Reproduction in Angiosperms

The flower is the primary reproductive organ of flowering plants. Flowers vary in size, shape, color, and arrangement patterns.

Parts of a Flower

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1. Peduncle/ Stalk: The stalk of the flower.
2. Receptacle: The small part at the base of the flower to which the stalk is attached.
3. Calyx/ Sepals: Small leaf-like parts at the base of the petals, forming the outermost layer (calyx). They protect the flower in its bud stage.
4. Corolla/ Petals: Brightly colored parts that attract pollinators like insects and butterflies. Collectively, they are called the corolla.
5. Androecium/ Stamens: The male reproductive parts, collectively known as the androecium. Each stamen consists of:
   • Anther: The head of the stamen that produces pollen.
   • Filament: A slender stalk that holds the anther.
6. Gynoecium/ Carpel (Pistil): The female reproductive part, collectively known as the gynoecium. It includes:
   • Stigma: The sticky tip that captures pollen.
   • Style: A tube-like structure that connects the stigma to the ovary.
   • Ovary: The base of the pistil that contains ovules. Fertilized ovules develop into seeds, and the ovary becomes the fruit.

Types of Flowers

• Hypogynous Flower: The ovary is superior, and other floral parts are arranged below it, e.g., Hibiscus.
• Perigynous Flower: The ovary is half-inferior and located in the center of a cup-shaped receptacle, e.g., Rose.
• Epigynous Flower: The ovary is inferior and embedded in the receptacle, with other floral parts above it, e.g., Sunflower.
• Inflorescence: A group of flowers attached to a common stalk.
• Solitary Flower: A single flower attached to a stem or leaf axis, e.g., Hibiscus.
• Perfect Flower: Contains both male (stamens) and female (carpels) parts, e.g., Pride of Barbados.
• Imperfect Flower: Missing either stamens or carpels, e.g., Maize.
• Complete Flower: Has all four floral parts (calyx, corolla, androecium, gynoecium), e.g., Hibiscus.
• Incomplete Flower: Lacks one or more floral parts, e.g., Maize.
• Regular Flower: Symmetrical flowers with identical whorl members, e.g., Hibiscus.
• Irregular Flower: Asymmetrical flowers with uneven whorl members, e.g., Pride of Barbados.
• Bisexual Flower: Contains both male and female parts, e.g., Hibiscus.
• Unisexual Flower: Contains only male or female parts, e.g., Maize.
• Monoecious Plant: Has both male and female flowers, e.g., Maize.
• Dioecious Plant: Male and female flowers are on separate plants, e.g., Pawpaw.

Types of Ovaries

1. Superior Ovary: Positioned above other floral parts, e.g., Hibiscus. These flowers are known as Hypogynous flowers.
2. Half-Inferior Ovary: Partially embedded in a cup-shaped receptacle, e.g., Rose. These flowers are known as Perigynous flowers.
3. Inferior Ovary: Fully embedded below other floral parts, e.g., Sunflower. These flowers are known as epigynous flowers.

Placentation

Placentation refers to the arrangement of ovules within the ovary. Ovules are attached to fleshy structures called placentae.

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Types of Placentation

1. Marginal: Ovules are attached along one side of the ovary, e.g., Cowpea.
2. Parietal: Ovules are attached to the ovary's walls, e.g., Pawpaw.
3. Free-Central: Ovules are attached to a central knob, e.g., Cana Lily.
4. Axile: Ovules are attached at the center where carpels meet, e.g., Tomato.
5. Basal: Ovules are attached at the base of the ovary, e.g., Sunflower.

Seeds

A seed is a fertilized and developed ovule.

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Parts of a Seed

1. Testa (Seed Coat): A tough outer covering that protects the embryo.
2. Cotyledon: Stores food for the embryo. Dicots have two cotyledons, while monocots have one.
3. Micropyle: A small opening that allows water and oxygen into the seed for germination.
4. Embryo: Contains:
   • Plumule: Develops into the shoot.
   • Radicle: Develops into the root system.

After fertilization, the petals, sepals, stamens, and style wither, leaving the ovary to develop into a fruit and the ovules into seeds.

Germination

Germination is the process where the embryo within a seed develops into a seedling or young plant.

Types of Germination

1. Hypogeal Germination: In this type of germination, the cotyledons remain in the soil as the epicotyl grows and elongates. Examples of plants with hypogeal germination include maize, gram seeds, and yam beans.
2. Epigeal Germination: This type of germination occurs when the cotyledons (seed leaves) are pushed above the soil surface. It is commonly seen in dicotyledonous plants like cowpea, groundnut, melon, and mango. In this process, the cotyledons appear flat, green, and leaf-like as they are elevated by the elongation of the hypocotyl.

Conditions Necessary for Germination

External conditions include:

1. Suitable Temperature: Enzymes are activated at the appropriate temperature, facilitating chemical reactions needed for cell formation. Each seed species has an optimum temperature for germination, typically within the range of 5–40 °C.
2. Water: Water is essential for initiating and sustaining germination. It softens the seed coat, activates biochemical reactions, and helps rupture the testa due to swelling. However, excess water may hinder germination by restricting oxygen supply.
3. Oxygen: Oxygen is required for aerobic respiration, which provides the energy needed for the growth of the embryo during germination.

Internal conditions include:

1. Enzymes: Enzymes accelerate the breakdown of stored food, releasing energy for the growing seed.
2. Stored Food: In dicot seeds, food is stored in the cotyledons, while in monocot seeds, it is stored in the endosperm.
3. Viability: A viable seed is one that is alive and capable of germinating under favorable conditions.