1. Agents of Pollination
Plants use three major agents:
Abiotic agents
- Wind
- Water
Biotic agent
- Animals
Majority of flowering plants are pollinated by biotic agents, especially insects.
Why do wind- and water-pollinated plants produce enormous amounts of pollen?
Because pollen reaching the stigma is largely a chance event, resulting in considerable pollen wastage. Therefore, they compensate by producing very large quantities of pollen grains.
2. Wind Pollination — Anemophily
Wind is the most common abiotic agent of pollination.
Adaptations of wind-pollinated flowers
- Pollen grains are light
- Pollen grains are non-sticky
- Stamens are well exposed for easy pollen dispersal
- Stigma is large and often feathery to trap airborne pollen
- Usually one ovule per ovary
- Flowers are often numerous and packed into an inflorescence
- Common in grasses
🌽 Example: Maize
The familiar corn cob has long, thread-like structures that are actually stigma and style, which help trap pollen carried by wind.
NEET fact:
Light + non-sticky pollen + exposed stamens + large feathery stigma → Wind pollination
3. Water Pollination — Hydrophily
Water pollination is very rare in flowering plants.
- Limited to about 30 genera
- Mostly found in monocotyledons
Examples
- Vallisneria
- Hydrilla
- Zostera (seagrass)
Important distinction
Not every aquatic plant is water-pollinated.
For example:
- Water hyacinth
- Water lily
Their flowers emerge above the water surface and are pollinated by insects or wind.
Vallisneria
- Female flowers reach the water surface through a long stalk.
- Male flowers/pollen grains are released onto the water surface.
- Pollen is carried passively by water currents.
- Eventually, pollen reaches the stigma of the female flower.
Seagrasses — Zostera
- Female flowers remain submerged.
- Pollen grains are released underwater.
- Pollen grains are often long and ribbon-like.
- They are carried passively by water currents.
In many water-pollinated species, pollen grains have a mucilaginous covering that protects them from wetting.
Why are wind- and water-pollinated flowers usually not colourful and nectar-producing?
Because they do not need to attract animals.
Therefore, they generally:
- are not brightly coloured
- do not produce nectar
- often lack fragrance
These features are important for animal-pollinated flowers, not for wind/water pollination.
4. Animal Pollination — Zoophily
Majority of flowering plants use animals as pollinating agents.
Common pollinators
- 🐝 Bees
- 🦋 Butterflies
- 🪰 Flies
- 🪲 Beetles
- Wasps
- Ants
- Moths
- Birds
- Bats
Insects, particularly bees, are the dominant biotic pollinating agents.
Even some:
- Primates such as lemurs
- Tree-dwelling rodents
- Reptiles such as geckos and garden lizards
can act as pollinators.
Adaptations of animal-pollinated flowers
Most insect-pollinated flowers are:
- Large
- Colourful
- Fragrant
- Rich in nectar
If flowers are small, many flowers are grouped into an inflorescence to make them conspicuous.
Attraction
Animals are attracted by:
- Colour
- Fragrance
Special example
Flowers pollinated by flies and beetles may produce foul odours to attract them.
Floral Rewards
Flowers provide rewards to encourage repeated animal visits.
Main rewards:
- Nectar
- Pollen grains
During feeding, the animal comes into contact with:
Anther → pollen sticks to animal body → animal visits another flower → pollen reaches stigma
Pollen of animal-pollinated flowers is generally sticky.
Special Pollination Relationships
Amorphophallus
Some flowers provide animals with a safe place to lay eggs as a floral reward.
Yucca and moth
A highly specific relationship exists between Yucca plant and a particular moth.
- Moth lays eggs in the ovary.
- Moth simultaneously pollinates the flower.
- Developing seeds provide food for the moth larvae.
Both organisms depend on each other to complete their life cycles.
Pollen/Nectar Robbers
Not every flower visitor is a pollinator.
Some insects may take:
- pollen
- nectar
without coming into contact with the anther and stigma.
Such visitors are called:
Pollen/nectar robbers
5. Outbreeding Devices
Why are outbreeding devices needed?
Most flowering plants have bisexual/hermaphrodite flowers.
Therefore, pollen can easily reach the stigma of the same flower.
Continuous self-pollination can cause:
Inbreeding depression
Hence, flowering plants have evolved mechanisms to discourage self-pollination and promote cross-pollination.
Important Outbreeding Devices
1. Dichogamy
Pollen release and stigma receptivity are not synchronised.
Two possibilities:
Protandry:
Anthers release pollen before stigma becomes receptive.
Protogyny:
Stigma becomes receptive before pollen is released.
Both prevent autogamy.
2. Herkogamy
Anther and stigma are positioned at different locations within the flower.
Therefore, pollen cannot easily reach the stigma of the same flower.
Prevents autogamy.
3. Self-incompatibility
A genetic mechanism that prevents self-pollen from fertilising the ovules.
It may inhibit:
- Pollen germination, or
- Pollen tube growth
Self-incompatibility prevents both self-pollination-related fertilisation from the same flower and, depending on the system, pollen from other flowers of the same plant.
4. Unisexual Flowers
Monoecious condition
Male and female flowers occur on the same plant.
Examples:
- Castor
- Maize
This prevents:
❌ Autogamy
But does not prevent:
Geitonogamy
Dioecious condition
Male and female flowers occur on different plants.
Example:
- Papaya
This prevents:
❌ Autogamy
❌ Geitonogamy
⭐ Papaya → dioecious → both autogamy and geitonogamy prevented
Pollen–Pistil Interaction
Pollination does not guarantee fertilisation.
Why?
Because the pollen landing on the stigma may be:
- Compatible → accepted
- Incompatible → rejected
Pistil recognition
The pistil recognises whether pollen is of the correct type through a chemical interaction between pollen and pistil components.
Compatible pollen
Pistil accepts it → pollen germinates → pollen tube develops.
Incompatible pollen
Pistil rejects it by preventing:
- pollen germination, or
- pollen tube growth through the style.
Pollen Tube Development
After compatible pollination:
Pollen grain → pollen tube → stigma → style → ovary → ovule
The pollen tube enters the ovule through the:
Micropyle
Then it enters one of the:
Synergids
through the filiform apparatus.
Filiform apparatus guides the entry of the pollen tube into the synergid.
Two-celled vs Three-celled Pollen
Two-celled pollen
Contains:
- Vegetative cell
- Generative cell
The generative cell divides during pollen tube growth to form two male gametes.
Three-celled pollen
Already contains:
- Vegetative cell
- Two male gametes
Therefore, the pollen tube carries the two male gametes from the beginning.
7. Artificial Hybridisation
Artificial hybridisation is an important method used in crop improvement.
Its purpose is to ensure that:
Only desired pollen reaches the stigma.
Two important techniques:
Emasculation
Removal of anthers from a bisexual flower before anther dehiscence.
- Done using forceps
- Prevents unwanted self-pollination
Bagging
The emasculated flower is covered with a suitable bag, usually made of butter paper.
Purpose:
Prevent contamination of stigma by unwanted pollen.
When the stigma becomes receptive:
- Desired pollen is placed on stigma.
- Flower is rebagged.
- Fruit is allowed to develop.
If Female Parent Has Unisexual Flowers
Emasculation is NOT required because there are no anthers in the female flower.
Instead:
Female flower bud → Bag before opening → Stigma becomes receptive → Desired pollen applied → Rebagging
NEET/JEE High-Yield Table
| Concept | Key point |
|---|---|
| Most common abiotic pollination | Wind |
| Most common biotic pollinators | Insects, especially bees |
| Wind pollen | Light, non-sticky |
| Animal pollen | Generally sticky |
| Wind stigma | Large, feathery |
| Water pollination | Rare |
| Vallisneria | Water pollination |
| Zostera | Water pollination |
| Water lily | Insect/wind, not water pollination |
| Dichogamy | Different timing of pollen release & stigma receptivity |
| Herkogamy | Different position of anther & stigma |
| Self-incompatibility | Genetic rejection of self-pollen |
| Castor & maize | Monoecious |
| Papaya | Dioecious |
| Monoecious prevents | Autogamy only |
| Dioecious prevents | Autogamy + geitonogamy |
| Pollen tube enters ovule through | Micropyle |
| Pollen tube enters | Synergid |
| Pollen tube guided by | Filiform apparatus |
| Removal of anthers | Emasculation |
| Covering flower | Bagging |
| Female unisexual flower | No emasculation required |
| Wrong pollen | Rejected by pistil |
| Compatible pollen | Accepted and germinates |
Sarvan sir- Chemistry For All 