1.0 Pollination: The First Step to Life
For a plant to produce seeds, the pollen grains from the anther must reach the stigma of a carpel. This transfer process is known as Pollination.
Pollination: The process of transfer of pollen grains from the anther to the stigma of the same flower or of another flower of the same species.
Types of Pollination
Pollination is broadly categorized into two types based on the "source" of the pollen:
- Autogamy: Pollination within the same flower.
- Geitonogamy: Pollination between different flowers of the same plant. (Functionally cross-pollination, genetically self-pollination).
Some flowers never open (e.g., Commelina, Oxalis). These are called Cleistogamous flowers. They can only undergo self-pollination. This is a favorite "Give Reason" question!
2.0 Agents of Cross-Pollination
Flowers adapt themselves in specific ways to suit their pollinating agents. These adaptations ensure that pollen transfer is successful.
1. Insect Pollination (Entomophily)
- Flowers are large and brightly colored to attract insects.
- Produce nectar and scent as rewards.
- Pollen grains are sticky or spiny to attach to the insect's body.
- Stigma is usually sticky and does not protrude out.
Examples: Hibiscus, Rose, Jasmine.
2. Wind Pollination (Anemophily)
- Flowers are small, dull, and produce no scent or nectar.
- Pollen grains are light, dry, and produced in large quantities.
- Anthers are versatile (swing freely on filaments).
- Stigmas are feathery to catch wind-blown pollen.
Examples: Maize, Grass, Wheat.
Quick Comparison: Insect vs. Wind Pollination
| Feature | Insect Pollinated | Wind Pollinated |
|---|---|---|
| Pollen | Sticky/Spiny, Fewer | Light/Smooth, Massive quantity |
| Stigma | Sticky, inside the flower | Feathery, hanging out |
| Petals | Large and Showy | Small and Greenish |
Water Pollination (Hydrophily) is rare and occurs in aquatic plants like Vallisneria. Remember: The pollen grains in these plants often have a waxy coating to prevent them from getting wet and rotting.
3.0 Contrivances for Cross-Pollination
Even in bisexual flowers (where both male and female parts are present), plants often use these five mechanisms to avoid self-pollination and favor cross-pollination.
1. Dichogamy (Difference in Timing)
The anther and stigma of the same flower mature at different times.
- Protandry: Anthers mature before the stigma.
Examples: Sunflower, Sweet Pea. - Protogyny: Stigma matures before the anthers.
Examples: Custard apple, Peepal.
2. Structural and Genetic Barriers
| Mechanism | Description | Example |
|---|---|---|
| Herkogamy | A physical barrier prevents pollen from reaching the stigma of the same flower. | Pansy, Iris |
| Heterostyly | Stigma and Anthers are placed at different heights. | Primrose |
| Self-Sterility | Pollen fails to germinate on the stigma of the same flower due to genetic reasons. | Orchids, Tea |
In Heterostyly, the difference in height ensures that an insect visiting the flower will pick up pollen on a part of its body that will exactly touch the stigma of a different flower.
Be very precise with Protandry and Protogyny.
Remember: Andro = Male (Anther) | Gyno = Female (Stigma). The prefix "Proto" means first. So, Protandry = Anther first!
4.0 Fertilization in Plants
Fertilization is the fusion of the male gamete with the female gamete to form a diploid zygote. This process occurs deep inside the ovary.
Step-by-Step Process
- Germination: The pollen grain absorbs sugary secretions from the stigma and swells.
- Pollen Tube Growth: A pollen tube grows out of the pollen grain, traveling through the style to reach the ovary.
- Entry: The tube usually enters the ovule through a tiny opening called the Micropyle.
- Gamete Release: The pollen tube carries two male gametes. One fuses with the Egg cell, and the other fuses with the Secondary Nucleus.
Double Fertilization: A unique process in flowering plants where two fusions occur:
1. Male Gamete + Egg → Zygote
2. Male Gamete + Secondary Nucleus → Endosperm
Post-Fertilization Changes
| Part Before Fertilization | Part After Fertilization |
|---|---|
| Ovary | Fruit |
| Ovule | Seed |
| Ovary Wall (Pericarp) | Fruit Wall |
| Integuments | Seed Coat (Testa & Tegmen) |
In "Name the Following," if asked for the nutritive tissue for the developing embryo, the answer is Endosperm. It is formed by the fusion of the second male gamete with two polar nuclei (Secondary Nucleus).
Fertilization was first discovered by Strasburger in 1884, while the concept of Double Fertilization was introduced by Nawaschin in 1898.