The plants in which the sex organs are carried within the flowers and the seeds are enclosed in a fruit are called angiosperms. Angiosperms are commonly known as flowering plants.
The flowering plants reproduce by ‘sexual reproduction’ method. This means that two sexes (male and female) are involved in reproduction in flowering plants. Like human beings, plants have also male and female sex organs, though they are different in form from humans. The sex organs (or reproductive organs) of a plant are in its flowers. In other words, flowers contain the sexual reproductive organs of a plant.
In most of the plants, the same flower contains the male organ as well as the female organ. In other words, the majority of plants are bisexual having the male and female reproductive organs in the same plant (or same flower). In fact, the reproductive part of higher plants is the flower.
The function of a flower is to make male and female gametes and to ensure that fertilisation will take place to make new seeds for the reproduction of plant. Sexual reproduction is the most common method of reproduction in flowering plants. From all this discussion we conclude that flowers are for sexual reproduction in plants. A flower makes both male and female gametes needed for sexual reproduction in plants.
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The sexual reproduction in plants takes place in the following steps:
1. The male organ of flower called ‘stamen’ makes the male gametes (male sex cells) of the plant. These male gametes are present in pollen grains.
2. The female organ of a flower called ‘carpel’ makes the female gametes (female sex cells) of the plant. These female gametes are present in ovules. The female gametes present in ovules are also called ‘ova’, ‘egg cells’ or just ‘eggs’.
3. The male gametes present in pollen grains fertilise the female gametes or egg cells present in ovules.
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4. The fertilised egg cells grow within ovules and become seeds.
5. The seeds produce new plants on germination (under suitable conditions of water, warmth, air and light, etc.).
We will now describe the various parts of a flower including the sexual reproductive organs. And then we will discuss the sexual reproduction in plants in detail.
The main parts of a flower are: Receptacle, Sepals, Petals, Stamen and Carpel. These main parts of a flower.
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1. Receptacle:
The base of a flower to which all the parts of a flower are attached is called receptacle.
2. Sepals:
The green, leaf-like parts in the outermost circle of a flower are called sepals. All the sepals taken together are called ‘calyx’. The function of sepals (or calyx) is to protect the flower in its initial stages when it is in the form of a bud.
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3. Petals:
The colourful parts of a flower are called petals. The petals lie inside the sepals. All the petals taken together are called ‘corolla’. The petals are usually scented. The function of petals (or corolla) is to attract insects (for pollination and to protect the reproductive organs which are at the centre of the flower.
4. Stamen:
The little stalks with swollen tops just inside the ring of petals in a flower are called stamens. Stamen is the male reproductive organ of the plant. Stamen produces pollen grains. The stamen is made of two parts a filament and an anther. The stalk of stamen is called filament and the swollen top of stamen is called anther.
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It is actually the anther of a stamen which makes the pollen grains and stores them (The pollen grains appear to be yellow, powder-like substance to us). Pollen grains contain the male gametes (or male sex cells) of the plant. It is clear from this discussion that the ma] gametes of a plant are made in the anther of stamen. Another point to be noted is that a flower usual has a number of stamens in it.
5. Carpel:
In the centre of a flower, there is a flask-shaped organ called carpel. Carpel is the female reproductive organ of the plant. A carpel is made of three parts: stigma, style and ovary. The top part of carpel is called stigma. Stigma is for receiving the pollen grains from the anther of stamen (during pollination).
Stigma is sticky so that pollen can stick to it. The middle part of carpel is called style. Style is a tube which connects stigma to the ovary. The swollen part at the bottom of a carpel is called ovary. The ovary makes ovules and stores them. Ovules contain the female gametes (or female sex cells) of the plant.
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There are usually many ovules in the ovary (but we have shown only one ovule in the ovary. Each ovule contains only one female gamete of the plant. The female gamete (or female sex cell) of the plant which is present inside the ovule is called ‘ovum’ or ‘egg’. It is clear from this discussion that the female gametes of a plant are made in the ovary of carpel. Please note that the female organ of a plant is known by two names: carpel and pistil. Another point to be noted is that the female organ called carpel is surrounded by a number of male organs called stamens in the flower.
The flowers which contain only one sex organ, either stamens or carpels, are called unisexual flowers. The flowers of papaya and watermelon plants are unisexual flowers. On the other hand, the flowers which contain both the sex organs, stamens as well as carpel, are called bisexual flowers. The flowers of Hibiscus and mustard plants are bisexual flowers.
A new seed of the plant is formed when the male gamete present in a pollen grain unites with the female gamete present in the ovule. This happens in two steps : pollination and fertilisation.
For the male gamete to be able to combine with the female gamete, it is necessary that first the pollen grains from the anther of stamen should be carried to the stigma of carpel. The transfer of pollen grains from the anther of a stamen to the stigma of a carpel is called pollination. Thus, pollination is said to take place when pollen grains are carried from the anther to the stigma of the flower.
Pollination is done by insects (like bees and butterflies), birds, wind, and water. Pollination can occur in two ways: self-pollination and cross-pollination. When the pollen grains from the anther of a flower are transferred to the stigma of the same flower (or another flower on the same plant), it is called self-pollination.
When the pollen grains from the anther of a flower on one plant are transferred to the stigma of a flower on another similar plant, it is called cross-pollination. Insects help in cross-pollination as follows:
When an insect sits on the flower of a plant for sucking nectar, then the pollen grains from the anther of this flower stick to its body. And when this insect now sits on another flower of another similar plant, then the pollen grains sticking to its body are transferred to the stigma of this second flower.
In this way the insect transfers the pollen grains from the anther of flower in one plant to the stigma of flower in another plant and causes cross-pollination. The blowing wind also carries pollen grains from one flower to another flower and helps in cross pollination.
After a pollen grain falls on the stigma, the next step is fertilisation. Fertilisation occurs when the male gamete present in pollen grain joins with the female gamete (of egg) present in ovule. This happens as follows. When a pollen grain falls on the stigma of the carpel, it bursts open and grows a pollen tube downwards through the style towards the female gamete in the ovary.
A male gamete moves down the pollen tube. The pollen tube enters the ovule in the ovary. The tip of pollen tube bursts open and male gamete comes out of pollen tube. In ovary, the male gamete of pollen combines with the nucleus of female gamete or egg present in ovule to form a fertilised egg (called zygote). And we say that fertilisation has taken place.