ICSE Class 6 Biology Chapter 5 The Structure and Functions of Plant Parts

The Structure and Functions of Plant Parts

We see plants of various types around us. Though these plants differ in many ways, for example, some may be small, some may be big, their leaves may be of different types and size, their flowers may also vary in colours, size and shape, etc. However, the overall structure of these plants is similar. All of them have two parts, one part above the ground, called the shoot system and one part below the ground, called the root system (Fig. 5.1).

The root system consists of the roots. The shoot system consists of the stem, branches, leaves, flowers and fruits.

The Root System

The roots may be of two types, namely, primary root and the secondary root. The primary root arises from the base of the stem. The primary root further bears side roots called secondary roots. The end parts of the primary or secondary roots have fine hair-like structures, called root-hairs. The primary root, the secondary root and the root hair together form the root system.

There are two types of root system, based on their structures. These are tap root system and fibrous root system.

Tap Root System

Some plants have a main root called a primary root that grows straight down into the soil. The root is broad at its origin, that is, at the base of the stem and thin at the tip. It develops many side roots called secondary roots (Fig. 5.2a). The primary root and the secondary roots together constitute the tap root system. Plants like gram, pea, mango and mustard have a tap root system.

Fibrous Root System

Plants like grass have a cluster of roots arising from the base of the stem. These roots are almost of the same size and thickness. These roots spread out in all directions and do not reach very far into the soil (Fig. 5.2b). Maize wheat and rice have a fibrous root system.

Go to your home garden or field and pull out a small Hibiscus plant. Remove the soil with water and observe its root system. You will observe a primary root, the secondary roots and the further branching roots. On careful examination you will find many hair-like outgrowths on the roots. These are the root hair.

Go to your home garden or field and pull out grass from the soil. Remove the soil with the help of water. What do you observe? You will observe a cluster of roots arising from the base of the stem. These are fibrous roots. Compare these roots with the tap root of the Hibiscus plant.

In some plants, roots grow from the base of the stem. They can grow out of other parts of stem or from the leaves also. Such roots are called adventitious roots.

Functions of the root

Roots fix the plant firmly to the ground.

Plants need water and minerals to manufacture food. Roots absorb these from the soil and transport them to the leaves and stem for manufacturing of food.

Soil particles remain stuck to the roots. This prevents the soil from being blown away by the wind or washed away by rain.

Modification of Roots

Besides these primary functions, roots of some plants are modified to perform other functions also. These are

to store food,

to provide additional support to the plant,

to help some plants in respiration, and

to absorb more nutrients and water.

For Storage of Food

Roots of some plants get modified to store food. These roots look swollen because of the food stored in them. They acquire different shapes. Some examples of primary roots modified to store food are carrots, radishes and turnips (Fig. 5.3).

Some adventitious roots also get modified to store food. Some examples are sweet potato and dahlia.

For Additional Support

In plants like banyan tree and screwpine, aerial roots grow vertically downward from the branches. Most of them become long enough to reach deep into the soil. These roots provide additional support to the heavy branches, and are called prop roots (Fig. 5.4).

In plants like sugar cane and maize where the stem is weak and tall, additional roots arise from the lower part of the stem and fix the plant firmly. Such roots grow obliquely and are called stilt roots.

In plants like money plants and Indian ivy having weak green stems, aerial roots develop in clusters from the stem. These roots help the plant to climb upwards by attaching themselves to a support. Such roots are called climbing roots (Fig. 5.5).

For Respiration

In certain plants like mangroves growing in waterlogged areas, cone-shaped roots grow vertically upwards and come out of the ground. These roots are known as pneumatophores and have pores on them for taking in air from the atmosphere. This air is used for respiration.

For Absorbing More Nutrients and Water

In plants like money plant, long, aerial, adventitious (fibrous) roots called feeder roots develop. These roots reach the soil and absorb more nutrients and water.

The Shoot System

The aerial part of the plant which grows above the soil is called the shoot system. It consists of any or all of the following parts, that is, stem, leaves, buds, flowers and fruits. The stem is the main part of the shoot system. We see around us mostly herbs, shrubs and trees. These plants have different types of stems (see Table 5.1).

The stem bears nodes, internodes and buds, and holds up the leaves, branches, flowers and fruits. The points on the stem from where leaves and branches arise are called nodes. The portion of the stem between two successive nodes is called an internode. Some small outgrowths can be seen on stems. These are called buds. The buds that grow at the tips of the stem or its branches are called apical buds or terminal buds. These buds have a young stem crowded with young leaves or flowers. The growth in a terminal bud leads to an increase in the length of the stem. Buds are also present in the axil of the leaf. An axil is the angle between a leaf and the stem. Such buds are called axillary buds. Axillary buds give rise to new branches.

Buds are of two types, that is, vegetative and floral. Vegetative buds give rise to branches and leaves whereas floral buds give rise to flowers.

Functions of the Stem

Stem (except in climbers and creepers) provides support to the branches, leaves, flowers and fruits, or the whole of the plant.

Stem conducts water and minerals upwards, that is, from the root to the leaves, and transports food from leaves to all parts of the plant.

In some plants, thick green stems rich in chlorophyll, carry out the function of manufacture food by photosynthesis.

Modification of Stems

Stems of some plants are modified to perform special functions such as making food, storing food and so on.

For Storage of Food - Underground Modifications of the Stem

In some plants, stems do not grow above the soil but they grow underground. Such stems are called underground stems. These stems store large quantity of food in them. Modified underground stems have different structures, and are therefore given different names, as discussed below:

Rhizome

Rhizomes are swollen stems that grow horizontally. They are thick, and branched. They have nodes, internodes, prominent buds and scaly leaves. New leaves and flowers develop from a bud. Some examples are ginger, turmeric and ferns.

Tuber

Tuber is an enlarged, roughly spherical-shaped stem storing food. Nodes and internodes are not distinct. Many buds, commonly called 'eyes' are present on these stems.

On sowing, these buds or 'eyes' can develop into new plants. Potato is the most common example of tuber.

Bulb

In a bulb, the stem is highly condensed into a flat disc at the bottom. It has a terminal bud which is covered by many scaly leaves. The inner scaly leaves are usually fleshy and store food and water. The outer scaly leaves are dry. Some examples are onion, tulip, lily and garlic.

To Provide Protection, Prepare Food, Provide Support - Aerial Modifications of the Stem

In some plants stems get modified to perform various functions. These are discussed here:

Stem Tendril - To Provide Support

In certain plants like passion flower and grapevine which have weak stems, some axillary buds grow into thin, thread-like coiled structures called tendrils. Tendrils coil around some nearby object like a wall or tree and help the plant to climb.

Thorn - To Provide Protection

Some buds of certain plants such as rose and Duranta get modified into hard, pointed structures called thorns. These thorns act as defensive organs to keep grazing animals away from the plants. Also, thorns prevent excessive loss of water during transpiration.

Phylloclade - To Prepare and Store Food

Stems of certain desert plants like Opuntia develop into thick, flat, short and green structures with nodes and internodes. Since the leaves in such plants are reduced to spines to check the loss of water, stems carry out the function of preparation and storage of food. The fleshy stem also stores water, which is used during long dry periods.

Desert lily has a bulb that sends up a stem in early spring which can be 1 to 4 feet high.

Take some specimens of underground stems such as potato, onion and ginger. Observe the nodes and internodes in them. Also draw their structures and compare them with each other.

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