Reproduction in Plants

"The process of producing young ones from their parents is known as reproduction."

1.0Introduction

All living organisms reproduce. What it means in simple terms is that living organisms produce young ones of their own kind. For example, a cat produces kittens which grow into adult cats. In plants too, seeds grow into young seedlings. The seedlings in due course of time develop into mature plants, as in a mango tree. Reproduction is one of the most important properties of living organisms. It means creating new life, producing young ones of their own kind.

Why living organisms need to reproduce?

All living organisms have a fixed life span. During their life span, the organisms perform various life functions including reproduction. In an annual plant, the life span is about one year, and in a perennial plant, it goes up to many years. Then the organisms die leaving behind individuals of their own kind. Thus, they ensure by reproducing during their life span. If they would not reproduce, then after their death, there would be no organisms left. The species would thus perish. Reproduction is, thus, the means of perpetuation of species.

2.0Modes of reproduction in plants

Flowers produce seeds which are formed by the fusion of a male gamete with a female gamete. We sow these seeds and grow more plants of the same type. This is the most common method of growing new plants. This type of reproduction is called sexual reproduction.

Can we grow new plants without seeds? Yes. There are some plants which do grow without seeds. These plants give rise to new plants from a part of stem, root, leaf or bud. These parts of the plant are called vegetative parts and hence the process, vegetative propagation. For example, a new plant of rose is produced by growing a cutting from one of its branches. This type of reproduction, which takes place without seeds, is called asexual reproduction. Sexual reproduction is characterized by the fusion of two cells (gametes) usually coming from two parents. New plants are produced from seeds. On the other hand, asexual reproduction does not involve the union of gametes. New individual is produced from a single parent.

3.0Methods of Asexual Reproduction

Budding:

Asexual mode of reproduction in which a small outgrowth (bud) appears on the body of the organism. Budding is commonly observed in yeast. A bulb-like projection, called the bud, is formed on the body. The nucleus of the body divides into two. Then, one of the two nuclei passes into the bud. The bud detaches itself from the parent body. It grows to full size and becomes a new individual.

Fragmentation:

In some filamentous organisms such as algae (slimy green patches in ponds or in stagnant water bodies), the filaments break up into two or more fragments. Each fragment or piece grows into a new individual. This process continues and cover a large area in a short period of time.

Spore formation:

In non-flowering plants (the plants which do not produce seeds- ferns, mosses), fungi (dung mould), formation of spores is a common method of reproduction. Spores are very small in size. They have thick walls. The thick walls help the spores to survive in adverse conditions like high temperature, low humidity scarcity of water and lack of food. The spores give rise to new organisms under favourable conditions.

Vegetative Propagation:

This is an asexual method of reproduction in plants where vegetative parts namely the root, stem, leaf or buds give rise to new plants. No reproductive organs take part in this method of reproduction and therefore, no seeds are produced.

(i) Natural vegetative propagation

(a) Vegetative propagation by roots - In sweet potato, dahlia etc., the swollen roots are present. New plants arise from these swollen roots buried in the soil.

(c) Vegetative propagation by leaves - In bryophyllum, plantlets develop from the margins of intact leaves. These plantlets on being detached, develop into independent plants. Buds present in individual notches along the margin of a bryophyllum leaf are capable of forming new plants.

(d) Vegetative propagation by buds - Flower buds which develop into flowers are called floral buds. There are buds in the axil (point of attachment of the leaf at the node) of leaves which develop into shoots. These buds are called vegetative buds. A bud has immature overlapping leaves around and such vegetative buds can

• Advantages of vegetative reproduction (i) It is an easier, rapid and less expensive method of propagation. Plants can be grown in much less time. (ii) Seedless plants can be raised. (iii) Plants produced by this method are identical copies of the parent plant and show no variations. (iv) Plants like banana, sugarcane, sweet potato, rose and jasmine do not produce viable seeds. Such plants can be easily grown by this method.

(ii) Artificial vegetative propagation

Because of the advantages offered by vegetative propagation, humans have used this method for artificial multiplication of useful plants. For this reason, such multiplication methods are called artificial methods of vegetative propagation. Some of these methods are as follows: (a) Cutting: Artificial method of vegetative propagation in which short lengths of plant stem or root with node are used for growing new plants.

• Stem cutting: Stem cuttings when put in the damp soil give rise to roots and shoots and form new plants. E.g. Rose, money plant and sugarcane.
  
• Root cuttings: In certain plants like lemon, tamarind, etc. root cuttings when put in the damp soil give rise to roots and shoots and form new plants.

4.0Sexual Reproduction

In nature, sexual reproduction occurs in plants as well as in animals. It is the most common method of reproduction. As stated earlier, two parents, one male and the other female are required for sexual reproduction. Two types of reproductive cells called gametes are produced from the reproductive organs of two parents. Male parent produces the male gamete and the female parent produces the female gamete. The fusion of the two gametes is called fertilization. The product of fusion of the two gametes is called zygote. The male gamete in a flowering plant is formed by the pollen grain whereas in animals, it is the sperm. The female gamete in plants is a large egg-cell in the ovule, while in animals, it is the ovum. After fertilization, the zygote undergoes cell division and growth. Ultimately, it forms the new individual. Embryo: A ball-like structure formed after divisions in the fertilized egg (zygote).

5.0Sexual reproduction in plants

A flower has commonly four parts. These parts, in order, from the outside to inside are sepals, petals, stamens and pistil. Out of these four parts; stamens and pistil are the important parts, as these take part in sexual reproduction. The stamens are the male reproductive part while the pistil (or carpel) is the female part.

Bisexual Flower: Flowers which contain both stamen and pistil are called bisexual flower. Examples - Rose, mustard and petunia.

Unisexual Flower: Flowers which contain either pistil or stamen are called unisexual flowers. Examples- papaya, corn and cucumber.

Pollination in Flowering Plants:

Pollination is the transfer of pollen grains from the ripe anther to the stigma. The transfer of pollen grains to the stigma can take place in two ways : (i) Self pollination: Within the same flower or between flowers of the same plant. (ii) Cross pollination: Between flowers of different plants of the same species.

Cross pollination often involves various external agencies to carry pollen grains from one flower to another one. These agencies may be air, water, insects or animals. Most flowers are pollinated by insects. When you visit a garden during flowering season, you observe many butterflies, bees and moths. What are the insects doing? Though these insects visit the flowers for nectar or honey, at the same time, they help in pollination.

When an insect visits a flower, the pollen grains get deposited on the body of the insect. When this insect visits another flower, the deposited pollen grains now get dusted on the stigma of the second flower, thus bringing about the transfer of pollen grains from the anther to the stigma (pollination). Pollen grains of all flowers are not carried by insects. In some cases, they are carried by wind (wind pollination). In case of water plants, pollen grains are carried by water (water pollination).

Fertilization in flowering plants:

Fertilization is a step between pollination and seed formation. The fusion of the male gamete with the female gamete is called fertilization.

During fertilization, the following events take place :

• The pollen grains germinate on the stigma and pollen tubes develop. The pollen tubes move downwards into the style. These tubes are the carriers of male gametes.
• One pollen tube finally enters the ovule where female gamete is located. Female gamete or egg cell is present inside the ovule.
• Finally the male gamete fuses with the female gamete. This completes the process of fertilization. The fusion product or the cell formed as a result of fusion of the two gametes is called zygote. The zygote soon develops into an embryo (baby plant).
  
  Process of fertilization

Formation of fruit and seed:

What happens to the flower after fertilization? If you observe carefully, you may see following changes in a flower :

• The flower loses its bright colour.
• The sepals, petals and stamens fall off.
• The ovary increases in size and becomes the fruit. The fruit thus is the ripened ovary.
• The ovary wall becomes the fruit wall.
• Inside the ovary, the ovules develop to form the seeds. The seeds develop from the ovules. The seed contains an embryo enclosed in a protective seed coat.

Some fruits are fleshy and juicy such as mango, apple and orange. Some fruits are hard like almonds and walnuts.

Functions of fruits

(i) The fruit wall gives protection to the seeds and therefore, to the embryo. (ii) The fruit is a store house of food material. (iii) The fruit helps in the dispersal of seeds.

Dispersal of seeds and fruits:

For a seed to give rise to a new plant, certain favourable conditions are necessary. A seed must fall on a suitable place where favourable conditions are present.

Let us study the following seeds or fruits to know about the mechanism of dispersal.

Collect seeds/fruits of drumstick, maple, calotropis (madar), sunflower, gokhru (tributus), okra (xanthium), urena and coconut.

Examine these seeds or fruits. What do you find?

You will find different situations :

(i) In some cases like maple, drumstick, winged seeds are present. Light seeds of grass or hairy seeds of madar (aak) and hairy fruit of sunflower dispersed by wind. (ii) In urena and xanthium, the seeds are thorny and stick to our clothes as we pass by them. These seeds also stick to bodies of animals passing nearby. Thus, animals as well as human beings help in dispersal. (iii) Some seeds are dispersed by water. These fruits or seeds usually develop floating ability in the form of spongy or fibrous coat as in coconut.

(iv) Some seeds are dispersed when the fruits burst with sudden jerks. The seeds are scattered far from the parent plant. This happens in the case of castor and balsam. Thus, seeds and fruits may be dispersed to places away from the parent plant by various means like wind, animals, or water. Seed dispersal help the plants to (i) prevent overcrowding. (ii) avoid competition for water, minerals and sunlight, and (iii) spread to new habitats.

6.0Building Concepts

Why do we need sexual reproduction?

• Sexual reproduction brings about the fusion of gametes from both the parents. The zygote formed possesses characters of both parents. This also helps to bring variations among new individuals. You can now understand why children of the same parents show variations.

What will happen if all seeds of a plant were to fall at the same place and grow there?

• If all seeds of a plant were to fall at the same place and grow, there would be severe competition for sunlight, water, minerals and space. As a result, the seeds would not grow into healthy plants.

7.0Biology Diagrams made Easy

8.0Chapter at a Glance

9.0SOME BASIC TERMS

• Asexual reproduction: The type of reproduction in which seeds do not form.
• Embryo: A ball-like structure formed after divisions in the fertilized egg.
• Fertilization: The fusion of male and female gamete is called fertilization.
• Hypha: Long filamentous structure of fungus.
• Pollen grain: Structure carrying male gametes.
• Pollen tube: Hollow tube-like structure formed from pollen.
• Pollination: Transfer of pollen grains from ripe anther to stigma.
• Spore: Reproductive cell capable of developing a new individual.
• Sporangium: An enclosed capsule containing spores.
• Zygote: Fertilized egg.