Evolution is one of the Biology Topics that has been debated and studied for centuries, exploring the process by which species change over time.
Sexual Reproduction in Flowering Plants: Introduction, Types, Steps and Examples
Sexual reproduction takes place through the combination of special reproductive cells called ‘sex cells’. Sex cells are of two types male sex cells and female sex cells (which come from two different parents: a male and a female). The sex cells are commonly known as gametes. Thus, the cells involved in sexual reproduction are called gametes. Gametes are of two types: male gametes and female gametes. In sexual reproduction, a male gamete fuses with a female gamete to form a new cell called a ‘zygote’. This zygote then grows and develops into a new organism in due course of time. Please note that the sex cells or gametes are also sometimes called germ cells. And there are two types of germ cells: male germ cells and female germ cells. In this chapter, we will be mostly using the term ‘gamete’. The students are, however, free to use the term ‘germ cell’ (in place of gamete).
The plants in which the sex organs are carried within the flowers and the seeds enclosed in fruit are called angiosperms. Angiosperms are commonly known as flowering plants. The flowering plants reproduce by the ‘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 (see Figure). 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 fertilization will take place to make new seeds for the reproduction of plants (see Figures). 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.
Flowers contain the sexual reproductive organs of a plant (This flower has been cut open to show the sexual reproductive organs inside). Flowers make fruits that contain seeds. These seeds can produce new plants.
Apples, oranges, lemons, and tomatoes, etc., are all fruits. They contain the seeds of their plants inside them. These seeds can be sown in the soil to grow more plants.
Pea pods are also fruits. The peas inside them are seeds of the pea plant. These peas (or seeds) can be sown in the soil to grow new pea plants.
Sexual reproduction in plants takes place in the following steps:
- The male organ of the flower called ‘stamen’ makes the male gametes (male sex cells) of the plant. These male gametes are present in pollen grains.
- 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’.
- The male gametes present in pollen grains fertilize the female gametes or egg cells present in ovules.
- The fertilized egg cells grow within ovules and become seeds.
- The seeds produce new plants on germination (under suitable conditions of water, warmth, air, 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 are shown in Figure.
The diagram to shows the Parts of a flower.
The base of a flower to which all the parts of a flower are attached is called a receptacle (see Figure).
The green, leaf-like parts in the outermost circle of a flower are called sepals (see Figure 43). 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 a7bud.
The colourful parts of a flower are called petals (see Figure). 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.
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 (see Figure). Stamen produces pollen grains. The stamen is made of two parts: a filament and an anther (see Figure). The stalk of the stamen is called filament and the swollen top of the stamen is called anther. It is actually the anther of a stamen that 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 male gametes of a plant are made in the anther of the stamen. Another point to be noted is that a flower usually has a number of stamens in it.
Stamen: Male Reproductive Organ of a Plant
In the centre of a flower, there is a flask-shaped organ called a carpel. Carpel is the female reproductive organ of the plant (see Figure). A carpel is made of three parts: stigma, style, and ovary (see Figure). The top part of the carpel is called the 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 the carpel is called style. Style is a tube that connects the stigma to the ovary. The swollen part at the bottom of a carpel is called the ovary (see Figure).
Carpel: Female reproductive organ of a plant (Carpel is also called Pistil).
The ovary makes ovules and stores them. Ovules contain the female gametes (or female sex cells) of the plant. There are usually many ovules in the ovary (but we have shown only one ovule in the ovary in Figure). 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 the 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 the carpel is surrounded by a number of male organs called stamens in the flower (see Figure).
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.
This picture shows the carpel of a tulip flower surrounded by many brown stamens.
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 fertilization.
1. Plant Pollination Process: Reproduction in Plants
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 the 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 [see Figure (a)].
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 [see Figure (b)]. 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 (see Figure). 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 (see Figure).
Insects (such as this bee) which sit on different flowers to suck nectar, help in pollination .by transferring pollen from the anther of one flower to the stigma of another flower.
The male flowers in this picture are shedding dust of their pollen into the air. These pollens are then carried away by the wind to the stigma of another flower for pollination.
2. Fertilization Mechanisms in Flowering Plants
After a pollen grain falls on the stigma, the next step is fertilization. Fertilization occurs when the male gamete present in the pollen grain joins with the female gamete (or egg) present in the 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 [see Figure].
Fertilisation in a flower.
A male gamete moves down the pollen tube. The pollen tube enters the ovule in the ovary [see Figure (b)]. The tip of the pollen tube bursts open and the male gamete comes out of the pollen tube. In the ovary, the male gamete of pollen combines with the nucleus of the female gamete or egg present in the ovule to form a fertilised egg (called a zygote) [see Figure (b)]. And we say that fertilisation has taken place.
Formation of Fruits and Seeds
The fertilised egg (or zygote) divides several times to form an embryo within the ovule. The ovule develops a tough coat around it and is gradually converted into a seed (containing the baby plant). In fact, all the eggs in the ovules present in the ovary of a flower get fertilised by male gametes from pollen grains and grow to become seeds. The ovary of the flower develops and becomes a fruit (with seeds inside it). The other parts of flowers like sepals, petals, stamens, stigma, and style dry up and fall off. Only the ovary is left behind. So, at the place on the plant where we had a flower originally, we now have a fruit (which is the ovary of the flower containing seeds). A fruit protects the seeds (see Figure). Some fruits are soft, sweet, and juicy like mangoes and oranges. But some fruits are hard, dry, and woody like peanuts and almonds, etc.
A fruit contains seeds of the plant inside it.
A seed is the reproductive unit of a plant (which can be used to grow a new plant). The seed contains a baby plant (or embryo) and food for the baby plant (see Figure).
Parts of a seed.
The part of the baby plant in the seed which develops into the shoot with leaves is called plumule and the part which develops into root is called radicle. The part of the seed which contains stored food for the baby plant is called a cotyledon. The wheat grains, gram (ichana), corn, peas, and beans, are all seeds (see Figure). The baby plant present inside a seed is in an inactive state (called a dormant state). When the seed gets suitable conditions like water, air, warmth, etc., it germinates and a new plant grows out of the seed. In this way, the parent plant reproduces more plants like itself by forming seeds through flowers.
These pictures show seeds of some common food crop plants. All these seeds can be used to grow new crop plants under suitable conditions.
Germination of Seeds
The seeds obtained from a plant are usually very dry. In this dry state, the seeds can remain alive but inactive for long periods. They are said to be dormant. When a seed gets water, air, warmth, etc., it begins to grow. When a seed begins to grow, it is said to germinate. Thus, the beginning of the growth of seeds is called the germination of seeds. Germination begins when the seed absorbs water, swells, and bursts through the seed coat. The water helps the enzymes to function in the seed. The enzymes digest the stored food in cotyledons and make it soluble. This soluble food makes the radicle and plumule present in the seed grow. The germination of a bean seed is shown in Figure.
The radicle of the seed grows first to form the root [see Figure (a)]. The root pushes down into the soil and begins to absorb water and minerals from the soil. After this, the plumule grows upwards to form the shoot [see Figure (b)], The shoot and root grow further [see Figure (c)], When the shoot comes up above the ground, it develops green leaves [see Figure (d)], The green leaves of the shoot begin to synthesise own food in the presence of sunlight. This seedling grows gradually and ultimately becomes a new plant.
Seeds germinate under suitable conditions to produce new plants. These pictures show the germination of a bean seed to form a new bean plant.