NCERT Class 9 Science Exploration Chapter 11 Reproduction How Life Continues PDF Download

Chapter 11: Reproduction: How Life Continues

You have learnt that one of the important characteristics of living beings is that they reproduce. Every organism has a definite life span - it is born, grows, matures, reproduces and eventually dies. Reproduction is a biological process by which living beings produce new individuals of their own kind. This is how life on the Earth continues to exist. For example, a mango tree may grow old and die, but its seeds continue to grow as new mango plants. Similarly, cows give birth to calves, dogs to puppies, cats to kittens and humans to children.

You have also learnt that living beings reproduce in two main ways - asexually, where a single parent produces offspring that are almost exact copies of the parent and sexually, where offspring inherit a mix of characteristics from the two individuals. This mixing of characteristics may lead to the small differences between parents and their young ones. Accumulated over many generations, such differences help living beings adapt to changing environments, and sometimes even give rise to new kinds of species. In this chapter, we will explore how reproduction takes place in different organisms, including humans.

11.1 Asexual Reproduction

Asexual mode of reproduction is seen in many unicellular organisms like bacteria, amoeba, yeast and simple multicellular organisms (hydra, sponge). It is also seen in many plants. Recall the examples you studied in earlier grades.

Many types of plants sprout new shoots and roots from their existing parts. For example, plants with fleshy underground stems, such as potato and ginger, sprout new plants without producing seeds. Money plant stem, sugarcane stem cuttings grow into a new plant, Bryophyllum (Fig. 11.1) leaves sprout tiny plantlets which eventually grow into new plants. All these are examples of vegetative propagation, which means that new plants arise from the vegetative parts, i.e., from growing parts of a plant. The key point about this type of reproduction is that it involves only one parent and hence, produces genetically identical individuals.

[Figure 11.1: Bryophyllum leaf sprouts tiny plantlet, See in your textbook]

Teacher's Note

When you see "genetically identical", it means every new plant has exactly the same DNA as its parent because only one parent was involved. This is different from sexual reproduction, where offspring will always be slightly different because they inherit traits from two parents.

11.1.1 How is vegetative propagation in plants helpful in agriculture?

Asexual reproduction produces genetically identical individuals. This natural process has been adapted by scientists and horticulturists to develop several methods of vegetative propagation, such as cutting, grafting, layering and tissue culture for growing plants. These methods are widely used to efficiently propagate plants, and have significantly improved agricultural and horticultural practices. Such methods enable farmers to cultivate desirable crops on a large scale.

Activity 11.1: Let us explore

1. Interact with gardeners working in your school garden or farmers working in a field.
2. Observe the techniques of cutting, grafting and layering followed by them. Discuss these techniques with them and record your observations in your notebook.

Cutting

1. Note the following points while observing the technique of cutting in the field.
   1. Does the gardener, scientist or horticulturist cut the overgrown branches of a plant at the end of its growing season? (Different plants have different growing seasons).
   2. Observe them prepare the cuttings from a plant for the purpose of growing new plants. Note the average length of the cuttings.
   3. Count the number of nodes and internodes on the cuttings.
2. Collect the cuttings of the shoots in the morning for planting.
3. Remove leaves from the lower half of each cutting.
4. Insert the cuttings up to approximately half of their length in the soil mixed with compost at an angle of about 45-60° from the soil surface (Fig. 11.2).
5. Water them regularly and observe the change, if any.

[Figure 11.2: Cutting, See in your textbook]

Grafting

1. For grafting, take a healthy rooted plant (Plant A) (Fig. 11.3a) (for example, a wild rose variety) and a healthy stem piece from another plant (Plant B) of other varieties (for example, a yellow rose plant and/or a pink rose plant) (Fig. 11.3c and Fig. 11.3d).
2. Create a wound or a slit on a twig of Plant A (Fig. 11.3b).
3. Insert and fit the cutting of stem of Plant B into the slit of stem of Plant A (Fig. 11.3e).
4. Protect the wound or slit by using a cotton cloth or by wrapping film to avoid pests entering the graft until it heals (Fig. 11.3f). Cut the other branches of Plant A.
5. Water the plant regularly and observe the growth of Plant B along with Plant A.

[Figure 11.3: Steps of grafting, See in your textbook]

Layering

1. For layering, select a flexible, thin twig of a tree or a shrub, such as a lemon and bury the middle part of the twig under the soil surface (Fig. 11.4).
2. Water it regularly and observe the growth of new leaves on the twig buried in the soil.
3. After 10-15 days, the roots will develop from the area of the twig buried in the soil.
4. Once roots have developed, cut the twig from the parent plant, so that it can grow as a new plant.

These are some of the methods of asexual reproduction through which different varieties of plants can be propagated.

[Figure 11.4: Layering, See in your textbook]

Teacher's Note

In cutting, grafting and layering, notice that the new plant grows from a piece of the parent plant, not from a seed. This is why all three methods produce offspring that are genetically identical to the parent. This is very useful in agriculture because if you find a crop plant with particularly good traits (like bigger fruit or disease resistance), you can multiply it exactly using these methods rather than hoping seeds will have the same quality.

Bridging Science and Society

Propagation of plants by the tissue culture technique has revolutionised farming practices like in banana farming. Farmers are now provided mass-produced healthy plantlets from the shoot tip (apical meristem) of several plants, which help eliminate virus-infected plants and ensures high yields. This is also an example of asexual reproduction in plants.

Various Krishi Vigyan Kendras (KVKs) under the Indian Council of Agricultural Research (ICAR) allow farmers to gain vocational skills in modern grafting that help them grow high-yield fruits, and boost their income by learning various marketing strategies and government programmes.

[Figure 11.5: Plant tissue culture, See in your textbook]

You have learnt about various methods of vegetative propagation in plants. Let us examine asexual reproduction in other organisms.

Key Points

• Reproduction is the process by which living beings produce new individuals of their own kind. Asexual reproduction involves only one parent and produces genetically identical offspring.
• Vegetative propagation uses parts of a plant (stems, leaves, or underground structures) to grow new plants without seeds. Common methods include cutting, grafting, layering and tissue culture.
• Scientists and farmers use vegetative propagation methods to grow desirable crop varieties on a large scale, ensuring consistent quality and high yields.
• Sexual reproduction involves two parents and produces offspring with a mix of characteristics from both parents, leading to variation that helps organisms adapt to changing environments.

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