Class 7 Science
Pollination: The transfer of pollen grains from anthers to stigma is called pollination. Pollination is of two types, viz. self pollination and cross pollination.
Self Pollination: When anthers of the same flower are transferred to the stigma, it is called self pollination.
Cross Pollination: When anthers from a different flower are transferred to the stigma, it is called cross pollination. The flowers can be on the same plant or on different plants. Cross pollination is the norm in most of the plants. Plants need help from various agents of pollination to carry out cross pollination. Wind, insects, birds and other animals play the role of agent of pollination.
Fertilization: The fusion of the male gamete and the female gamete is called fertilization. When pollen settles on top of the stigma, it germinates to produce a pollen tube. The pollen tube enters the ovary through the style. Male nuclei are transferred to the ovary, through the pollen tube. The cell formed, just after fertilization, is called zygote. Zygote develops into embryo. Each embryo develops into a seed. The seed is an embryo which is enclosed in a protective coat. The ovary gets transformed into fruit.
If all the seeds were to germinate near the mother plant, the new plants shall not get adequate resources, like sunlight, air and nutrients. For proper growth of the new plants, it is necessary that they develop away from the mother plant. For this, it is necessary that seeds are spread far and wide. The process of spreading seeds to different places is called dispersal of seeds. Many agents of seed dispersal assist the plants in this process.
Dispersal by Wind: Seeds of some plants are light-weight and some hair-like or wing-like structures are present on them. Such seeds float on air and are thus dispersed by wind. Example: Dandelion, maple, drumstick, etc.
Dispersal by Water: Dispersal by water takes place in some aquatic plants and in some which grow near a water body. Seeds of water lily float and thus dispersed by water. The coconut seed has a tough fibrous covering which has plenty of air inside. This helps the coconut seeds in floating on water.
Dispersal by Animals: Some seeds have spine like structures on them. They get stuck to the fur of animals and thus get spread to different places. Examples: Beggar tick, Xanthium, Urena, etc. Some seeds are swallowed by birds and animals along with fruits. These seeds get dispersed with bird or animal droppings.
Dispersal by Bursting: Some fruits burst open when they mature. The force of bursting is enough to spread the seeds. Examples: Ladyfinger, castor, balsam, etc.
Dispersal by Humans: Human beings also help in dispersal of seeds, especially during farming.
What is pollination?
Answer: Transfer of pollen grains from anther to stigma is called pollination.
Give two examples of seeds which are dispersed by wind.
Answer: Dandelion, maple
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