🌸 The Flower
A flower is the reproductive part of an angiosperm, meant for sexual reproduction. It develops from a floral bud and is typically borne on the peduncle. A typical flower has four kinds of whorls arranged successively on the swollen end of the stalk (pedicel), called the thalamus or receptacle.
🌼 Parts of a Typical Flower
A typical flower consists of four concentric whorls:
- Calyx: The outermost whorl, composed of units called sepals. Sepals are typically green, leaf-like, and protect the flower in the bud stage. The calyx can be gamosepalous (sepals united) or polysepalous (sepals free).
- Corolla: Composed of units called petals. Petals are usually brightly coloured to attract insects for pollination. The corolla can be gamopetalous (petals united) or polypetalous (petals free). The shape of the corolla varies greatly.
- Androecium: Composed of stamens, the male reproductive organ. Each stamen consists of a stalk (filament) and an anther (which produces pollen). Stamens may be free or united in various ways (monadelphous, diadelphous, polyadelphous). Epipetalous stamens are attached to the petals (e.g., Solanum).
- Gynoecium: The female reproductive part, composed of one or more carpels. Each carpel has three parts: stigma (receptive tip for pollen), style (stalk connecting stigma to ovary), and ovary (enlarged basal part containing ovules).
- Apocarpous: Gynoecium with two or more free carpels (e.g., Lotus, Rose).
- Syncarpous: Gynoecium with two or more fused carpels (e.g., Mustard, Tomato).
This can be summarized into following table:
| Whorl | Function | Example |
|---|---|---|
| Calyx (Sepals) | Outermost whorl; protects the flower in bud | Hibiscus |
| Corolla (Petals) | Colored whorl attracting pollinators | Rose, Sunflower |
| Androecium (Stamens) | Male reproductive whorl; stamens = filament + anther | Mustard |
| Gynoecium (Carpels) | Female reproductive whorl; ovary, style, stigma | Pea, Lily |
🌺 Essential and Non-essential Whorls
- Essential whorls: Androecium and Gynoecium (reproductive)
- Non-essential whorls: Calyx and Corolla (accessory)
🌷 Types of Flowers
Flower Types (based on floral appendages)
- Trimerous: Floral appendages are in multiples of 3 (e.g., Monocots).
- Tetramerous: Floral appendages are in multiples of 4.
- Pentamerous: Floral appendages are in multiples of 5 (e.g., Dicots).
Flower Types (based on ovary position)
Based on the position of the calyx, corolla, and androecium with respect to the ovary on the thalamus:
- Hypogynous: The gynoecium occupies the highest position, while other parts are situated below it. The ovary is said to be superior. Examples: Mustard, China rose, Brinjal.
- Perigynous: The gynoecium is situated in the center, and other parts are located on the rim of the thalamus almost at the same level. The ovary is said to be half-inferior. Examples: Plum, Rose, Peach.
- Epigynous: The margin of the thalamus grows upward, enclosing the ovary completely and fusing with it. The other floral parts arise above the ovary. The ovary is said to be inferior. Examples: Guava, Cucumber, Ray florets of sunflower.
🌼 Symmetry of Flowers
- Actinomorphic (Radial Symmetry): A flower can be divided into two equal radial halves in any radial plane passing through the center. Examples: Mustard, Datura, Chilli.
- Zygomorphic (Bilateral Symmetry): A flower can be divided into two similar halves by only one particular vertical plane. Examples: Pea, Gulmohur, Bean, Cassia.
- Asymmetric: A flower cannot be divided into two similar halves by any vertical plane. Example: Canna.
🌿 Aestivation
Aestivation refers to the arrangement of sepals or petals in a floral bud with respect to the other members of the same whorl before it opens.
| Type | Description | Example |
|---|---|---|
| Valvate | Sepals/petals just touch each other; no overlap | Calotropis |
| Twisted (or Contorted) | Each petal overlaps the next one | China rose |
| Imbricate | Irregular overlapping; one outer, one inner, others in between | Cassia |
| Vexillary (Papilionaceous) | Large standard overlaps wings and keel | Pea, Bean |
🌸 Placentation
The arrangement of ovules within the ovary.
- Marginal: Ovules are borne on a ridge along the ventral suture of the ovary (e.g., in a simple pistil). Example: Pea.
- Axile: Ovules are attached to a central axis in a multilocular (multi-chambered) ovary. Example: China rose, Tomato, Lemon.
- Parietal: Ovules develop on the inner wall (periphery) of the ovary. The ovary is one-chambered, but may become two-chambered due to a false septum (replum). Example: Mustard, Argemone.
- Free-central: Ovules are borne on a central axis, and septa are absent. Ovary is one-chambered. Example: Dianthus, Primrose.
- Basal: A single ovule is attached to the base of the ovary. Example: Sunflower, Marigold.
🌹 Importance
- Understanding floral structure helps in taxonomy and plant identification.
- Aestivation and placentation are key features for floral formula derivation.
💡 Questions and Answers
Q1. What is aestivation? Name any two types.
A1. Arrangement of sepals/petals in floral bud. Types: Valvate (Calotropis), Twisted (Hibiscus).
Q2. Define placentation. Name one plant showing axile placentation.
A2. Arrangement of ovules inside ovary. Axile placentation – Hibiscus, Lemon.
Q3. Differentiate between actinomorphic and zygomorphic flowers.
A3. Actinomorphic – radial symmetry (Mustard), Zygomorphic – bilateral symmetry (Pea).
Q4. Describe the arrangement of floral members in relation to their insertion on thalamus (Ovary Position).
A4.
- Hypogynous: Ovary is superior. The thalamus is convex, and the calyx, corolla, and androecium are inserted below the ovary (e.g., Mustard, China rose).
- Perigynous: Ovary is half-inferior. The thalamus forms a cup-like structure, and the other whorls are inserted on its rim, around the ovary (e.g., Rose, Plum).
- Epigynous: Ovary is inferior. The thalamus grows up and completely encloses the ovary, fusing with it. The other whorls are inserted above the ovary (e.g., Guava, Cucumber).