Class 7 Reproduction in Plants Notes for exams

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Class 7 Science: Reproduction in Plants (Notes for Exams)

Chapter 12: Reproduction in Plants

Reproduction

Reproduction is the process by which living organisms produce more of their kind. Plants reproduce either sexually or asexually.

I. Asexual mode of reproduction

It is the process which involves a single parent in giving rise to offspring. It is the growth of a new plant from a part of the plant other than the seed.

Modes of asexual reproduction in plants

Plants reproducing by the asexual mode do not produce gametes. A single parent multiplies to give rise to new individuals. The different modes of asexual reproduction in plants are explained below.

1. Vegetative propagation is the production of new plants from the vegetative parts of the plant. Roots, stems and leaves are called the vegetative parts of a plant. Vegetative propagation takes place by different methods like leaf propagation, root propagation, underground stem propagation, etc.
   1. Leaf propagation: Bryophyllum propagates vegetatively by the formation of leaf buds on the margins of a leaf. When the buds come in contact with moist soil, each bud is capable of growing into a new plant.
   2. Root propagation: Sweet potato and dahlia root tubers are cut into many pieces and are covered with mud. Each piece of the root tuber is capable of growing into a young plant.
   3. Stem propagation: In the money plant, the stem is cut and one side of it is buried in the soil. This stem from the parent plant grows into a new plant.
   4. Underground stem tuber propagation: Potato is a stem tuber growing underground. It bears special structures called eyes. When a potato is cut and buried under the soil, each eye having a bud develops into a new plant.
   5. Underground stem propagation: Ginger is an underground stem called a rhizome. It has a lot of scaly leaves at the nodes. These, when buried under the soil, give rise to new plants.
2. Budding involves the growth of a small bulb-like projection called a bud. This bud grows and detaches itself from the parent cell to grow independently into a new organism.
   1. Yeast reproduces by budding.
   2. Yeast is a unicellular organism belonging to the category of fungi.
   3. It reproduces a new individual by the process of budding.
   4. A little amount of cytoplasm forms a protuberance.
   5. The nucleus divides into two. One nucleus migrates into the bud.
   6. The bud gets detached from the parent to develop into a new individual.
   7. During budding, yeast respires and releases carbon dioxide.
   8. The carbon dioxide released helps the dough to puff up and become spongy. Hence, yeast is used to bake cakes and to make dough.
3. Fragmentation involves the breaking down of filaments into fragments that grow into young ones.
   1. Algae are green, thread-like plants that grow in stagnant water, ponds and lakes.
   2. They float on the surface of the water, causing algal bloom.
   3. Algae reproduce by the method of fragmentation.
   4. As it is a multicellular organism, each cell strip breaks off as a fragment and falls on the substratum.
   5. It gets its strength to develop into a new individual.
4. Spore formation is the method of developing new individuals by forming reproductive structures called spores.
   1. A spore is a small spherical or oval structure which protects the future individual in a thick protective covering.
   2. A spore germinates on a substratum under favourable conditions.
   3. Some organisms like ferns and some groups of fungi reproduce by spore formation.
   1. Ferns reproduce by releasing spores that germinate into young ones.
   2. Fungus reproduces by means of spores. A fungus like bread mould produces spores which germinate on moist organic surfaces. The cottony white mass on bread formed by the fungus after spore germination is called a mould. Spores can survive in extreme conditions because of the protective hard coat.
   3. Moss also reproduces by spores.

II. Sexual mode of reproduction

This type of reproduction involves two organisms of opposite sex, the male and the female. Reproductive organs in plants produce gametes: ovules (eggs) and pollen grains.

1. The egg is the female gamete, produced in the ovule inside the ovary.
2. Pollen grains are produced by the stamens; they carry the male gametes.
3. A zygote is the future individual formed by the fusion of an egg and a male gamete from a pollen grain. The zygote develops into a seed.
4. Seeds enclose the embryo, the future individual, until favourable conditions prevail for the development of the embryo into a plantlet.

Reproductive structures of a plant

Flowers are the reproductive structures which help the plant to undergo the process of sexual reproduction. These are the most attractive parts of the plant. A flower may have a stalk-like structure called the pedicel, which helps in its attachment to the plant.

Structure of a flower

A complete flower is made up of four whorls. These are the sepals, petals, stamens and the pistil. These four whorls are attached to the flattened tip of the flower called the receptacle.

1. Sepals collectively form the calyx. These protect the inner parts of the flower in its bud condition.
2. Petals collectively form the corolla. These are the coloured structures which attract insects and birds for pollination.
3. Stamens are the male reproductive parts. A stamen is made up of two structures, namely a filament and an anther. The filament is a tubular structure. The anther is a knob-like structure containing pollen grains. Pollen grains carry the male gametes (male reproductive cells).
4. The pistil is the centrally located female reproductive part of a flower. It is made up of the stigma, style and ovary. The stigma is a flattened structure which receives the pollen. It is sticky in nature. The style is a long tubular structure which conveys pollen to the ovary. The ovary contains numerous ovules. Each ovule contains a female gamete or egg cell.

Types of flowers

Flowers, based on the reproductive whorls they carry, can be classified into unisexual flowers and bisexual flowers.

1. A unisexual flower is a flower which consists of either stamens or a pistil. A flower with a whorl of stamens is called a male flower. A flower with just a pistil is called a female flower.
2. A bisexual flower is a flower which consists of both the whorls, stamens and pistil, i.e. male and female reproductive structures.

Pollination

Pollination is the transfer of pollen from the stamen to the stigma.

1. The different factors which help in the transfer of pollen are called agents of pollination. Agents of pollination include air, water, birds, insects, animals, etc.
2. Pollination can be of two types, namely self-pollination and cross-pollination.
3. Self-pollination is the transfer of pollen from the stamen to the stigma of the same flower. Cross-pollination is the transfer of pollen from the stamen of one flower to the stigma of another flower on the same plant or a different plant of the same kind.

Fertilization

1. The stigma of the flower secretes sugary substances.
2. As the pollen grains from the stamen fall on the stigma, they recognise the sugary substance and germinate to produce a structure called the pollen tube.
3. The pollen tube grows through the style to reach the egg in the ovary.
4. As it reaches the ovule, the tip of the tube opens and releases male gametes to fertilise the egg cell.
5. The fusion of a male gamete and a female gamete produces a zygote, a single cell.
6. The zygote develops into an embryo.
7. The fertilized ovary becomes a fruit.
8. The ovule develops into a seed.

Formation of seed

There occur some changes in the flower after the process of fertilization.

1. Sepals, petals, stigma and style fall off, leaving the fertilized ovary.
2. The zygote utilises the reserved food in the ovule to divide and transform into an embryo.
3. The walls of the ovule develop the hard layers (coats) of the seed.
4. A seed contains a young plant and stored food in the form of cotyledons.

Formation of fruit

The ovary grows in size due to the formation of seeds. This well-grown fertilized ovary becomes the fruit. A fruit can have any number of seeds in it. Fruits can be fleshy or dry in nature.

it is used to bake a cake and to make dough.

3. Fragmentation

Fertilizaion