CBSE Class 6 Science Advanced Biology Getting to Know Plants Notes Set 01

Getting to Know Plants

Introduction

Plants are found everywhere around us. They are living things that stay fixed at one place, take in water and minerals through their roots and make their own food with sunlight and a green pigment called chlorophyll. Like animals, plants have organs and organ systems, but these are quite different from ours.

1.1 Different Types of Plants

On the basis of height, the kind of stem and the level at which branches arise, plants are placed in five groups.

(a) Herbs. These are small plants with soft, green, easily damaged stems. They are usually less than one metre tall and live for one or two seasons. Examples: grass, maize, rice, mint, coriander, mustard.

(b) Shrubs. They have hard, woody stems, and several branches of nearly equal size come out from just above the ground, so they look like bushes. They are medium-sized and live longer than herbs but not as long as trees. Examples: China rose, lemon, pomegranate, jasmine, nerium.

(c) Trees. They are tall and big, with one thick, hard, woody stem called the trunk. Branches, twigs and leaves arise at some height above the ground. Trees live for many years. Examples: mango, neem, banyan. Coconut and palm are trees too, although their trunk does not branch.

(d) Creepers. Their stems are weak and cannot stand up, so they spread along the ground. Examples: sweet potato, pumpkin, water melon, bottle gourd, bitter gourd.

(e) Climbers. Their stems are also weak, but they climb up by taking support from a nearby object. Examples: betel, grape, money plant, beans.

1.2 Parts of a Flowering Plant

A flowering plant has two main systems: the root system below the soil and the shoot system above it.

(a) Root System

The root system is the underground, non-green part of the plant. It is made of a main root and its branches.

Features of roots. They grow from the radicle of the seed, move towards soil and water and away from light, have no nodes or internodes, and give out many side branches (secondary and tertiary roots).

Parts of a root.

1. Primary root: the main root, thick, growing straight down from the radicle.
2. Secondary roots: side branches coming out of the primary root.
3. Tertiary roots: branches of the secondary roots.
4. Root cap: a cover over the tip of every root branch. It guards the soft, growing tip while the root pushes through soil.
5. Root hairs: very fine hairs on the roots. They take in water and minerals from the soil.

Types of Root System

Tap root system. The primary root stays thick and long and goes straight down. It is also called the true root. It is found in dicots, for example mustard, gram, bean, pea, mango, carrot and radish.

Fibrous root system. The primary root lives only for a short time and is replaced by a bunch of thin, fibre-like roots that spread from the base of the stem. It is found in monocots, for example maize, wheat, rice, barley and grasses.

Modifications of Roots

Besides holding the plant and absorbing water, the roots of some plants change their form to do other jobs.

1. Storage of food. The taproots of carrot, radish, turnip and beetroot become swollen and fleshy because they store food. Dahlia and sweet potato store food in root tubers, which are swollen adventitious roots.

2. Support. Two kinds of roots give extra support.
Stilt roots: in bamboo, maize and sugarcane, extra roots grow slanting down from the lower nodes and brace the plant.
Prop roots: in banyan and the rubber tree, rope-like roots grow down from the horizontal branches, enter the soil and work like pillars to carry the heavy branches.

3. Climbing. Betel, black pepper and money plant have climbing roots from their nodes. They stick to a wall, rock or tree and help the plant to go up.

4. Parasitic roots. A plant like dodder sends special roots into its host plant and sucks water and food from it.

5. Multiplication. The roots of dahlia and asparagus can grow into new plants.

6. Pneumatophores (breathing roots). Plants of marshy places, such as mangroves, have aerial roots that stand out of the mud and take in air for gaseous exchange.

Functions of Roots

Fixation. Roots hold the plant tightly in the soil.

Activity 1. Aim: to show that roots anchor a plant in the soil. Sow a few gram seeds in a pot of garden soil and water them daily. After a week try to pull out a seedling and notice the force needed. Keep the pot for one more week and try again with a bigger plant. Observation: a larger force is needed the second time, as the root of the grown plant is bigger and has gone deeper. Conclusion: roots fix a plant in the soil.

Absorption. The root hairs take in water and minerals from the soil.

Activity 2. Aim: to show that roots absorb water. Place a radish in a glass of red-coloured water and leave it for a day. Next day cut the radish lengthwise. Observation: red colour has spread through the radish, with red lines along its length. Conclusion: the red colour shows the path of water, so roots absorb water.

(b) Shoot System

The shoot system is the part of the plant above the ground. It includes the stem, branches, leaves, flowers and fruits, and it develops from the plumule of the seed.

(i) Stem

The stem is the main axis of the shoot system. It is green in herbs but hard and woody in shrubs and trees. The stem of a tree is called the trunk, and it is covered by bark.

Features of the stem. It grows towards light, away from soil and water. It has nodes and internodes. It bears leaves, branches, buds, flowers and fruits. Buds in the angles of leaves (axillary buds) grow into new branches. Young stems are green; old stems are woody and not green.

Functions of the stem.

1. Support: it holds up the branches, leaves, flowers and fruits and keeps leaves spread out to receive sunlight.
2. Conduction: it carries water and minerals from the roots up to every part, and it carries the food made in leaves to other parts.

Activity 3. Aim: to show that water is conducted through the stem. Materials: a glass, water, red ink and a twig with a white flower (such as balsam or petunia). Procedure: fill a third of a glass with water, add a few drops of red ink and stir. Cut the twig slantwise and put it in the glass. Leave it overnight and look the next morning. Observation: the petals and the veins of the leaves have turned red. Conclusion: coloured water has gone up through the stem to the leaves and flowers. The thin red lines are the xylem vessels, which carry water from roots to the rest of the shoot.

Modifications of the Stem

Some stems change to do special work besides support and conduction.

1. Making food. The green stems of herbs make food by photosynthesis. In cacti and other succulents, the stem becomes green and flat like a leaf and prepares the food, while the leaves are reduced to spines.

2. Storing food. Several underground stems are swollen with stored food.

Rhizome: in ginger the underground stem swells into an irregular shape. It has nodes, internodes, buds and scale leaves.
Tuber: a swollen underground stem used to store food, which helps the plant live through winter or bad conditions. It carries buds (the eyes of a potato).
Corm: a special tuber, round and compact, covered with scale leaves, for example gladiolus and zimikand.
Bulb: in onion and garlic, fleshy leaves store food. They overlap one another and are attached to a disc, which is the very short stem.

3. Stem tendrils. In plants such as grapevine and passion flower the stem is weak, and some branches turn into thin, thread-like coils called stem tendrils. They twist around a support and hold the plant up.

Activity 4. Aim: to show that a potato has buds that grow into shoots. Look at a potato and find the small hollows (eyes) that hold buds. Keep it for some days or place it in moist soil for a week, then take it out. Observation: the buds in the eyes have started to grow into shoots above ground. Conclusion: a potato bears buds that grow into new shoots.

(ii) Leaves: the Food Factories of the Plant

Parts of a leaf.

1. Petiole: the stalk by which the leaf is joined to the stem.
2. Lamina: the broad, flat, green blade of the leaf.
3. Midrib: the thick vein running through the middle of the lamina.
4. Veins: the lines spread over the lamina.

Leaf venation. The design made by the veins in a leaf is its venation.

Reticulate venation: the veins form a net-like pattern on both sides of the midrib, as in mango and pea leaves.
Parallel venation: the veins run side by side, as in grass leaves.

Functions of a Leaf

1. Making food (photosynthesis). A green plant makes its own food by photosynthesis. In this process the leaves combine carbon dioxide and water, using chlorophyll and sunlight, to make food, and they release oxygen. Oxygen and water are the by-products. As a word equation: carbon dioxide + water, in sunlight and chlorophyll, gives glucose + oxygen.

2. Transpiration. The roots take in more water than the plant needs. The extra water leaves the leaf surface as vapour through tiny pores called stomata. This loss of water vapour is transpiration, and plants give out a large amount of water in this way.

3. Gaseous exchange. During photosynthesis and respiration, gases pass in and out of the leaf through the stomata.

Types of Leaf

Simple leaf. The leaf has one undivided lamina. Examples: mango, guava, money plant.

Compound leaf. The lamina is cut into several small parts called leaflets, all attached to the same petiole. The leaflets may be set on the two sides of the petiole as in rose and neem, or may spring from one point at the tip of the petiole as in palm.

Modifications of Leaves

1. Leaf tendrils: in climbers such as sweet pea, leaves or leaflets turn into spring-like threads that coil round objects and help the weak stem climb.

2. Leaf spines: in prickly poppy the leaf margins have spines that protect the plant from grazing animals. In cacti the whole leaf is a spine, which prevents loss of water by transpiration.

3. Insect-eating (insectivorous) plants: the leaves change into a pitcher, a bladder or a flytrap to catch and digest insects, as in pitcher plant and Venus flytrap.

4. Vegetative propagation: in plants like Bryophyllum and Begonia, buds appear in the notches of the leaf margin and grow into new plants.

(iii) Flower

A flower is a changed shoot in which nodes and internodes are squeezed very close. It grows from a floral bud and is the place where sexual reproduction takes place in plants.

Parts of a flower. Flowers differ in size, shape and colour, but all have the same basic parts: pedicel and thalamus, calyx, corolla, androecium (stamens, male part) and gynoecium (pistils, female part).

1. Pedicel and thalamus. The flower is borne on a stalk, the pedicel. The swollen upper end of the pedicel is the thalamus, which carries all four whorls.

2. Calyx. It is the first, outermost whorl, made of leaf-like sepals. Sepals are usually green and can make food. They also guard the delicate inner parts while the flower is a bud.

3. Corolla. It is the second whorl, made of petals. Petals are usually brightly coloured because of pigments. Their colour, sweet smell and nectar attract insects that help in pollination.

4. Androecium (stamens). It is the third whorl and is made of one or more stamens. A stamen has three parts: the filament (stalk), the connective (joins the anther to the filament) and the anther at the top. The anther has two lobes, so it is bilobed, and each lobe has two pollen sacs holding millions of tiny pollen grains (microspores), which look like yellow dust.

5. Gynoecium (pistils). It is the fourth whorl, at the centre of the flower, and is made of one or more carpels. Free carpels are called pistils. A pistil has three parts.

Ovary: the swollen base. It carries the ovules on a raised tissue called the placenta, and each ovule holds a female reproductive cell.
Style: the long stalk rising from the ovary and joining it to the stigma. It lifts the stigma up.
Stigma: the tip of the style. It is generally rough, hairy and sticky and receives the pollen grains in pollination.

Types of Flowers

Complete flowers have all four whorls, for example China rose, mustard, rose and pea. Incomplete flowers lack one of the four whorls, for example pumpkin, water melon, wheat and grass.

Functions of Flowers

1. Flowers are the reproductive organs of the plant.
2. The ovary becomes the fruit and the ovules become the seeds.
3. Honey bees collect the nectar to make honey.
4. Perfumes are made from flowers.
5. Cloves are dried flower buds, used to flavour food and to prepare several Ayurvedic medicines.

Add to Your Knowledge

1. Photosynthesis produces oxygen and water in plants.
2. Monocots have fibrous roots and dicots have tap roots.
3. Essential and non-essential whorls: stamens and pistil are the essential whorls, because they are the male and female reproductive organs and take part directly in reproduction. Calyx and corolla are accessory whorls, because they are not directly involved.
4. Pollination: moving pollen grains from the anther to the stigma. In self-pollination the pollen goes to the stigma of the same flower; in cross-pollination it goes to the stigma of a different flower.
5. Fertilization: the joining of a male gamete with a female gamete. A pollen grain carries two male gametes.
6. After fertilization the ovule turns into the seed and the ovary wall into the fruit wall.
7. The covering of the seed has an outer hard layer, the testa, and an inner layer, the tegmen.
8. A seed has cotyledons (store food) and an embryo that has a radicle (future root) and a plumule (future shoot).
9. Seed dispersal is the carrying of seeds away from the parent plant. There are five ways: wind, water, animals, explosion and gravity.

Summary

1. Plants are grouped as herbs, shrubs, trees, creepers and climbers by height, stem type and where branches arise.
2. A root system has a primary root with secondary and tertiary branches; root hairs absorb water and minerals and the root also fixes the plant.
3. The two root types are tap root and fibrous root.
4. The shoot is made of stem, leaves, flowers and fruits; the stem carries water and minerals from the roots to all parts.
5. A leaf has a petiole and a lamina with a midrib and veins; veins may be reticulate or parallel. Plants with parallel venation have fibrous roots, and those with reticulate venation have tap roots.
6. Green plants make food by photosynthesis from carbon dioxide and water with chlorophyll and sunlight; extra water leaves as vapour by transpiration.
7. A flower has a pedicel and thalamus bearing the calyx (sepals), corolla (petals), androecium (stamens) and gynoecium (pistils).

Concept Map