CBSE Class 7 Science Advanced Biology Reproduction in Plants Notes Set 01

Reproduction in Plants

Reproduction

Every living thing can make new individuals like itself. This act is called reproduction, and it keeps a species alive from one generation to the next. The new individuals are the offspring.

A group of organisms or cells that arises from a single parent without any mixing of genes, and so is genetically the same as that parent, is called a clone.

Methods of Reproduction

Depending on how many parents take part, reproduction is of two kinds.

1. Asexual reproduction: one parent alone produces the young ones, which are exact copies of it (clones).
2. Sexual reproduction: a male parent and a female parent take part. It goes through these stages.
(a) The male parent makes male gametes and the female parent makes female gametes. A gamete is a reproductive cell carrying half of the parent's genetic material.
(b) A male gamete joins a female gamete. This joining is fertilization, and the cell formed is the zygote.
(c) The zygote grows into the young one, which resembles its parents but is not an exact copy.

Comparison of the two methods

Reproduction in Plants

Plants can reproduce in both ways. The chart below shows how the modes are classified.

Asexual Reproduction in Plants (Vegetative Propagation)

In this method a new plant grows from a vegetative part of the old plant, such as a root, stem, leaf or bud, and the flower is not used. It is therefore also called vegetative propagation. It is of two types, natural and artificial.

Natural Vegetative Propagation

Here the plant parts give rise to new plants on their own. A part that can separate from the parent and grow into a new plant is called a vegetative propagule.

1. By roots: the roots of some plants hold a lot of stored food and develop buds that grow into new plants. Example: sweet potato.

2. By stems:

Bulb: a stem that stores food and carries buds, which can sprout into new plants. Examples: onion and garlic.
Stem tuber: the potato is an underground stem that has swollen with stored food. It has small scars called eyes (nodes) holding buds, and each bud can grow into a plant. (The potato is a stem and not a root.)
Runner: a weak, creeping stem that puts out roots wherever it touches the ground. When the older part between dies away, the rooted pieces live as separate plants. Examples: grass and wild strawberry.

3. By leaves: in some plants buds appear on the leaf edge and grow into small plants. Example: Bryophyllum.

Artificial Vegetative Propagation

Gardeners use these methods to get quickly many plants and to combine good qualities of different varieties.

1. Cutting: a piece of stem is cut and planted in soil, where it grows into a new plant. Example: rose.
2. Grafting: done between closely related plants. The shoot of the superior plant is the scion and the rooted plant is the stock. The scion is fixed on the stock and tied firmly; they grow as one plant showing qualities of both. Examples: grafted rose and mango.

3. Budding: a bud with a small piece of bark of the wanted plant is slipped into a slit cut in the bark of another plant and tied. Example: plum rose.
4. Layering: a branch is bent down and a part of it is covered with soil. Roots form there and it can then grow into a separate plant. Example: climbing rose.

Sexual Reproduction in Plants

The flower is the reproductive organ of a flowering plant. Its work is to bring male and female gametes together.

Structure of a Flower

A typical flower has four whorls.

Calyx: the lowest, usually green and leaf-like whorl made of sepals; it protects the bud.
Corolla: the whorl of petals, which are brightly coloured modified leaves.
Stamen (androecium): the male part. The anther holds the pollen grains and the filament is the stalk that carries it.
Pistil (gynoecium): the female part, made of the stigma (top), style (the tube below it) and ovary (the swollen base). The ovary contains ovules, in which the egg develops.

Types of Flowers

Complete (bisexual) flowers have all four whorls, so both male and female parts are present, for example rose and lily.
Incomplete (unisexual) flowers lack one of these whorls, either the stamens or the pistil, for example papaya, watermelon and maize. A flower with only stamens is a staminate flower and one with only a pistil is a pistillate flower.

Pollination

Pollination is the movement of pollen grains from the anther to the stigma, of the same flower or of another flower. It may be natural or artificial.

Natural Pollination

1. Self-pollination: pollen goes from the anther to the stigma of the same flower or of another flower on the same plant.
2. Cross-pollination: pollen goes to the stigma of a flower on a different plant of the same kind. Carriers of pollen are called pollinating agents: insects, water and wind.

By insects: the most common way. Insects such as bees and butterflies visit flowers for nectar; pollen sticks to their bodies and rubs off on the stigma of the next flower. Such flowers (rose, marigold, night jasmine) are large, bright and often scented, and their pollen and stigma are usually sticky (strawberry, mustard).

By water: seen in water plants such as Hydrilla and Vallisneria. Their flowers are light, float on the water surface and make no nectar.

By wind: pollen is blown to the stigma. Much pollen is made so that some of it reaches a stigma. These flowers are small and have dry, light pollen, as in rice, maize and grasses.

Artificial Pollination

People transfer pollen by hand between two chosen varieties to get a new variety with the useful features of both, for example one that resists disease and one that gives a high yield. It increases the variety of genes in the offspring. Cross-pollination of rice in agriculture is a common example.

Fertilization

Steps after pollen is released:
1. Pollen grains are made in the anther. When the ripe anther splits, yellow powdery pollen is freed.
2. Pollination brings the pollen to the sticky stigma of a pistil.
3. From the stigma the pollen goes down the style, the tube that links the stigma with the ovary.
4. It reaches the egg in the ovule and fuses with it. This is fertilization.
5. Then the ovary becomes the fruit and the ovule becomes the seed.

Post-Fertilization Events

The fertilized egg grows into the embryo. Around it is a food-rich tissue, the endosperm, which feeds the developing embryo. The ovule turns into the seed, while the sepals, petals and stamens dry up and fall off.

Seed and its Dispersal

A seed is a ripened ovule that can grow into a new plant when conditions suit. It has a protective seed coat and seed leaves called cotyledons that store food. Flowering plants have either one cotyledon (monocots) or two (dicots).

A plant cannot walk, so its seeds must be carried away from it. This is seed dispersal, and it helps the young plants to avoid crowding. Living and non-living agents are involved.

Seed Germination

After dispersal seeds wait for favourable conditions. Germination is the sprouting of the seed into a seedling, which becomes a young plant. The radicle comes out first and fixes the seedling in the soil. The plumule develops next and takes food from the cotyledons. Seeds need three things to germinate: water, a suitable temperature and oxygen.

Points to Remember

1. Reproduction gives rise to young ones and continues the generations.
2. Plants reproduce asexually (vegetative propagation) and sexually (by flowers).
3. Asexual reproduction needs no male or female reproductive parts.
4. Sexual reproduction ends in fruits that contain seeds.
5. Pollination is the transfer of pollen from anther to stigma and is followed by fertilization.
6. The embryo is nourished by the endosperm.
7. A seed is a matured ovule.
8. Seed dispersal is the carrying of seeds away from the parent plant.
9. Seed germination is the sprouting of a seedling from a seed.