Grade 7, Science Olympiad (CBSE) - Sexual reproduction 

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Grade 7  |   Science  |   Sexual reproduction, Reproduction in plants, Olympiad, CBSE, ICSE, SOF, ITO

Sexual reproduction

Plants may reproduce either sexually or sexually. In the sexual mode of reproduction, two parents are involved. Here, the two individuals produce male and female gametes which on fusion forms a single cell zygote and this process is called fertilization

This zygote by further division forms an embryo and then develops into an individual organism. Plants have a sexually reproductive part which we commonly called a flower. 

There are two types of flowers-

1. Bisexual flowers- such flowers contain both the state and the pistil. For example- Rose and Mustard.

2. Unisexual flowers such flowers either have stamens or the pistol within them. For example- maize, watermelon, cucumber, etc.

Flower

It is a special organ of a plant which consists of different parts. 

1. Calyx- It is consists of sepals( green colored structures) which is the outermost whorl. 

2. Corolla- It is the second whorl which consists of colorful petals to attract animals and insects towards them for pollination. 

3. Androecium- It is the third whorl which consists of male reproductive part -Stamen.

Stamens have two part - anther and filament. The anther is a capsule which has the pollens ready for pollination. The filament is the long tube which attaches the anther to the base of the flower.

4. Gynoecium- This whorl is present at the center of the flower. Its base is swollen which we call ovary and the long tube is called style. The stigma is the flat surface at the mouth of style which acts as a platform for the pollens to land. 

Mechanism of sexual reproduction

It occurs in following steps-1. Pollination 

2. Fertilization

3. Seed formation

4. Fruit formation

5. Seed germination

Pollination is the process of transfer of pollen grains from the anther of stamens to the stigma of pistil or carpel. There are two types of pollinations - Self-pollination and cross-pollination.

Self-pollination is the transfer of pollen grains from the anther of shaken to the stigma of pistol or carpel within the same flower only.

Cross-pollination is the transfer of pollen from the anther to the stigma of pistil from another flower of the same plant of the other plant. 

Many agents help in the process of publication like wind, insects, water, birds, animals, etc. 

Fertilization 

It is the process of fusion of male and female gametes. Pollen settles on the surface of stigma and starts forming a pollen tube. This tube extends towards the ovary where the ovule is present. This tube carries the male gametes inside the ovary to fertilize the ovule. The fertilization results in the formation of a zygote. This zygote by further development develops into an embryo. Later on, this embryo develops into a seed and the ovary dehisces to form a fruit. 

We can say that seed is an embryo which is covered by the protective layers. 

Seed coat- it is the protective covering of the seed. 

Cotyledons- the food is stored in the cotyledons which helps the young plant to grow till it becomes able to make it own food. 

Germination is the growth of seed which under favorable conditions activates the embryo in the seed and slowly it develops into a new plant. 

Factors required for successful germination are water, Oxygen, and Temperature.

Dispersal of seeds

Seed dispersal is very important as the mother plant does not want the baby plants to grow beneath it as they won't get the adequate resources to grow. To facilitate the seed dispersal process, plants have developed various features within them. Wind, water, animals, explosion helps the plants to disperse their seeds to vast localities. 

1. Dispersal by wind

The seed which is to be dispersed by wind is light weighted and some hair like or wing-like structures are present on them which helps the seed to float in the air. Examples - Dandelion, drumstick,  Maple, etc.

2. Dispersal by animals

Many seeds have a spine like structures on them which gets stuck to the furs of the animals and gets spread to different places. Examples- Xanthium, Beggar tick, Urena etc.

3. Dispersal by water

 Plants which disperse their seeds with the help of water grow near the water bodies. Like the seeds of coconut and lilies are spread with the help of water only.

4. Dispersal by explosion

Their are the pods in which seeds are stored in many plants. When the sun dries this pod, it causes the pod to burst with great force releasing the seeds to vast distances. Examples- Beans, Lady finger, Mustard, Castor etc. 




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