ICSE Class 6 Biology Chapter 4 The Plant Structure and Functions

Unit 4: The Plant Structure and Functions

Syllabus

The structure, functions and some modifications of plant parts:

1. Root - Storage roots: e.g. carrot, radish, turnip, beetroot (without mentioning the terms - fusiform, napiform)

Support - prop roots - banyan

2. Stems - Underground stems: tuber, bulb, rhizome - suitable examples

Stem tendrils, thorns, climbing stems (with examples)

3. Leaves - venation, simple/compound leaves, arrangement

Some modification with examples (e.g. prickly pear)

4. Flowers - parts and their functions.

Pollination - agents and type of pollination

Fertilization - fruit, seed dispersal

Parts of a flower - using specimens like lily/hibiscus. Students to observe/draw parts.

Practical observation and drawing of the different parts of plants available in school garden/park, the variations in them (E).

(Teacher may also draw attention of students to the shape, texture, edge of leaves)

Modifications - specimens - to be observed and drawn (E)

Introduction to Plant Parts

You see a variety of small and large plants in your surroundings such as, the rose, balsam, mango, neem, peepal, mint, rose, balsam, mango, neem, peepal, mint, etc. They have variations in height, shape of leaves, colour of flowers, etc. Despite these variations they have some structural similarities for example, majority of plants have roots.

Activity 1

Carefully dig out a small plant like balsam from the soil. Gently wash the portion, which was under the ground, with water.

Take a large-sized beaker or tumbler containing some water. Keep the plant in it with its underground parts submerged in water.

Observe the plant carefully and compare its parts. Identify the two main parts of the plant - one which was under the ground known as the root system, and the other which was above the soil known as the shoot system.

Parts of a Plant

Root system | Shoot system

Let us discuss both the systems in detail.

A. The Root System

The underground part of the plant is called root. It has the following main characteristics:

1. It is not green in colour.

2. It grows downward into the soil away from the sunlight.

3. It never bears leaves, flowers, fruits or buds.

4. It has one main thick primary root with many side branches. The end parts of the root branches bear fine hair-like processes called root-hairs. The primary root with all its branches and the root-hairs is called the root system.

Types of Root System

The root system is of two types -

(i) tap root system

(ii) fibrous root system.

(i) Tap Root System

It has a thick main root called primary root and bears many side branches called secondary roots. Such a root system is found in dicot plants such as gram, pea, etc.

(ii) Fibrous Root System

It has a cluster of roots of the same thickness and size arising from the base of the stem. This type of root system is found in monocot plants such as maize.

There is another way of classifying the roots on the basis of their origin - True roots and Adventitious roots.

Teacher's Note

Plants in your home garden like tomato or mint show these different root systems. Dig carefully to observe the roots without damaging them.

True roots develop from the radicle (Fig. 4.4) of the germinating seed e.g., pea, carrot.

Adventitious roots grow from the stem or leaves, e.g., banyan tree (Fig. 4.5).

Modifications of the Root

The roots undergo modifications to perform mainly the following two functions:

(i) Storage of food

(ii) To provide additional support.

(i) Roots for Storage of Food

The primary roots usually get thickened to store food. They acquire several shapes. For example, roundish (turnip), conical (carrot) or spindle-like (radish) (Fig. 4.6).

The adventitious roots are sometimes swollen to store food. Examples are:

(1) tuberous roots (as in sweet potato, tapioca, etc.) (Fig. 4.7a).

(2) nodu lose roots (as in turmeric, arrow root, etc.) (Fig. 4.7b).

(3) fasciculated roots (arranged in bundles as in dahlia, asparagus, etc.) (Fig. 4.7c).

(ii) Roots for Additional Support

Sometimes, the roots such as prop roots, stilt roots and climbing roots get modified to give support to the plant.

(a) Prop roots - These are aerial roots which grow vertically downward from the horizontal branches. As they from the horizontal branches. As they touch the ground, they go deep into the touch the soil. They look like ropes or pillars, e.g., banyan, screw pine, etc. (Fig. 4.8a).

(b) Stilt roots - Plants with weak stem such as sugarcane, wheat, maize, etc., give rise to adventitious roots from the lower nodes. They grow obliquely downwards like stilts (Fig. 4.8b).

(c) Climbing roots - Weak green stem develops a cluster of aerial roots from their nodes or internodes. They attach themselves to a support and help the plant to climb upwards, e.g., money plant, Indian ivy, etc. (Fig. 4.8c).

Roots for Special Functions

(i) Nitrogen fixing roots (Fig. 4.9a): In leguminous plants (gram, pea, etc.), roots develop small swellings called nodules which contain bacteria. These bacteria fix nitrogen of the soil into soluble nitrates which increases the fertility of the soil.

(ii) Breathing roots or pneumato- phores (Fig. 4.9b): In plants which are found in marshy or water logged land, the roots come above the ground like cones. They have minute pores through which they take atmospheric air for respiration e.g., white mangrove.

(iii) Aerial or epiphytic roots (Fig. 4.9c): These roots are present in those plants which grow aerially on the host plant, e.g., orchids. They have hanging roots which draw moisture from the air through spongy tissues found on their tips.

(iv) Sucking roots (haustoria) (Fig. 4.9d): They are found in parasite plants, e.g. dodder (Cuscuta). These plants send adventitious roots into the host to suck nutrients.

Functions of the Root

1. Fixes the plant in the soil.

2. Absorbs water and minerals from the soil for the growth of the entire plant.

3. Acts as a storage part for food materials for certain plants.

4. Binds the soil together so that it does not get washed away during rain or blown away by the wind.

B. The Shoot System

The part of a plant which grows above the soil forms the shoot system. The shoot bears stem, buds, leaves, flowers and fruits.

(i) The Stem

The stem is the main aerial part of the shoot system. The points on the stem from where the leaves and branches originate are called nodes. The part of the stem between two nodes is called an internode. The undeveloped shoot called bud is present at the tip of the stem. The growing tip of the shoot is called apical bud or terminal bud.

The buds present between the stem and the base of leaves are called axillary buds. The axillary buds produce new branches and help in growth.

Modifications of Stem

Like roots, the stem also undergoes three categories of modifications to perform various functions - (a) under-ground, (b) sub-aerial and (c) aerial.

(a) Underground Modification of Stem

Stems of most plants grow above the ground. But some stems grow below the ground also, and these underground stems. The underground stems store food in large quantity. Some of their examples are as follows:

(1) Tuber (Fig. 4.11a): Potato is an example of a tuber. The lower branches of its stem grow underground and swell up at the tips to store food. It has a number of buds called 'eyes'. Each "eye" can produce a shoot of the new plant on sowing.

(2) Rhizome (Fig. 4.11b): Rhizome is irregular in shape and grows horizontally below the soil. It has scaly leaves, buds, nodes and internodes. Examples: Ginger, banana, ferns.

(3) Bulb (Fig. 4.11c): It has a condensed disc-shaped stem and a terminal bud overlapped by numerous scaly leaves. Inner leaves are fleshy, thick and store food. Outer dry and scaly leaves are protective. Examples: Onion, garlic, lily.

(4) Corm (Fig. 4.11d): It is somewhat round, condensed, flattened from top to bottom and is covered by thin scaly leaves. Examples: Gladiolus, Amorphophallus (zimikand).

(b) Sub-aerial Modifications of Stem

Certain stems, which are weak and green, grow horizontally on the surface of the soil. Some of their types are as follows.

(1) Runner (Fig. 4.12a): A runner is a soft, thin slender branch which creeps on the ground e.g., doob grass and wood-sorrel.

(2) Stolon (Fig. 4.12b): Stolon grows upwards above the ground to some distance, then bend down to the ground, e.g., mint and wild strawberry.

(3) Offset (Fig. 4.12c): Offsets are like runners. They arise from the axil of the leaf and have thick internodes, e.g. water lettuce and water hyacinth.

(c) Aerial Modifications of Stem

Certain stems modify aerially to perform various functions like protection, preparation of food, support and vegetative reproduction. Vegetative and floral buds grow into tendrils and thorns. Examples are given below:

(1) Tendril (for support) (Fig. 4.13a): The stem of some plants occur in the form of thin thread-like leafless branch called tendril. The tendril coils around any object near it and helps the plant to climb up. For example, grapevine, gourds, etc.

(2) Thorn (for protection) (Fig. 4.13b): In some plants, a bud is modified into a hard, point like small structure called thorn. The thorns protect these plants from the grazing animals. For example, rose, lemon, duranta, etc.

(3) Twiner (for climbing) (Fig. 4.13c): The stems of some plants like morning glory twine around their own stem. This twining helps the stem to climb up easily.

(4) Phylloclade (Fig. 4.13d): In cactus like opuntia, the stem is thick, flat, green and with nodes and internodes. In such stems, food is prepared and stored, and leaves are thin and reduced in the form of spines to prevent loss of water. Examples: cactus, ruscus, etc.

Functions of Stem

1. Stem bears all the aerial parts of the plant-leaves, flower and fruits in their suitable positions.

2. Stem helps in the upward movement of water and minerals absorbed by the roots and transports them upto the leaves, flower and fruits.

3. Food prepared by the leaves is conducted downwards to the root by the stem.

4. Stem also manufactures food when green and young.

(ii) The Leaf

The flat, green and broad parts of the shoot, located on the node of the stem, are the 'leaves'. Leaves develop from the nodes. There is always a bud in the axil of a leaf (axil is the angle between the leaf and the stem). Leaves do not continuously grow like the stem but stop growing on attaining full size.

Parts of a Leaf

A typical leaf has the following parts:

Leaf base (Petiole): The basal part of a leaf is like a stalk; with this, it attaches itself to the stem at the node.

Sometimes, leaves are directly attached to the stem without a leaf stalk. Such leaves are called 'sessile', e.g., zinnia.

Leaf blade or Lamina: The green, flat and broad part of the leaf is called 'lamina' or 'leaf blade'.

Midrib: Petiole enters into the leaf like axis, called 'midrib'. This bears branches called 'veins'. Petiole, midrib, veins and veinlets (further branches of veins) conduct water. Veins also provide a skeleton or a supportive framework to the leaves.

Types of Leaves

Leaves can be classified in various ways:

On the Basis of Shape

(i) Needle shaped, e.g., pine, onion.

(ii) Oval, e.g., guava, apple.

(iii) Heart-shaped, e.g., peepal.

(iv) Oblong, e.g., banana.

(v) Circular, e.g., lotus, nasturtium.

(vi) Tapering, e.g., eucalyptus, ashoka.

On the Basis of Margin

(i) Complete or entire margin, e.g., peepal.

(ii) Toothed or serrate margin, e.g., china rose, rose.

(iii) Wavy margin, e.g., ashoka, mango

Teacher's Note

Look at leaves on plants near your home - the margins and shapes vary greatly, helping plants adapt to their environment.

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