Category : 7th Class
Each living organism has a definite span of life. During this period it produces new offsprings which are similar to itself. Thus each organism is survived by its offspring. This is made possible by the process of multiplication. This is also called reproduction which can be defined as "the ability of living organisms to produce new ones of their own kind." Reproduction is mainly of two types - sexual and asexual. In sexual reproduction, the male and the female gametes fuse to form seeds that eventually develop into new plants. The seeds are formed inside the fruit. On the other hand, in asexual reproduction, a new plant is grown from any part of a plant other than the seeds.
Asexual reproduction is a mode of reproduction by which offspring arise from a single parent, and inherit the genes of that parent only, it is reproduction which does not involve meiosis or fertilization. It refers to reproduction without the fusion of gametes. Asexual reproduction is the primary form of reproduction for single-celled organisms such as the bacteria, and protests. Many plants and fungi reproduce asexually as well. All prokaryotes reproduce asexually (without the formation and fusion of gametes), A lack of sexual reproduction is relatively rare among multi cellular organisms, for reasons that are not completely understood. Many hypotheses suggest that asexual reproduction may have short term benefits when rapid population growth is important or in stable environments, while sexual reproduction offers a net advantage by allowing more rapid generation of genetic diversity, allowing adaptation to changing environments.
Characteristics of Asexual Reproduction
Do you know?
Organisms like Hydra use regenerative cells for reproduction in the process of budding
Modes of Asexual Reproduction
Following means of asexual reproduction are used by plants:
They are vegetative propagation, budding, fragmentation and spore formation. The vegetative parts of a plant are the roots, stems and leaves. When new plants are produced from these parts, the process is called vegetative propagation.
When a new plant is developed by a vegetative part; such as root, stem or leaf; it is known as vegetative propagation. For example:
• The leaf of bryophyllum produces new plants through its notches.
When the buds come in contact with moist soil, each bud is capable of growing into a new plant.
Do you know?
Bryophyllum can regenerate itself from any of its parts. A leaf, or stem or root or fruit of a Bryophyllum plant can individually grow into a new plant.
Plants that use buds for vegetative propagation are potato, ginger and turmeric, amongst others. Roots that store food are known as tubers. Such roots are the food storage tanks for the plants.
Do you know?
The word potato comes from the Spanish word Potato
Budding: This method is used by unicellular plants; like yeast. Yeast is a fungus and fungi are also known as non-green plants. An ingredient used to bake a cake is a single celled plant. Yeast reproduces by a process called budding. The small bulb-like projection coming out from the yeast is known as a bud. It detaches
Do you know?
Yeast is commonly used in baking (baker’s yeast) and brewing (brewer’s yeast). It is very popular in study of unicellular organisms because it can be cultured easily, it grows rapidly and its entire genome is known.
from the parent plant and develops into a new plant. The bud gets its own nucleus. The bud develops to certain size and detaches from the mother cell to produce the new yeast.
In some simple plants, the plant body is divided into smaller fragments. Each fragment then develops into a new plant. Example: Spirogyra.
Fragmentation is spirogyra
Algae, the simplest green plants, reproduce by an asexual method known as fragmentation. An alga breaks up into smaller fragments. Each of these smaller fragments grows into a new independent alga.
Special spore-bearing organs are present in some plants; especially in fungi and algae. These are called sporangiophore. The sporangiophore bears spores. The spores germinate to develop a new plant. The white mass and cottony like structure on the bread is called mould.
The process of producing young ones that look like the parents is called reproduction. Plants reproduce either sexually or asexually. When two parents are involved in the process, it is called sexual reproduction. Two gametes, viz. male and female gametes, are formed. The fusion of male and female gametes is called fertilization. Zygote is formed after fertilization. The zygote develops into an embryo and finally into a new individual. Plants reproduce sexually by the most attractive part, called the flower.
Do you know?
Stamens of lotus are dried to produce fragrant teas while lotus seeds can be eaten raw or popped like popcorn
Do you know?
The agave also known as century plant spends many years without growing flowers after which it grows one single bloom and dies.
Flower: Flower is a special organ of flowering plants (angiosperms) which works as the reproductive system. A flower is composed of four distinct whorls.
(1) The outermost whorl is called calyx. It is composed of green leaf- like structures; called sepals.
(2) The second whorl is called corolla. It is composed of colorful structures; called petals. Petals are colourful so that insects and other animals can be attracted towards them. This is necessary for pollination.
(3) Androecium: The third whorl is called androecium. It is composed of stamens. Stamen has two main parts. The tube-like portion is called filament. The capsule like structure at the top is called anther. The anther produces pollen grains; which are the male gametes.
(4) Gynoecium: The whorl at the center is called gynoecium. It has a swollen base; called ovary and a tube-like structure; called style. The top of the tube is somewhat flattened and is called stigma. Ovary produces the eggs or female gametes.
Do you know?
The transfer of pollen grains from anthers to stigma is called pollination. Pollination is of two types, viz. self pollination and cross pollination. Pollen grains transfer from one flower to another by pollinating agents.
The transfer of pollen grains from the anther of one flower, to the stigma of another flower on another plant of the same type, is known as 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 put cross pollination. Wind, insects, birds and other animals play the role of agent of pollination.
The transfer of pollen grains from the anther to the stigma of the same flower or to the stigma of another flower on the same plant, it is termed self-pollination.
Some of the features of flowers pollinated by wind and insects are tabulated as below:
Anemophilous flowers (wind pollinated)
Entomophilous flowers (insect pollinated)
Flowers are dull coloured, without scent and nectar.
Flowers are bright coloured, scented and secrete nectar.
Pollen grains are small, light and smooth.
Pollen grains are large, sticky and spiny
Stigma long, feathery and sticky
stigma small and often deep in corolla
Pollen grains are produced in
Less pollen grains are
Flowers are small
Flowers are large
Stamens are long to be exposed to wind
Stamens are small and may be within corolla tube
Water: Some aquatic plants like sea grass are pollinated by water. The flowers release their pollen grains into water which are carried by the water currants to other flowers.
Mammals: Bats and rodents are agents of pollination in some flowers. Such flowers have following features: Flowers have strong scent. Those that are attracted by mice have yeasty odour. They are often brown or white in colour. These are quite sturdy in order to allow the mammals to carry out their feeding.
Birds: Some flowers are even pollinated by birds. Two most common birds are sunbirds and humming birds. These birds have long sucking beaks that allow them to suck nectar. Such flowers have following features: These are brightly colored so as to attract the birds. The petals are usually red, orange or yellow in color. They are not scented because birds lack sense of smell.
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. It may be either fleshy or dry.
The seed of a dicotyledonous plant has three main parts: Seed coat-a tough protective outer covering
Embryo- consisting of the young roots and shoot which will develop into the adult plant
Food store -a store of food (starch) for the young plant to use until it is large enough to make its own food Germination is the start of growth in the seed. The process by which the embryo in the seed becomes active and begins to grow into a new plant is called germination.
Three factors are required for successful germination:
Dispersal of Seeds
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. Seeds and fruits are dispersed by agents like wind, water, animals and humans.
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
The pods present in these seeds dry up in the sun. This causes the pod to split with great force, thereby dispersing the seeds away from the parent plant. This method is called explosion. Such fruits are called dehiscent fruits. Examples are mustard, ladies finger, peas, bean, pod and castor.
Dispersal by Humans
Human beings also help in dispersal of seeds, especially during fanning.
It is a state of rest of the seeds to tide over unfavorable conditions. There are several ways that keep a seed in the dormant stage. Some seeds develop a thick seed coat that does not allow the entry of water or oxygen. In some germination starts only after the seeds are exposed to light.
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