Pollination and Fertilization

Pollination is the fundamental first step in the sexual reproduction of flowering plants, defined as the transfer of pollen grains from the anther of a flower to the stigma of the same or a different flower. This process is crucial as it brings the male gametes, contained within the pollen grains, to the female part of the flower. Self-pollination involves the transfer of pollen to the stigma of the same flower or a genetically identical flower on the same plant, ensuring genetic consistency but reducing variation. Cross-pollination, the transfer of pollen to the stigma of a flower on a different plant of the same species, is more common and promotes genetic diversity, leading to healthier offspring. Plants have developed remarkable adaptations to prevent self-pollination, such as having male and female flowers on different plants (dioecy) or mechanisms where the anther and stigma of a flower mature at different times.

Since plants are stationary, they rely on various external agents to act as pollinators, primarily leading to anemophily (wind pollination), entomophily (insect pollination), and hydrophyly (water pollination). Wind-pollinated flowers, like those of grasses or corn, are typically small, inconspicuous, and produce enormous amounts of light, dry pollen to increase the chance of reaching a stigma. In contrast, insect-pollinated flowers, such as sunflowers or roses, are brightly colored, possess a pleasant fragrance, and produce nectar to attract insects like bees and butterflies. Their pollen grains are often sticky and spiny to easily cling to an insect’s body, and their stigmas are sticky to capture the pollen carried by these visitors. This intricate relationship highlights a key example of mutualism in nature.

The successful landing of a compatible pollen grain on the stigma sets the stage for fertilization. The pollen grain absorbs moisture and germinates, growing a long pollen tube that travels down the style, guided by chemicals, towards the ovary. The male gametes travel inside this tube. Meanwhile, inside the ovule, an embryo sac contains the egg cell. Upon reaching the ovule through the micropyle, the tip of the pollen tube bursts, releasing the two male gametes. The other male gamete fuses with two polar nuclei in the embryo sac to form the triploid endosperm, which provides nourishment for the developing embryo. This crucial double fusion event is unique to flowering plants and is known as double fertilization. Following fertilization, the ovule develops into the seed, the ovary matures into the fruit, and other floral parts typically wither and fall away.

• Multiple choice type

Question 1. 

Which one of the following is one of the characteristics of self-pollinated flowers?

1. Flowers are large and showy
2. Flowers remain closed and do not open
3. Stigma and anthers mature at the same time
4. Pollen is produced in very large quantities

Question 2. 

Exine and intine are the parts of

1. Embryo sac
2. Pollen grain
3. Stigma
4. Seed

• Very short answer type

Question 1. 

1. State the name of the chief pollinating agent against the corresponding plant by choosing from those given in brackets. Dahlia_______ .

1. crow
2. butterflies
3. mosquito

2. State the name of the chief pollinating agent against the corresponding plant by choosing from those given in brackets. Maize_______ .

1. bees
2. locusts
3. rain
4. wind

3. State the name of the chief pollinating agent against the corresponding plant by choosing from those given in brackets. Vallisneria ______ 

1. wind
2. water
3. ants
4. rabbits

Question 2. 

Match the items in Column A with those in Column B.

Column A	Column B
(a) Generative nucleus	(i) Pollen tube
(b) Germ pore	(ii) Endosperm nucleus
(c) Exine	(iii) Testa
(d) Secondary nucleus	(iv) Fertilization
(e) Integument	(v) Male nuclei
(f) Egg nucleus	(vi) Rough

Ans:

Column A	Column B	Rationale
(a) Generative nucleus	(v) Male nuclei	The generative nucleus (or cell) divides to form the two male gametes (nuclei) that participate in fertilization.
(b) Germ pore	(i) Pollen tube	The germ pore is the thin region on the pollen grain wall through which the pollen tube emerges during germination.
(c) Exine	(vi) Rough	The exine is the tough, outer wall layer of the pollen grain and is often sculptured, thick, and rough.
(d) Secondary nucleus	(ii) Endosperm nucleus	The secondary nucleus fuses with the second male gamete during double fertilization to form the triploid Endosperm Nucleus.
(e) Integument	(iii) Testa	The integuments are the protective outer layers of the ovule; after fertilization, they harden and develop into the testa (seed coat).
(f) Egg nucleus	(iv) Fertilization	The egg nucleus fuses with the first male gamete during fertilization, leading to the formation of the zygote.

Question 3. 

1. Fill in the blanks with suitable words. Transference of pollen grains from anthers to stigma of the same flower is called ________. 

2. Fill in the blanks with suitable words. Different timings for the maturation of gynoecium and androecium is called ________. 

3. Fill in the blanks with suitable words. _______ is a water-pollinated flower.

Ans:

1. Transference of pollen grains from anthers to stigma of the same flower is called self-pollination (or autogamy).
2. Different timings for the maturation of gynoecium and androecium is called dichogamy.
3. Vallisneria (or Hydrilla) is a water-pollinated flower.

Question 4. 

1. Name of the part of the ovary which gives rise to: Seed ______  

2. Name of the part of the ovary which gives rise to: Fruit ______ 

3. Name of the parts of the ovary which give rise to: Fruit wall ______

Ans:

Here are the parts of the ovary that give rise to the specified structures after fertilization:

1. Seed: Ovule
2. Fruit: Ovary (The entire ovary wall and contents mature to form the fruit.)
3. Fruit wall: Ovary wall (The ovary wall develops into the pericarp, which is the fruit wall).

Question 5. 

1. Give one word/term for the following: A flower containing both male and female parts. 

2. Give one word/term for the following: Arrangement of flowers on a twig/stem. 

3. Give one word/term for the following: When pollen grains of a flower reach the stigma of the same flower.

4. The maturation time of reproductive parts in a flower is different. 

5. Give one word/term for the following: When stigma and anthers do not grow up to the same height, which favors only cross-pollination. 

6. Give one word/term for the following: Pollination of flowers by insects.

7. Give one word/term for the following: Pollination of flowers by birds.

Ans:

Here are the single words or terms for the given definitions:

• A flower containing both male and female parts: Bisexual (or Perfect or Hermaphrodite)
• Arrangement of flowers on a twig/stem: Inflorescence
• When pollen grains of a flower reach the stigma of the same flower: Self-pollination (or Autogamy)
• The maturation time of reproductive parts in a flower is different: Dichogamy
• When stigma and anthers do not grow up to the same height, which favors only cross-pollination: Heterostyly
• Pollination of flowers by insects: Entomophily
• Pollination of flowers by birds: Ornithophily
• Short answer type

Question 1. 

1. Explain the Following Term: Ornithophily 

2. Explain the following term: Elephophily 

3. Explain the following term: Artificial pollination

Ans:

Here are the explanations for the given terms related to pollination:

1. Ornithophily 

Ornithophily is the term for pollination carried out by birds. Flowers that utilize birds as their primary agents for pollen transfer are called ornithophilous flowers.

• Characteristics of Ornithophilous Flowers: They are typically large, brightly colored (often red or yellow), lack fragrance (as birds have a poor sense of smell), and produce abundant, watery nectar to reward the bird pollinators. Examples include the flowers of the Bottlebrush and Hummingbird Sage.

2. Elephophily 

Elephophily is the rare term used to describe pollination carried out by elephants.

• Mechanism: This method is believed to occur in certain large, robust plants with flowers located close to the ground (basal flowering). The flowers may possess characteristics—such as a strong, low-level scent and large size—that attract elephants, which then transfer pollen by rubbing against the flowers or consuming parts of them. A classic, though rare, example is the species Rafflesia.

3. Artificial Pollination 

Artificial pollination is the process where humans manually transfer viable pollen from the anther of one flower to the stigma of the same or another flower.

• Purpose: This technique is primarily used in plant breeding and hybridization programs to create new varieties of crops with desirable traits (like higher yield, disease resistance, or better quality). It ensures controlled cross-pollination when natural methods are ineffective or undesirable.
• Method: A breeder typically collects pollen using a brush or forceps and dusts it onto the receptive stigma.

Question 2. 

1. What happens to the following after fertilization? Ovules 

2. What happens to the following after fertilization? Calyx

3. What happens to the following after fertilization? Petals 

4. What happens to the following after fertilization? Stamens

Ans:

Here’s what generally happens to the specified flower parts after successful fertilization:

1. Ovules: The ovules, which contain the egg cell, develop into the seeds . Their protective layers (integuments) harden to form the seed coat (testa).
2. Calyx (Sepals): The sepals usually wither and fall off shortly after fertilization. However, in some plants (like the tomato, brinjal, and strawberry), the calyx may persist (remain attached) and sometimes even enlarge to protect the developing fruit.
3. Petals (Corolla): The petals usually wither, change color, and drop off (fall away) because their function of attracting pollinators is complete.
4. Stamens (Androecium): The stamens, having released their pollen, usually wither and drop off from the flower.

Question 3. 

Mention any two contrivances in flowers which favour cross-pollination.

Ans:

Two key contrivances (adaptations) in flowers that favour cross-pollination (allogamy) are Dichogamy and Self-sterility.

1. Dichogamy

Dichogamy is the condition where the male and female reproductive organs (stamens and pistil) in a bisexual flower mature at different times. Since the pollen is released when the stigma is not receptive (or vice-versa), self-pollination is prevented.

Dichogamy has two forms:

• Protoandry: Anthers mature first (e.g., Sunflower).
• Protogyny: Stigma matures first (e.g., Plantain).

2. Self-Sterility (or Self-Incompatibility) 

Self-sterility is a genetic mechanism where the pollen grain, even if deposited on the stigma of the same flower, fails to germinate or the pollen tube fails to grow.

• This is controlled by specific genes (S-genes) that prevent fertilization between genetically similar gametes, thus mandating cross-pollination to set seed. (e.g., Passion Flower, some species of tobacco).

Other major contrivances include:

• Unisexuality: The flower only possesses male or female parts (e.g., Maize, Papaya).
• Heterostyly: Stamens and pistils are of unequal lengths, physically preventing pollen transfer (e.g., Primrose).
• Long answer type

Question 1. 

1. What are the advantages of the following in the flower to the plant concerned?

Long and feathery stigma 

2. What are the advantages of the following in the flower to the plant concerned?

Brightly coloured petals 

3. What are the advantages of the following in the flower to the plant concerned?

Smooth and light pollen 

4. What are the advantages of the following in the flower to the plant concerned?

Protruding and easily movable anthers 

5. What are the advantages of the following in the flower to the plant concerned?

Fragrant nectar

Ans:

Here are the advantages of the specified flower characteristics to the plant:

1. Long and Feathery Stigma
   • Advantage: It increases the surface area available to efficiently trap light, wind-borne pollen grains. This feature is typical of wind-pollinated (anemophilous) flowers, maximizing the chance of successful pollination despite the random nature of the wind.
2. Brightly Coloured Petals
   • Advantage: They serve as visual attractants to specific animal pollinators, such as insects (bees, butterflies) and birds. The conspicuous colors make the flower easily visible against the green foliage, guiding the pollinator directly to the reproductive parts.
3. Smooth and Light Pollen
   • Advantage: These characteristics are essential for wind pollination. Being light allows the pollen to be easily carried long distances by air currents, and being smooth helps prevent clumping, ensuring the grains remain airborne and disperse effectively.
4. Protruding and Easily Movable Anthers
   • Advantage: This positioning allows the anthers to hang out of the flower (exposed to the air). This arrangement facilitates the easy shaking out and dispersal of pollen by wind or ensures that pollen brushes directly onto the body of an approaching animal pollinator.
5. Fragrant Nectar
   • Advantage: Nectar acts as a reward for animal pollinators (insects, bats, birds). The fragrance serves as a long-distance scent beacon, guiding these animals to the flower so that pollen transfer occurs while they feed.

Question 2. 

Describe the advantages and disadvantages of cross-pollination to the plant.

Ans:

Cross-pollination (or allogamy) is the transfer of pollen between different plants of the same species. It offers significant evolutionary benefits, but also comes with certain risks and costs.

Advantages of Cross-Pollination

1. Genetic Variation (Hybrid Vigor): The primary advantage is genetic recombination, which introduces new gene combinations. This leads to hybrid vigor (heterosis), meaning the offspring are generally more robust, healthier, and show greater yield than the parents.
2. Adaptability: The increased genetic diversity allows the species to better adapt to changes in the environment (e.g., climate change, new diseases, pests), making the population more resilient and increasing its chances of survival.
3. Elimination of Defects: It can help mask or eliminate undesirable recessive traits that might otherwise become fixed in a self-pollinating population.

Disadvantages of Cross-Pollination

• Inefficiency and Wastage: Cross-pollination requires external agents (wind, water, insects, animals). It is often inefficient, as a large quantity of pollen is wasted because it doesn’t reach a suitable stigma.
• Dependence on External Agents: The plant is entirely dependent on a successful interaction with an external pollinator. If the pollinator population declines or weather conditions are poor, successful pollination and seed set may fail entirely.
• High Energy Cost: To attract pollinators, cross-pollinating flowers must invest significant energy into producing showy petals, nectar, and fragrances.
• Risk of Undesirable Crosses: There is a chance of interspecific pollination (pollination between different species), which usually results in no fertilization or the creation of sterile hybrids.

• Structured/Application/Skill Type

Question 1. 

What is the function of the pollen tube? Explain it with the help of a diagram. 

Ans:

The pollen tube is a temporary, tubular structure formed by the germinating pollen grain, and its primary function is to serve as the conduit for delivering the male gametes to the female gamete (egg cell) for fertilization.

Function of the Pollen Tube

The main function of the pollen tube can be broken down into two essential steps:

1. Transport of Male Gametes: The pollen tube grows down the style toward the ovary. It carries the two non-motile male nuclei (gametes), which originated from the generative nucleus of the pollen grain.
2. Penetration and Discharge: The tube eventually reaches the ovule and penetrates the embryo sac (usually through the micropyle). Once inside, it ruptures and discharges the two male gametes near the egg apparatus. One male gamete fuses with the egg nucleus (fertilization), and the other fuses with the secondary nucleus (forming the endosperm).

In essence, the pollen tube replaces the need for water as a transport medium for the male gametes, which is a key evolutionary feature of seed plants (spermatophytes).

Explanation with a Diagram

The diagram below illustrates the process:

Step	Description (Referring to the Diagram)
1. Germination	A pollen grain lands on the receptive stigma (S). It absorbs moisture and nutrients, causing the intine (inner pollen wall) to bulge out through the germ pore, forming the pollen tube.
2. Growth through Style	The pollen tube grows downward through the tissues of the style (St), guided by chemical signals. The tube nucleus is usually at the tip, guiding the growth, while the two male nuclei follow behind.
3. Entry into Ovule	The tube reaches the ovary (O), finds the ovule (Ov), and typically enters through the small pore called the micropyle (M).
4. Discharge	The pollen tube enters the embryo sac and bursts, releasing the two male nuclei (gametes), which then proceed to fuse with the egg cell and the secondary nucleus (double fertilization).

Question 2. 

1. Given ahead is a diagrammatic sketch of the sectional view of a germinating pollen grain. Study the same and then answer the question that follows:

Name the parts labelled 1, 2, 3, 4 and 5. 

2. Given ahead is a diagrammatic sketch of the sectional view of a germinating pollen grain. Study the same and then answer the question that follows:

Where does the germination of the pollen grain take place and how? 

3. Given ahead is a diagrammatic sketch of the sectional view of a germinating pollen grain. Study the same and then answer the question that follows:

What is the function of the part labelled ‘4’?

4. Given ahead is a diagrammatic sketch of the sectional view of a germinating pollen grain. Study the same and then answer the question that follows:

What happens to the part labelled ‘5’ during the process?

Ans:

1. Name the Parts Labelled 1, 2, 3, 4, and 5

Label Number	Name of Part
1	Exine (Outer wall)
2	Intine (Inner wall)
3	Pollen Tube
4	Tube Nucleus (or Vegetative Nucleus)
5	Male Gamete Nuclei (or Generative Nuclei)

2. Germination Location and Process

The germination of the pollen grain takes place on the stigma of the pistil.

Process:

1. Absorption: When a compatible pollen grain lands on the stigma, it absorbs the sugary, nutrient-rich fluid secreted by the stigma.
2. Growth: The intine (2), swells and pushes out through one of the thin spots in the exine (1), known as the germ pore.
3. Tube Formation: This outgrowth forms the pollen tube (3), which grows downward through the style towards the ovary, guided by chemical signals.

3. Function of Part Labelled ‘4’ (Tube Nucleus)

The primary function of the part labelled 4 (Tube Nucleus) is to guide the growth and direction of the pollen tube through the style towards the ovule.

It essentially controls the metabolic functions necessary for the pollen tube to penetrate the stigma and style tissue.

4. Fate of Part Labelled ‘5’ (Male Gamete Nuclei)

The part labelled 5 (Male Gamete Nuclei), which usually consists of two nuclei derived from the generative cell, are carried by the pollen tube to the ovule.

• Function in Fertilization: Once the pollen tube reaches the embryo sac inside the ovule, the two male nuclei are released, where they participate in double fertilization:
  • One male nucleus fuses with the egg cell nucleus to form the Zygote (which develops into the embryo).
  • The other male nucleus fuses with the secondary nucleus to form the Primary Endosperm Nucleus (which develops into the endosperm/food reserve).

Question 3. 

1. Given ahead is a diagrammatic representation of the process of fertilization. Study the same and then answer the question that follows: 

Name the parts labelled 1, 2, 3, 4, 5 and 6. 

2. Given ahead is a diagrammatic representation of the process of fertilization. Study the same and then answer the question that follows: 

What happens to

i) Ovary

ii) Ovule after fertilisation 

3. Given ahead is a diagrammatic representation of the process of fertilization. Study the same and then answer the question that follows:

What is the function of the synergids? 

4. Given ahead is a diagrammatic representation of the process of fertilization. Study the same and then answer the question that follows:

What part does the stigma play in the process of fertilisation?

Ans:

1. Name the Parts Labelled 1, 2, 3, 4, 5 and 6

The labels point to the structures involved in the fertilization process:

• 1: Style
• 2: Pollen Tube
• 3: Polar Nuclei (or Central Cell Nuclei)
• 4: Integuments
• 5: Male Gametes (or Male Nuclei)
• 6: Micropyle

2. Fate of Ovary and Ovule after Fertilization

After successful fertilization, significant transformations occur in the flower parts:

• i) Ovary: The ovary develops into the fruit. It undergoes rapid growth and modification to protect the developing seeds and aid in their dispersal.
• ii) Ovule: The ovule develops into the seed. The outer layer (integuments) hardens to become the seed coat (testa), and the fertilized egg develops into the embryo.

3. Function of the Synergids

The synergids are two specialized cells that flank the egg cell within the embryo sac.

• Their main function is to guide the growth of the pollen tube towards the egg apparatus using chemical signals.
• One synergid degenerates just before or after the pollen tube enters the embryo sac, allowing the pollen tube to discharge the two male gametes near the egg cell.

4. Role of the Stigma in Fertilization

The stigma is the receptive tip of the pistil and plays the crucial initial role in the entire fertilization process:

• Reception: It is the surface that receives the pollen grains during pollination.
• Selection: The stigma secretes a sticky, sugary substance that helps trap the pollen and stimulates it to germinate, ensuring only viable and compatible pollen can proceed further by forming a pollen tube.