Chapter: 02. The Flower
The Flower
Learning Outcomes
After studying this document, you will be able to:
Recognize that flowers are of various shapes, sizes, and colours and are an important part of the plant. Explain the structure and function of each whorl of a complete flower. List the agents of cross-pollination. Learn the process of seed germination and list the conditions required for germination.
Structure of a Flower
A flower is the most attractive and colourful part of a flowering plant. It develops from a floral bud and is a modified shoot. Flowers contain sex organs and play a crucial role in sexual reproduction.
Pedicel: The stalk that attaches a flower to the stem or branch. Thalamus: The upper swollen end of the pedicel to which all floral parts are attached. Sessile flowers: Flowers that do not have a pedicel. Parts of a Flower
A flower has four main parts, arranged in whorls from outermost to innermost:
Fig. 2.1 Structure of a flower
1. Calyx
Description: The outermost whorl of the flower. Composition: Formed of small, green, leaf-like structures called sepals. Function: Sepals enclose and protect the inner whorls of a flower when it is in the bud stage. 2. Corolla
Description: The second whorl from the outside. Composition: Formed of petals. Characteristics: In most flowers, petals are large and brightly coloured. Function: They attract insects for pollination.
3. Androecium (The Male Reproductive Whorl)
Description: The third whorl of a flower, located inner to the petals. Composition: Formed of stamens. Structure of a Stamen: Each stamen has two main parts: Filament: A long, thread-like stalk. Anther: A bilobed, sac-like structure attached to the tip of the filament. Each lobe of the anther contains two pollen sacs. Pollen grains: Yellow, powdery grains formed within the pollen sacs of the anther. Function: Pollen grains contain male gametes which are essential for fertilisation.
Fig. 2.2 Structure of a stamen
4. Gynoecium (The Female Reproductive Whorl)
Description: The fourth and innermost whorl of a flower. Composition: Formed of a single flask-shaped structure called the pistil or carpel. It is the female reproductive part of a flower. Structure of a Pistil: A pistil has three main parts: Ovary: The swollen basal part of the pistil. Each ovule contains a female gamete called the egg or ovum. Fate after fertilisation: The ovary matures into a fruit, and ovules form seeds. Style: The long, thread-like middle part of the pistil. Function: The pollen tube grows through the style towards the ovary. Stigma: The swollen, knob-like tip of the style. Characteristic: It is sticky. Function: Traps pollen grains brought by pollination.
Fig. 2.3 Structure of a pistil
Essential and Accessory Whorls
Floral whorls can be categorized based on their role in reproduction:
Calyx and Corolla are called accessory whorls. Reason: They primarily help in pollination but do not directly participate in reproduction (fertilisation and seed formation). Androecium and Gynoecium are called essential parts or essential whorls. Reason: They are the reproductive organs of a flower and directly participate in fertilisation and seed formation. Studying Flower Parts (Observation from Activities)
When studying different types of flowers, you can observe and record details about their whorls. For instance, in a Hibiscus flower:
Types of Flowers
Flowers can be classified in several ways:
A. Based on Presence of Reproductive Parts
Bisexual Flowers (Hermaphrodite Flowers): Definition: Have both male reproductive organs (stamens) and female reproductive organs (pistil). Examples: China rose (Hibiscus), Pea, Mustard, Gram, Petunia, Rose. Definition: Have either the male reproductive part (stamens) or the female reproductive part (pistil). Examples: Cucumber, Pumpkin, Papaya, Maize, Coconut. Further Classification based on Plant Type:
Dioecious Plants: Bear male flowers on one plant and female flowers on a separate plant. Monoecious Plants: Bear both male and female flowers on the same plant. Examples: Maize, Coconut. B. Based on Presence of Floral Whorls
Definition: Have all four whorls present: sepals, petals, stamens, and pistils. Examples: China rose (Hibiscus), Petunia, Mustard, Pea. Definition: Have one or more whorls missing. Examples: Mulberry, Pumpkin, Papaya. Flower Arrangement:
Solitary Flower: A single flower on a twig at a specific position. Inflorescence: A group or cluster of flowers borne on a twig or branch.
Functions of a Flower
Flowers serve multiple important functions for plants and ecosystems:
Flowers are the reproductive organs of a plant. They produce male and female gametes. Ovules within the flower form seeds. Seeds germinate into new plants, ensuring the continuation of the species. Flowers are widely used for decoration due to their beauty. They are also valued for their fragrance. Smallest flower: Duckweed plant (Wolffia), about 0.5 mm in diameter. Largest flower: Rafflesia, about 1 metre in diameter, found in Indonesia.
A Rafflesia flower The ovary of the flower develops into fruits, and its ovules form seeds. Both fruits and seeds are widely consumed as food by humans and animals. Nectar produced by flowers serves as food for bees, moths, and butterflies, which in turn helps in pollination. Cloves are dried floral buds used for flavouring food and as medicine. Flowers are used for worship in many cultures. They are also used for making garlands and offerings.
Mechanism of Sexual Reproduction in Plants
Sexual reproduction in plants involves the fusion of male and female gametes to form seeds and fruits. It includes the following sequential steps:
Formation of fruits and seeds Dispersal of fruits and seeds Pollination
Definition: The process of transfer of pollen grains from the anther to the stigma of a flower. Significance: It is the first crucial step for the formation of seeds. Types of Pollination
Pollination can occur in two main ways: Self-pollination and Cross-pollination.
Self-pollination: Pollination occurs in the flowers on the same plant. It can happen in two ways: Within the same flower: Transfer of pollen grains from the anther to the stigma of the same flower. Between flowers on the same plant: Transfer of pollen grains from the anther of one flower to the stigma of another flower borne on the same plant. Cross-pollination: Pollination occurs between flowers of different plants of the same species. There is a transfer of pollen grains from the anthers of one flower to the stigma of another flower on two different plants.
Fig. 2.4 Different types of pollination Differences between Self-pollination and Cross-pollination
Agents of Cross-pollination
Wind, water, insects, and animals help transfer pollen grains from the anther of one flower to the stigma of another flower. These are called agents of cross-pollination or pollinators.
Pollination by Wind (Anemophily)
Process: Pollen grains are transferred by wind. Examples: Maize, Wheat, Pine, Palm, grasses. Characteristics of Wind-pollinated Flowers: Colourless or dull, tiny, and without fragrance. Pollen grains: Colourless, tiny, dry, light, and produced in large numbers. Stigma: Large and feathery to easily catch pollen.
Fig. 2.5 Wind pollination in maize Pollination by Water (Hydrophily)
Process: Pollen grains are transferred by water, common in aquatic plants. Examples: Vallisneria, Hydrilla. Male and female flowers are on separate plants and remain underwater. On maturing, male flowers detach and float to the surface. Female flowers rise to the water surface and float. When male flowers contact female flowers, pollen grains stick to the stigma, completing pollination. Characteristics of Water-pollinated Flowers: Pollen grains are light and produced in large numbers. Male flowers are very small.
Fig. 2.6 Water pollination in Vallisneria Pollination by Insects (Entomophily)
Process: Most flowers are pollinated by insects like honeybees, wasps, moths, and butterflies. When an insect visits a flower for nectar, pollen grains stick to its body. When the same insect visits another flower, these pollen grains are brushed off onto the stigma. Examples: Rose, Marigold, Salvia, Night jasmine, Chrysanthemum, Dahlia. Characteristics of Insect-pollinated Flowers: Have bright colours (e.g., red, orange, yellow) to attract insects. Some flowers are sweet-smelling (e.g., Night jasmine) to attract nocturnal insects. Some flowers produce nectar to attract insects. They have sticky pollen grains and a sticky stigma. Note: Lotus and Trapa are aquatic plants, but their flowers are exposed to air and are pollinated by insects, not water.
Fig. 2.7 Insect pollination in Salvia Differences between Insect-pollinated flowers and Wind-pollinated flowers
Pollination by Animals (Zoophily)
Some animals like birds and mammals (e.g., Squirrel, Bat, and even humans) also assist in cross-pollination.
Fertilisation
Upon reaching the stigma, a pollen grain germinates and forms a pollen tube. The pollen tube grows downwards through the style and enters the ovary to reach an ovule. Inside the ovule, the pollen tube releases male gametes. One male gamete fuses with the female gamete (egg) inside the ovule. This fusion produces a zygote. Definition: The fusion of male and female gametes is called fertilisation. Outcome: After fertilisation, the ovary develops into a fruit, and the ovules develop into seeds.
Fig. 2.8 Germination of pollen grain into a pollen tube that carries male gametes to the egg cell to complete the process of fertilisation
Formation of Fruit (Fate of Each Whorl of Flower)
After fertilisation, significant changes occur in the flower, leading to fruit and seed formation:
The ovary remains attached to the stalk of the flower and grows into a fruit. The ovules inside the ovary develop into seeds. The wall of the ovary develops into the fruit wall. Other parts of the flower, such as sepals and petals, generally fall off. The style and stigma also fall off. The stamens also fall off. Note: In some flowers like tomato, brinjal, lady’s finger, sepals may remain attached even after fruit formation.
Parts of a Fruit
A fruit is a ripened ovary. It primarily consists of two main parts:
Pericarp (Fruit wall)
Development: The pericarp develops from the ovary wall. Thickness: It can be thick (in fleshy fruits) or thin (in dry fruits). Layers: It has three distinct layers: Epicarp: The outermost thin covering of the fruit (e.g., skin of a mango). Mesocarp: In fleshy fruits, this is the sweet, pulpy, and fleshy middle layer of the pericarp (e.g., the edible part of a mango). Endocarp: The inner hard part of the pericarp (e.g., the hard shell around the mango seed).
Fig. 2.9 Fruit wall of a fleshy fruit (mango) Dry Fruits and Fleshy Fruits
Fruits are broadly categorized based on their pericarp:
Pericarp: Dry, thin, and non-edible. 
Self Study
Fig. 2.1 Structure of a flower
Fig. 2.2 Structure of a stamen
Fig. 2.3 Structure of a pistil
Fig. 2.4 Different types of pollination
Fig. 2.5 Wind pollination in maize
Fig. 2.6 Water pollination in Vallisneria
Fig. 2.7 Insect pollination in Salvia
Fig. 2.8 Germination of pollen grain into a pollen tube that carries male gametes to the egg cell to complete the process of fertilisation
Fig. 2.9 Fruit wall of a fleshy fruit (mango)