact between the male and female flowering parts. If the flower is mono sexual(having either male or female parts) chances of reproduction is not by direct contact, but vectors like honey bee are used as a media. The stamen is the male reproductive part and pistil is called a female reproductive part in a flower. Below is the description of parts of a flowering plant in detail.
Parts of a flower
Flowers can be unisexual or bisexual. A typical bisexual flower has the male and female parts within it. Some additional parts which support reproduction are the calyx, Corolla, Androecium, and Gynoecium, which are explained below. Calyx is thecollection of sepals. Sepals are the first layer from the base of the flower. They usually appear in green colour but rarely sepals can have different colours. The coloured sepals are called petaloid help in protecting the flower during the budding stage. Corolla is an orderly arranged collection of petals placed just above the sepals. The petals are the most colourful part of a flower aids in attracting the insects and birds to the flower so that pollination becomes successful. Androecium is the third layer of the flower situated just above the Corolla. Corolla is used to indicate the male parts of the sexual reproduction of a plant. The androecium has a bunch of stamens with each stamen contains anther and filament. Anther is at the tip of the filament, inside are the Pollen grains and the filament- is a stalk that will hold the anther straight. Gynoecium is the 4th layer having a collection of carpels. It again has three parts namely stigma, style, and ovary. Stigma is a small and sticky landing spot on which pollen grains stick. Style is a thin stalk-like structure assist in holding the stigma and Ovary. At the base of the style, many ovules are present. Ovules contain the female gametes.
Mechanism of reproduction in a flowering plant
As mentioned above, the flower can be unisexual or monosexual (papaya, watermelon) if it contains only stamens or only carpels. If a single flower has both stamen and carpels, it is a bisexual flower (Hibiscus, mustard). Stamen produces yellowish pollen grains ( it is a male part). If you just touch a flower, you will see that some yellowish materials stuck to the hand called pollen. The carpel- female is present in the middle of the flower. A carpel has bottom part -ovary, the middle elongated portion- style and the last part - a sticky stigma. The ovary contains ovules - egg cells found in each individual ovule. The male germ-cell produced in the pollen grain comes in contact with the ovule resulting in the fusion of the germ-cells called fertilization. In order to for the fusion of male and female parts, the pollen must be transferred from the stamen to the stigma. If the transfer happens within the same flower, it is a self-pollination and if it happens between 2 different flowers, it is a cross-pollination. Pollen transfer can happen through various vehicles such as wind, rain, insects, birds, animals, etc. Once the pollen lands on a suitable stigma, it should reach the female germ-cells which are in the ovary. To facilitate this transfer, a small tube-like structure is developed out of the pollen grain and travels through the style of the flower to reach the ovary. After the fusion (fertilization), the zygote multiplies several times to form an embryo within the ovule. In order to protect the embryo, the ovule develops a tough covering layer which later become the seed. The ovary grows rapidly and ripens to form a fruit. Meanwhile, the petals, sepals, stamens, style, and stigma may shrivel and fall off. The seed contains the future plant or embryo which develops into a seedling under appropriate conditions. The formation of the seed is known as the germination.
Stages of sexual reproduction in plants
1. Pollination: It is a natural or artificial process in which the pollen grains will be transferred to the stigma of the same flower or to the stigma of different flowers. There are two types of pollination-, namely, self-pollination (this occurs within a flower) and a cross-pollination (occurs between different flowers). Pollination takes place in plants with the help of wind, water, and insects like honey bee (see the picture above). The fusion of male and female gametes is called fertilization.
2. Zygote formation: After the transfer of pollen grains, the male gamete is transferred down through the style of the pistil to the ovary where the male gamete is fused with the female gamete to form a zygote.
3. Fruit and seed formation; after the fusion of male and female gametes (fertilization), the ovary gets ripened and formed into the fruit. The seeds develop from the ovules of the ovary. Each seed contains an embryo enclosed in a protective covering around the seed. The fruits can be soft like mango, orange, or they can be hard nuts like almond and walnuts. Below is the picture showing the complete process of reproduction in a plant.
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