Flower Chapter Notes | Biology Class 9 ICSE PDF Download

Introduction

The flower is the reproductive organ of flowering plants, playing a vital role in the survival of the species through reproduction. It is typically the most visually striking and prominent part of a plant, designed to facilitate the formation of seeds for the next generation.

Structure of a Bisexual Flower

• Flower is a specialized shoot with leaves modified into floral structures.
• Supported by a stalk called the pedicel; flowers without a stalk are sessile.
• The tip of the pedicel may expand into a cup-shaped structure called the thalamus or receptacle, which bears floral parts.
• Floral parts are arranged in four whorls on the thalamus:
  • First whorl (Calyx): Consists of green sepals, collectively called the calyx.
  • Second whorl (Corolla): Comprises large, brightly colored petals, collectively called the corolla.
  • Third whorl (Androecium): Male reproductive parts, consisting of stamens. Each stamen has a filament and a bilobed anther, collectively called the androecium (male symbol: ♂).
  • Fourth whorl (Gynoecium): Female reproductive parts, consisting of one or more carpels forming the pistil, collectively called the gynoecium (female symbol: ♀). Each carpel includes an ovary, style, and stigma.

Complete and Incomplete Flowers

• Complete (Perfect) Flower: Contains all four floral whorls (calyx, corolla, androecium, gynoecium).
• Incomplete (Imperfect) Flower: Lacks one or more floral whorls.
• Essential Parts: Stamens (male) and carpels (female), directly involved in reproduction.
• Non-Essential (Accessory) Parts: Sepals and petals, which protect reproductive parts or attract pollinators.
• Perianth:When petals and sepals are not differentiated, they are collectively called perianth.
  • Petaloid Perianth: Non-green, resembling petals.
  • Sepaloid Perianth: Green, resembling sepals.
• Bracts: Leaf-like structures in whose axils flowers arise; may be green or colorful, e.g., large colorful bracts in Bougainvillea, often mistaken for petals.
• Nectaries: Groups of cells secreting nectar (sweet liquid), usually at the base of the pistil or petals, attracting insects for pollination, e.g., prominent in Nasturtium.

Sexuality in Flowers

• Stamens: Produce pollen grains containing male gametes.
• Carpels: Contain ovules with female gametes (egg cells).
• Bisexual (Hermaphrodite/Perfect) Flower: Contains both stamens and carpels.
• Unisexual (Imperfect) Flower:Contains either stamens or carpels.
  • Staminate Flower: Contains only stamens (male flower).
  • Pistillate Flower: Contains only carpels (female flower).
• Neuter Flower: Lacks both male and female reproductive organs, e.g., ray florets of sunflower.
• Monoecious Plants: Bear both male and female flowers on the same plant, e.g., maize, cucumber, pumpkin.
• Dioecious Plants: Bear male and female flowers on different plants, e.g., palm, papaya.

General Description of the Floral Parts

(a) Calyx (Sepals):

• Usually five sepals, but the number may vary.
• Polysepalous: Sepals are free.
• Gamosepalous: Sepals are fused.
• Epicalyx: An additional series of sepals in some flowers, e.g., Hibiscus (shoe-flower).
• Sepals may fall off or persist when the flower opens.
• Sepals are usually green but can be colorful (petaloid), e.g., red in Gul Mohur.
• Functions: Protect the flower bud; green sepals may perform photosynthesis.

(b) Corolla (Petals):

• Arranged in a single whorl, double whorl (e.g., Poppy), or spiral (e.g., Water lily).
• Polypetalous: Petals are free.
• Gamopetalous: Petals are fused, forming a tube or funnel shape.
• Functions: Attract insects for pollination; protect stamens and pistils, especially in tubular forms.

(c) Androecium (Stamens):

• Number of stamens varies from a few to many.
• Each stamen consists of a filament and a bilobed anther with four pollen sacs.
• Pollen sacs contain pollen grains, which are powdery, vary in shape and size, and are released when sacs rupture.
• Cohesion of Stamens:
  • Polyandrous: Stamens are free, e.g., Petunia.
  • Monadelphous: Filaments united in one group, anthers free, e.g., China rose, cotton (staminal tube).
  • Diadelphous: Filaments united in two bundles, e.g., pea (9 stamens form a tube, 1 is free).
  • Polyadelphous: Filaments united in several groups, e.g., Bombax.

(d) Gynoecium (Carpels/Pistil):

• Composed of one or more carpels; collectively called the pistil.
• Each carpel has:
  • Stigma: Terminal, knob-like part, often lobed or feathery, covered with hair or glandular papillae; serves as the landing place for pollen during pollination.
  • Style: Tubular stalk connecting stigma to ovary.
  • Ovary: Swollen basal part with one or more chambers (locules) containing ovules.
• Placenta: Cushion-like region in the ovary attaching ovules to the ovary wall.

Inflorescence and Placentation

• Inflorescence refers to the arrangement of flowers on a plant’s axis. Flowers can be organized in various patterns across different plant species, and this arrangement is called inflorescence. Flowers may appear singly at the top of the main stem or at the ends of lateral branches, or they may grow in clusters from the leaf axils. In some plants, such as sunflowers, the axis flattens into a disc, with the youngest flowers at the center and the oldest at the edges.
• Placentation describes how ovules are attached to the ovary wall via a tissue called the placenta. Ovules develop into seeds within the mature fruit, which is the transformed ovary. Different placentation patterns can be observed in fruits like peas, tomatoes, mangoes, and lotuses.

Key Points to Understand

• A flower is a reproductive structure.
• The essential parts of a flower are the stamens (male reproductive organs) and carpels (female reproductive organs).
• Sepals protect the flower bud, petals attract pollinators, stamens produce pollen, and the carpel’s ovary develops into the fruit containing seeds (ovules).
• Essential parts (stamens and carpels) are reproductive, while non-essential parts (sepals and petals) are non-reproductive.
• Nectaries, which produce nectar, are often found in brightly colored, insect-pollinated flowers.
• Sepals may sometimes be brightly colored, resembling petals, and are then called petaloid.
• Pollen grains are fine, powdery particles that vary in shape.
• The arrangement of flowers on a plant is known as inflorescence.