What do we observe first in a plant? Flowers, right? They are bright, colourful and so beautiful. We use them for so many occasions. But did you know that they are not just pretty things to look at? They are responsible for Sexual Reproduction. Yes, you heard right. This means that they are ‘mothers’. Sounds interesting, right? So let’s learn about how flowers reproduce sexually.
The type of reproduction in which the fusion of male and female gamete occurs is known as sexual reproduction. Flowers produce male and female gametes in the reproductive organs.
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Comparison of Asexual and Sexual Reproduction
|In asexual reproduction, only one parent is necessary.||In sexual reproduction, two parents are necessary.|
|The process of gamete formation and fertilization does not occur.||The process of gamete formation and fertilization occurs.|
|Offsprings are exact copies of their parents.||Offsprings are not the exact copies of parents.|
|It generally occurs in lower organisms.||It generally occurs in higher organisms.|
Reproductive Part of a Plant
Flowers are the reproductive parts of a plant. Male and female gametes are produced in the flower.
The flower consists of four main parts-
- Sepals: They are the green leaf-like structures which protect the flower in the bud stage.
- Petals: They are the coloured structures. They are the most beautiful part of the flower that attracts the birds and insects for pollination
- Stamens: They are the male reproductive parts. Stamens consist of two parts – anther and filament. Anther is a swollen part of the stamen. The filament is a long, slender stalk which attaches the stamen to the flower. Pollen grains are formed inside the anther which produces the male gametes.
- Pistil or Carpel: It is the female reproductive part of the flower. It consists of three parts – stigma, style and the ovary. The stigma is at the top of the pistil. It receives the pollen grains during pollination. The style is a long, tube-like structure which passes the pollen grains to the ovary. The ovary is the swollen part present at the basal part of the pistil. The ovary consists of the ovule which bears the female gamete.
Uni-sexual and Bisexual Flowers
Flowers can be of two types depending upon whether both the sexual parts – male and female are present in the same flower or different flower.
- Bisexual flowers: Flowers which contain both the stamens and the pistil are known as bisexual flowers. For example- Rose, Hibiscus, Gulmohar, Mustard etc.
- Uni-sexual flowers: The flowers which contain either the stamens or the pistil are known as unisexual flowers. For example- Maize, Papaya, Cucumber etc.
Transfer of pollen grains from the anthers to the receptive stigma is known as pollination. Pollination is of two types:
- Self-pollination: In this process pollen grains from the anthers pollinate the stigma of the same flower.
- Cross-pollination: In this case pollen grains of one flower fall on a different flower of either the same plant or another plant.
Agents of Pollination
Pollination occurs through some external agents like,
The pollen grains which are very light in weight can be passed on by wind from one flower to another or to the stigma of the same flower. This is known as wind pollination. Examples of wind pollination are pollination in wheat, rice, maize etc. Wind-pollinated flowers have the following characteristics:
- Wind-pollinated flowers are mostly small and not showy.
- They produce lightweight pollens in large number and do not produce nectar.
- The stigma is sticky.
When an insect sits on the flowers to collect the nectar, the pollen grains stick on their legs and wings. When they travel to another flower, they carry the pollens with them. In this way, they carry out pollination. Examples are a sweet pea, orchids, sunflower, buttercup. Characteristics of insect-pollinated flowers:
- These flowers have bright colours, are large and showy.
- They have nectaries, sticky stigma and sticky pollen grains.
- Some birds like Sunbird and hummingbirds and mammals like Squirrel also help in pollination.
Aquatic plants carry out water pollination. In this, the pollen grains are released in water passively and are carried out by water currents to the other flowers for pollination. Examples of water-pollination are seagrass, hydrilla etc.
Fertilization is the fusion of male and female gametes. It results in the formation of single-celled zygote.
Germination of Pollen Grain
The pollen grain develops pollen tube through the style and reaches the ovary. It enters the ovule from a small opening. Pollen tube carries the male gamete. Male gamete fuses with the female gamete to form the zygote.
Following changes take place in the flower after fertilization. The fertilized zygote grows into an embryo. The embryo has two parts – plumule which grows into shoot and radical which grows into the root.
- The ovary grows into fruit.
- The other parts of the flower fall off.
- The ovule develops into a seed.
Structure of Seed
The seed contains the embryo covered by the seed coat. Cotyledon present in the seed helps in the nourishment of the developing baby plant on germination of the seed.
Learn more about Vegetative Propagation here.
Questions For You
Q1: Which of the following statements is/are true for sexual reproduction in plants?
- Plants are obtained from seeds
- Two plants are always essential
- Fertilization can occur only after pollination
- Only insects are agents of pollination
Solution: Statements (a) and (c) are true.
Q2. In the diagram given below, label the parts marked (a), (b) and (c).
- Pollen grain
- Pollen tube passing through the stigma