Fertilisation in Flowering Plants & Double Fertilisation - Biology Class 12 - NEET

1. POLLINATION - The Precursor to Fertilisation

1.1 Definition

Pollination is the mechanism to achieve the transfer of pollen grains (shed from the anther) to the stigma of a pistil for fertilisation to occur.

1.2 Types of Pollination

Based on Source of Pollen:

A. Autogamy (Self-Pollination)

• Transfer of pollen grains from the anther to the stigma of the same flower
• Requirements for complete autogamy:
  • Synchrony in pollen release and stigma receptivity
  • Anthers and stigma must lie close to each other
• Chasmogamous flowers: Similar to flowers of other species with exposed anthers and stigma
• Cleistogamous flowers: Do not open at all; anthers and stigma lie close to each other
  • Invariably autogamous
  • Produce assured seed-set even in absence of pollinators
  • Examples: Viola (common pansy), Oxalis, Commelina

B. Geitonogamy

• Transfer of pollen grains from the anther to the stigma of another flower of the same plant
• Although involves a pollinating agent, it is genetically similar to autogamy
• Pollen grains come from the same plant

C. Xenogamy (Cross-Pollination)

• Transfer of pollen grains from anther to the stigma of a different plant
• Only type that brings genetically different types of pollen grains to the stigma
• Most important for genetic diversity

2. AGENTS OF POLLINATION

2.1 Classification

Plants use two abiotic (wind and water) and one biotic (animals) agents to achieve pollination.

2.2 Wind Pollination

• More common amongst abiotic pollinations
• Wind pollination requires:
  • Light and non-sticky pollen grains that can be transported in wind currents
  • Well-exposed stamens so pollens are easily dispersed into wind currents
  • Large, often-feathery stigma to easily trap air-borne pollen grains
• Wind-pollinated flowers often have:
  • Single ovule in each ovary
  • Numerous flowers packed into an inflorescence
  • Example: corn cob (ears are the stigma and style waving in wind to trap pollen grains)
• Quite common in grasses

2.3 Water Pollination

• Quite rare in flowering plants
• Limited to about 30 genera, mostly monocotyledons
• Water is a regular mode of transport for male gametes among lower plant groups (algae, bryophytes, pteridophytes)
• Examples: Vallisneria, Hydrilla (fresh water); several marine sea-grasses such as Zostera

Mechanism in Vallisneria:

• Female flower reaches surface of water by long stalk
• Male flowers or pollen grains released on surface of water
• Carried passively by water currents; some eventually reach female flowers and stigma

Variation in seagrasses:

• Female flowers remain submerged in water
• Pollen grains released inside the water
• Long, ribbon-like appearance
• Carried passively inside water by currents
• Some reach stigma and achieve pollination

Protection mechanism:

• Pollen grains in many water-pollinated species protected from wetting by a mucilaginous covering

Why are wind and water pollinated flowers not very colourful and do not produce nectar?

• Because they don't need to attract animal pollinators

2.4 Animal Pollination (Biotic)

Majority of flowering plants use a range of animals as pollinating agents.

Common pollinators:

• Bees
• Butterflies
• Flies
• Beetles
• Wasps
• Ants
• Moths
• Birds (sunbirds and humming birds)
• Bats

Even larger animals reported:

• Some primates (lemurs)
• Arboreal (tree-dwelling) rodents
• Reptiles (gecko lizard and garden lizard)

Adaptations for Animal Pollination:

Flower Characteristics:

• Often flowers of animal-pollinated plants are specifically adapted for a particular species of animal
• Large, colourful, fragrant flowers
• Rich in nectar
• When flowers are small, they cluster into an inflorescence to make them conspicuous

Attraction Mechanisms:

• Animals attracted to flowers by colour and/or fragrance
• Flowers pollinated by flies and beetles secrete foul odours to attract these animals

Reward System:

• To sustain animal visits, flowers must provide rewards
• Nectar and pollen grains are the usual floral rewards
• For harvesting the reward(s), the animal visitor comes in contact with the anthers and the stigma
• Body of animal gets a coating of pollen grains (generally sticky in animal pollinated flowers)
• When animal carrying pollen on its body comes in contact with stigma, it brings about pollination

Special Relationships:

Floral rewards providing safe places to lay eggs:

• Example: tallest flower of Amorphophallus (about 6 feet in height)
• A similar relationship exists between a species of moth and the plant Yucca where both species cannot complete their life cycles without each other

Mutualistic Relationship:

• Moth and Yucca plant - moth deposits its eggs in locule of ovary and flower gets pollinated by moth
• Seeds start developing; larvae also come out of eggs
• Larvae feed on developing seeds
• Both species - moth and the plant - cannot complete their life cycles without each other

3. OUTBREEDING DEVICES

3.1 Purpose

Majority of flowering plants produce hermaphrodite flowers and pollen grains are likely to come in contact with the stigma of the same flower. Continued self-pollination results in inbreeding depression. Flowering plants have developed many devices to discourage self-pollination and to encourage cross-pollination.

3.2 Types of Outbreeding Devices

1. Asynchrony in Pollen Release and Stigma Receptivity

• Either the pollen is released before the stigma becomes receptive
• OR stigma becomes receptive much before the release of pollen
• Both prevent autogamy

2. Physical Barriers

• In some other species, the anther and stigma are placed at different positions
• Pollen cannot come in contact with the stigma of the same flower
• Both these devices prevent autogamy

3. Self-Incompatibility

• Genetic mechanism
• Prevents self-pollen (from the same flower or other flowers of the same plant) from fertilising the ovules
• Inhibits pollen germination OR pollen tube growth in the pistil

4. Production of Unisexual Flowers

• If both male and female flowers are present on the same plant such as castor and maize (monoecious), it prevents autogamy but not geitonogamy
• In several species such as papaya, male and female flowers are present on different plants (dioecious)
• This condition prevents both autogamy and geitonogamy

4. POLLEN-PISTIL INTERACTION

4.1 Definition and Importance

Pollination does not guarantee the transfer of the right type of pollen (compatible pollen of the same species as the stigma). Often, pollen of the wrong type, either from other species or from the same plant (if it is self-incompatible), also land on the stigma.

The pistil has the ability to recognise the pollen, whether it is of the right type (compatible) or of the wrong type (incompatible).

4.2 Mechanism

If pollen is of the right type:

• The pistil accepts the pollen
• Promotes post-pollination events that lead to fertilisation

If pollen is of the wrong type:

• The pistil rejects the pollenby:
  • Preventing pollen germination on the stigma, OR
  • Pollen tube growth in the style

Nature of Interaction:

• This dialogue between pollen grain and the pistil is mediated by chemical components of the pollen interacting with those of the pistil
• Only in recent years botanists have been able to identify some of the pollen and pistil components and the interactions leading to recognition, followed by acceptance or rejection

Significance:

• The knowledge gained in this area would help the plant breeder in manipulating pollen-pistil interaction, even in incompatible pollinations, to get desired hybrids

5. POLLEN GERMINATION

5.1 Process

Following compatible pollination, the pollen grain germinates on the stigma to produce a pollen tube through one of the germ pores.

5.2 Pollen Tube Growth

Path of Growth:

• Pollen tube grows through the tissues of the stigma and style
• Reaches the ovary

Entry into Ovule:

• After reaching the ovary, enters the ovule through the micropyle
• Then enters one of the synergids through the filiform apparatus

Cell Division during Growth:

• In plants which shed pollen at 2-celled condition (a vegetative cell and a generative cell):

  • The generative cell divides during growth of pollen tube in the stigma
  • Forms two male gametes during the growth of pollen tube in the stigma

• In plants which shed pollen in the 3-celled condition:

  • Pollen tubes carry the two male gametes from the beginning

5.3 Observing Pollen Germination

You can easily study pollen germination by:

• Dusting some pollen from flowers such as pea, chickpea, Crotalaria, balsam and Vinca on a glass slide
• Containing a drop of sugar solution (about 10 per cent)
• After about 15-30 minutes, observe the slide under the low power lens of the microscope
• You are likely to see pollen tubes coming out of the pollen grains

6. FERTILISATION PROCESS

6.1 Entry into Embryo Sac

Discharge of Male Gametes:All these events - from pollen deposition on the stigma until pollen tubes enter the ovule - are together referred to as pollen-pistil interaction.

Many recent studies have shown that filiform apparatus present at the micropylar part of the synergids guides the entry of pollen tube.

After reaching the embryo sac, the pollen tube enters into one of the synergids and discharges the two male gametes there.

6.2 Double Fertilisation

The Unique Phenomenon

Double fertilisation is an event unique to flowering plants. Since two types of fusions - syngamy and triple fusion - take place in an embryo sac, the phenomenon is termed double fertilisation.

First Fusion - Syngamy

• One of the male gametes moves towards the egg cell
• Fuses with its nucleus thus completing the syngamy
• This results in the formation of a diploid cell, the zygote

Second Fusion - Triple Fusion

• The other male gamete moves towards the two polar nuclei located in the central cell
• Fuses with them to produce a triploid primary endosperm nucleus (PEN)
• As this involves the fusion of three haploid nuclei, it is termed triple fusion

Summary of Double Fertilisation:

1. Syngamy: Male gamete (n) + Egg cell (n) → Zygote (2n)
2. Triple Fusion: Male gamete (n) + Two polar nuclei (n+n) → Primary Endosperm Nucleus (3n)

6.3 Post-Fertilisation Changes

Central Cell:

• The central cell after triple fusion becomes the primary endosperm cell (PEC)
• Develops into the endosperm

Zygote:

• The zygote develops into an embryo

KEY TERMS AND DEFINITIONS

IMPORTANT DIAGRAMS TO REMEMBER

1. Types of flowers - chasmogamous vs cleistogamous
2. Wind vs Animal pollinated flowers - structural differences
3. Structure of pistil - stigma, style, ovary
4. Ovule structure - integuments, nucellus, embryo sac
5. Mature embryo sac (7-celled, 8-nucleate) - showing all cells
6. Pollen tube growth - path through stigma, style to ovule
7. Double fertilisation - syngamy and triple fusion

EXAM-FOCUSED POINTS

Numerical Information to Remember:

1. Pollen grain diameter: 25-50 micrometers
2. Embryo sac: 7-celled, 8-nucleate structure
3. Cells at micropylar end: 3 (2 synergids + 1 egg cell = egg apparatus)
4. Cells at chalazal end: 3 (antipodal cells)
5. Central cell nuclei: 2 (polar nuclei)
6. Pollen condition at shedding: 2-celled (60% angiosperms) OR 3-celled (remaining)
7. Zygote ploidy: Diploid (2n)
8. PEN/Endosperm ploidy: Triploid (3n)
9. Seed moisture content at maturity: 10-15%
10. Oldest viable seed record: 10,000 years (Lupinus arcticus)
11. Recent old seed record: 2,000 years (Phoenix dactylifera)

Process Sequences to Remember:

A. Microsporogenesis:Microspore mother cell (PMC) → Meiosis → Microspore tetrad → Individual microspores → Pollen grains

B. Megasporogenesis:Megaspore mother cell (MMC) → Meiosis → 4 megaspores → 3 degenerate, 1 functional → Embryo sac (female gametophyte)

C. Pollen-Pistil Interaction:Pollination → Pollen recognition → Pollen germination → Pollen tube growth through stigma and style → Entry into ovary → Entry through micropyle → Entry into synergid → Discharge of male gametes

D. Double Fertilisation:

1. Syngamy: Male gamete + Egg → Zygote (2n)
2. Triple fusion: Male gamete + 2 Polar nuclei → PEN (3n)

E. Post-fertilisation:Zygote → Embryo PEN → Endosperm Ovule → Seed Ovary → Fruit