Plants

UNIT 3

Plants

Learning Objectives

After completing this lesson, students will be able to

* know about types of pollination and the agents of pollination.

* understand the life cycle of flowering plants.

* identify different types of soil.

* know how the honey bee, earthworm, dragon fly are useful to farmers.

Introduction

Plants are useful to us in a number of ways. Plants produce their own food. At the same time they are used as food by men and animals. Plants are not only used as food but also as medicine. Almost all parts of the plants are useful to us. As a natural resource they are beautiful and pleasing to our eyes. They release oxygen, which is essential for our survival, into the atmosphere. Some plants grow in our surrounding naturally and some are grown by us. We need to know about plants which are useful to us in many ways. Let us study in this lesson about the life cycle of plants, agriculture and the types of soil.

I. Reproduction in Plants

Reproduction is the process by which new individuals of the same species are produced. Both plants and animals reproduce. The flowers perform the function of reproduction in plants. There are two kinds of reproduction that take place in plants. They are sexual reproduction and asexual reproduction. In asexual reproduction, new plants are produced from roots, leaves, stems and buds. In sexual reproduction, new plants emerge from seeds.

1. Flower

Flower is the reproductive part of a plant. It is a modified shoot. Flowers have four important parts. They are:

* Sepal

* Petal

* Androecium

* Gynoecium

* Sepal

It is the outer part of the flower. Usually it is small and green in colour. It protects the bud in the early stage.

* Petal

It is often colourful and it attracts the insects.

* Androecium

It is the male reproductive part of the flower. It is composed of stamens. Each stamen consists of a stalk called filament and a small bag like structure called anther at the tip. The pollen grains are produced in the anther within the pollen sacs.

*Gynoecium

It is a female part of the flower. It has three parts. They are: ovary, style and stigma. The ovary contains the ovules.

The flowers which contain either androecium or gynoecium are called unisexual flowers. E.g. Corn, Papaya, Cucumber. The flowers which contain both androecium and gynoecium are called bisexual flowers. E.g. Mustard, Rose.

To attract the pollinators (E.g. Insects) plants are bright in colour and produce smell in their flowers. Plants which are pollinated by the honey bees and butterflies have sweet scents and bigger colourful petals. E.g. Sunflower, Pumpkin. Pigments present in petals give them different colours. Plants which are pollinated by the moth and bats release their fragrance mostly at night and have colourless petals. E.g. Mango, Banana, Guava, Jasmine etc. The following table gives the names of the pigment present in petals.

Activity 1

Take a hibiscus flower or a rose flower. Display the parts like sepal, petal, gynoecium and androecium in a chart paper and note down its colour and shape.

Do you know?

Kurinji or Neelakurinji (Strobilanthes kunthianu) is a shrub that is found in the Chola forests of the Western Ghats in South India. Nilagiri Hills, which literally means the blue mountains, got their name from the purplish blue flowers of Neelakurinji that blossoms once in 12 years.

2. Pollination

The transfer of pollen grains from the anther to stigma of a flower is called pollination. Pollination is the first important event in the development of fruit and seed. Pollination is followed by fertilization. Two types of pollination take place in the flowering plants. They are self pollination and cross pollination.

The transfer of pollen grains from the anther of a flower to the stigma of the same flower is called self pollination. The transfer of pollen grains of a flower to the stigma of another flower of a different plant of the same species is called cross pollination.

In self pollination, seeds produce weak plants and new varities of plants cannot be produced. In cross pollination, seeds produce good plants and new verities of plants can be produced. Pollination takes place through different agents. They are explained below.

* Pollination by Wind (Anemophily)

The flowers pollinated by wind are mostly small in size and do not have any attractive colour, smell and nectar. The pollen grains are non-sticky, dry, light and powdery. Hence, they are easily carried by the wind. E.g. Grass, Maize, Pine.

* Pollination by Water (Hydrophily)

The flowers of water plants are not colourful and they have no nectar. Pollen grains of these plants have mucilaginous covering to protect them from getting wet. They float in water and reach the other plant. E.g. Vallisneria, Hydrilla, Zosteria.

* Pollination by Insects (Entamophily)

This is the most common type of pollination in plants like sunflower, ladies finger, brinjal and pumpkin. Some flowers are large in size and they have sweet smell. Some of these flowers produce nectar. They attact insects like butterflies and honey bees.

Do you know?

Fruit bats, humming birds and ants may also act as pollinating agents. Pollination by birds is known as Ornithophily.

Activity

Classify the plants based on the pollination methods.

Vallisneria, Hydrilla, Sunflower, Grass, Brinjal, Maize, Pumpkin.

3. Fertilization

The process of fusion of male (pollen grains) and female (stigma) gametes is called fertilization. The cell which results after fusion of the gametes is called a zygote. The zygote develops into an embryo.

4. Fruits and Seed formation

After fertilization, the ovary grows into a fruit and other parts of the flower fall off. The seeds develop from the ovules. The seed contains an embryo enclosed in a protective seed coat. Based on the number of cotyledons in the seed, the angiosperm plants have been divided into two groups namely, dicotyledon and monocotyledon.

Activity 3

Collect seeds of different plants from your area. Find out whether they are monocotyledons or dicotyledons.

Dicotyledons have seeds with two cotyledons. E.g. Pea, Bean, Castor. They have leaves with netted venation and taproot system.

Mono cotyledons have seeds with one cotyledon. E.g. Maize, Rice, Wheat. They have leaves with parallel venation and fibrous root system.

II. Life Cycle of a Flowering Plant

The major stages of the flowering plants are the germination of seed, growth, flowering, re-production (pollination), seed formation and seed spreading. Every seed has minute plant called the embryo. Under favourable conditions like sunlight, water and soil, embryo is grown up into a new plant. This new plant bears fruits with seeds and multiplies. This cycle continues forming the life cycle of flowering plants.

1. Dispersal of Seeds

Spreading of seeds from one place to another with the help of agents like air, water, animals and birds is known as dispersal of seeds. A single plant produces a large number of seeds. If all these seeds fall directly below the parent plant, the seedlings would have to compete for space, water, oxygen, minerals and sun light. When the seedlings are grouped together in one place, they can easily be destroyed by grazing animals. But, by nature the seedsand fruits of plants are distributed far and wide through various agencies.

* Dispersal by Wind (Anemochory)

The seeds which are smaller, lighter and tiny float in air over long distance. Some of them proceed with hairs and membranous wing like structures and so they are carried away easily. E.g. Cotton seed, Drumstick.

* Dispersal by Water (Hydrochory)

Fruits which are dispersed by water have outer coats modified to enable them to float. The mesocarp (middle layer) of coconut is fibrous and is easily carried away by water. They reach different places and grow into a new plant. E.g. Lotus, Coconut.

* Dispersal by Animals (Zoochory)

Some fruits have hooks, spines, bristles, stiff hair etc, on their outer coat. These fruits stick on the furry coats or skins of some animals and are carried from one place to another. E.g. Xanthium, Achyranthus.

* Dispersal by Birds

While eating fruits like tomato and guava, birds eat seeds also along with the edible portion and they are passed out in the excreta later. These types of seeds are protected from the digestive juices by their seed coat.

* Self Dispersal Method (Autochory)

Some fruits disperse their seeds in the wind through an explosive mechanism and spread them. E.g. Ladies finger, Balsam.

Activity 4

Collect variety of seeds with hair, wings, hooks and spines. Keep them in a card board box separately. Name them and collect information on how they are dispersed.

Do you know?

Man is also responsible for the dispersal of many fruits and seeds. Useful plants like cinchona, rubber and eucalyptus have been successfully introduced by man, to the new surroundings far away from their original habitat.

2. Germination of Seeds

The seed is a fertilized ovule. It consists of embryo, food materials which are protected by the seed coat. During favourable conditions, the seed germinates and gives rise to a new seedling. During the early stages of germination, the seedlings get the food required for its growth from the cotyledons. After the food stored in the cotyledons has been used up, the seedling gets its food from the soil. The seedling absorbs water and nutrients from the soil with the help of its roots. It develops leaves and grows into a plant.

III. Agriculture

Man started practicing agriculture thousands of years back. This was one of the developments of civilization. In the modern days agriculture is practiced on a large scale due to the advancement of science and technology. Application of modern technologies like plant breeding and usage of chemicals like fertilizers and pesticides have increased the yield. Major agricultural products are cereals, vegetables, fruits and oil seeds. They are cultivated not only for our basic needs but also for commercial purpose.

1. Soil

Soil is one of the most important natural resources. It is essential for agriculture. It supports the growth of plants by holding the roots and supplying water and nutrients. It is the home for many organisms. Soil is formed by the breaking of rocks by the action of wind, water and climate. The mixture of rock particles and humus is called the soil. The soil is classified on the basis of the proportion of various sizes.

* Sandy soil

It contains greater proportion of big particles. They cannot fit closely together. Water can drain quickly through the spaces between the sand particles. So, sandy soils tend to be light, well aerated and dry.

* Clay soil

It contains greater proportion of fine particles, packed tightly together, leaving little space for air. It can retain a lot of water in the tiny gaps between the particles. Plants like paddy grow well in this soil.

* Loamy soil

It contains large and fine particles in almost same proportion. The best top soil for growing plants is loam. It is a mixture of sand, clay and another type of soil particle known as silt. Silt occurs as a deposit in river beds. It has right water holding capacity for the growth of plants. Clay and loamy soil are suitable for growing wheat, gram and paddy.

Activity 5

Take a little amount of soil and powder it. Put this soil in a glass tumbler. Mix it with water; stir it well with a small stick to dissolve the soil. Let it undisturbed for some time. Now you can see different layers of soil. The rotting matter floating on the water is called humus. The other layers are clay, sand and gravel. From this we can see that the soil is a mixture of various particles.

2. Classroom Agriculture

Classroom agriculture creates the basic understanding about agricultural practices in the classes. Through this, we come to know about the values and importance of agriculture. To mould us into better members of the society, this programme teaches the connections between agriculture and the environment, food, energy, animals, society, economy, science and technology.

IV. Friends of Farmer

Insects are generally considered to be harmful. But, many of them are helpful to us in many ways. Earthworm, honeybee and dragonfly are useful to plants and farmers.

* Earthworm

Earthworms help to increase the amount of air and water that gets into the soil. They break down organic matters like leaves and grass into smaller particles that plants can use. When they eat them, they leave behind castings that are a type of fertilizer. The process of decomposing bio-degradable wastes by earthworms is known as vermicompost.

Activity 6

Visit a nursery garden near your area and observe how the varieties of saplings are growing there. Prepare a report about it.

* Honey bee

Honey bees are helpful for cross pollination in flowers. They are attracted by the colour and smell of the flowers. They convert the pollen which is the only natural protein source for them into honey. Honey is used as food and also for medicinal purposes. Bees also produce wax, which is used for making candles.

* Dragon fly

It destroys the egg and larva of harmful insects and mosquitoes and prevents the spreading of diseases. It is also helpful in cross fertilization.