ICSE Class 8 Biology Oxford Chapter 02 Growth Development

Growth, Development, and Reproduction

Learning Outcomes

Reproduction in Plants

Germination of Seeds

Reproduction in Animals

Reproduction in Humans

Growth and Development

Characteristics of Adolescence

All organisms produce living beings similar to themselves, as shown in pictures (a) to (c). This ability, called reproduction, is one of the seven basic characteristics shown by living organisms, which you must have studied in lower classes. Thus, reproduction may be defined as the ability of living organisms to produce living beings similar to themselves.

What are the different means of reproduction? What is its importance to a living organism? We will discuss these aspects in this chapter.

As you are aware, organisms exhibit great diversity, from microorganisms to large plants and animals. All of them reproduce because only then can they ensure that their kind continues to exist. Reproduction may be sexual or asexual. Sexual reproduction involves the formation of special reproductive cells called gametes - male gametes produced in the male reproductive organs and female gametes produced in the female reproductive organs. Asexual reproduction does not involve the formation of gametes. In this type of reproduction, new organisms are formed either by division of the parent body or by differentiation of the parent body. Let us examine different modes of reproduction in plants and animals one by one.

Fact File

In organisms such as humans, male and female gametes are of different sizes. The female gametes are large and non-motile whereas the male gametes are much smaller and motile.

Reproduction In Plants

Plants show both sexual and asexual modes of reproduction. Let us discuss asexual reproduction first.

Asexual Reproduction in Plants

In plants, asexual reproduction takes place through spore formation and vegetative propagation. Let us understand what happens in each mode.

Spore Formation

Lower plants such as ferns, mosses, fungi, and lichens reproduce asexually through spores. Figure 2.1 shows spore formation in bread mould, a fungus. Spores are similar to seeds in higher plants.

Vegetative Propagation

Budding and fragmentation are vegetative means of reproduction in lower plants.

Budding

Budding is commonly seen in fungi (e.g., yeast) (Fig. 2.2). A small, bulb-like outgrowth (bud) emerges from the body of the parent and continues to increase in size. The nucleus divides into two, one of which moves into the bud. The bud breaks off from the parent once it is capable of existing independently.

Fragmentation

Fragmentation is another common form of asexual reproduction in the plant kingdom. An adult organism simply breaks up into two or more fragments, each of which grows to become a new individual. Spirogyra (Fig. 2.3) reproduces by fragmentation. However, it must be noted that fragmentation in filamentous algae such as Spirogyra is more or less accidental.

Tech File

Artificial methods of vegetative reproduction are popular among farmers and gardeners. Vegetative reproduction is especially useful for growing plants whose seed production or germination is low or whose seeds are very expensive. Cutting, grafting, and layering are commonly practiced artificial methods of vegetative reproduction.

Higher plants show a tendency to multiply using their vegetative parts, namely the root, the stem, or the leaf.

Roots

The tuberous roots of dahlia (Fig. 2.4), sweet potato, and asparagus are true roots modified to store food in them. In some cases, such as dahlia, the tuberous root needs to be planted along with the bud for it to develop into a new plant. The bud is present where the stem joins the root. In sweet potato the tuberous root develops adventitious shoots called slips, which can be broken off and planted.

Stems

Vegetative propagation through stems, which is very common in plants, can be considered under two heads: sub-aerial stems and underground stems.

Sub-aerial stems

Runners and stolons - Plants such as strawberry, oxalis (Fig. 2.5), and grass grow stems that run parallel to the ground and produce a new plant at each node. The new plants may then be detached from the parent plant. Both runners and stolons are horizontal stems, the only difference being that stolons are usually arched while runners extend straight on.

Sucker - In certain plants, horizontal stems arise from the base of the shoot, grow horizontally in the soil for some length, and then come out to form shoots. The suckers then break away to form independent plants.

Offset - Offset means a side shoot emerging from a bud on the main stem. The offset falls off easily in most cases and continues to develop as an independent plant after rooting. An offset is shorter and thicker than a runner. Water lettuce (Fig. 2.6) and water hyacinth are examples of plants that reproduce by this method.

Underground stems

Rhizome - A rhizome is an underground stem that produces roots and shoots at the nodes, giving rise to new plants. It is commonly seen in ginger, turmeric, and banana.

Bulb - It is a very short vertical stem which is surrounded by scale leaves (e.g., onion) (Fig. 2.7). Scale leaves are fleshy as they accumulate food.

Tuber - Tubers are underground stems formed by swelling of their tips due to food accumulation (e.g., potato) (Fig. 2.8). Each 'eye' of a potato contains a bud which has the capability to develop into a new plant. These new plants use up the food stored in the underground stem to grow.

Corm - A corm is usually round in shape and is borne in clusters (e.g., Gladiolus) (Fig. 2.9). Roots develop from the base of the corm, sometimes also from the sides.

Leaves

In plants such as Bryophyllum, rows of buds develop at the leaf margins, each at the end of a vein (Fig. 2.10). These buds may drop off from the leaf and grow into new plants. Sometimes the leaf itself may drop off, and the buds continue to grow in the soil.

Sexual Reproduction in Plants

Sexual reproduction is observed in flowering plants and many non-flowering plants. In this chapter, we will only consider sexual reproduction in flowering plants, which make up roughly 90% of the plant kingdom. Flowering plants reproduce sexually. The reproductive organs in flowering plants are flowers, from which the group derives its name. Flowers may be bisexual (with both male and female organs), as in rose, or unisexual (with only male or female organs), as in papaya.

Activity

Aim: To observe vegetative reproduction in plants.

Materials required: Leaves of Bryophyllum, rhizome of ginger, stem tuber of potato, and a bulb of onion.

Procedure:

1. Fill four medium-sized flower pots with garden soil. Place the Bryophyllum leaf on the surface of the soil in one pot and place all other specimens just below the soil level.

2. Adventitious buds on the leaves of Bryophyllum will give rise to new plants in a few weeks' time. Ginger, potato, and onion need to be watered and looked after for about six months to see the establishment of a new set of underground stems.

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