Chapter 1: Sexual Reproduction in Flowering Plants Case Study Questions | biology Class 12 CBSE

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Chapter 1: Sexual Reproduction in Flowering Plants Case Study Questions | biology Class 12 CBSE | ByteNotes.in

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Riya was observing a transverse section of an anther under a compound microscope during her biology practical. She noticed that the anther had a distinct bilobed structure with four microsporangia, two in each lobe. The outermost layer appeared thin and protective, followed by the endothecium, then middle layers, and finally the innermost nutritive layer surrounding a mass of homogenous cells at the centre. Her teacher explained that these central cells would undergo a special type of cell division to produce microspores, which would eventually develop into pollen grains. Riya was fascinated to learn that the innermost wall layer plays a critical role in nourishing developing pollen and often contains cells with more than one nucleus.

Question 1: Name the innermost wall layer of the microsporangium and state its primary function.

The innermost wall layer is called the tapetum.
Its primary function is to nourish the developing pollen grains (microspores).

Question 2: What is the name of the central homogenous cells Riya observed, and what process do they undergo to form microspores?

The central homogenous cells are called sporogenous tissue.
They undergo meiotic divisions (microsporogenesis) to form microspore tetrads.

Question 3: How do tapetal cells become bi-nucleate? Describe the structure of a mature pollen grain, mentioning the role of each wall layer.

Tapetal cells become bi-nucleate either by cell fusion or by nuclear division without cytokinesis (endomitosis).
A mature pollen grain has two wall layers: the outer exine made of sporopollenin, which is highly resistant and helps in protection and fossilisation; it has germ pores for pollen tube emergence.
The inner layer is the intine, made of cellulose and pectin, which is thin and continuous.
The pollen grain contains a vegetative cell (large, with food reserves) and a generative cell (small, spindle-shaped, gives rise to two male gametes).

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During a field trip, students collected pollen samples from various flowering plants. Their teacher explained that pollen grains are excellent fossil indicators because of a special substance in their outer wall. The teacher also noted that pollen of certain plants, like Parthenium (carrot grass), can cause severe allergic reactions including asthma and bronchitis. The students further learnt that pollen viability varies greatly: rice and wheat pollen lose viability within 30 minutes of release, while members of Rosaceae can maintain it for months. For conservation purposes, pollen can be stored in liquid nitrogen at –196°C in pollen banks, similar to seed banks, for use in crop breeding programmes.

Question 1: Why are pollen grains well-preserved as fossils? Name the substance responsible.

Pollen grains are well-preserved as fossils because the outer wall (exine) is made of sporopollenin.
Sporopollenin is one of the most resistant organic materials known; it can withstand high temperatures, strong acids, and alkali, and no enzyme is known to degrade it.

Question 2: What is a pollen bank? At what temperature is pollen stored, and what is its significance?

A pollen bank is a facility where pollen grains of a large number of species are stored for long-term use.
Pollen is stored in liquid nitrogen at –196°C; it can be used in crop breeding programmes similar to how seed banks are used.

Question 3: What are germ pores? Explain the significance of pollen viability in the context of fertilisation, and mention two factors affecting viability.

Germ pores are prominent apertures in the exine where sporopollenin is absent; they serve as sites for pollen tube emergence during germination.
Pollen grains must land on the stigma before losing viability for fertilisation to occur.
Pollen viability is critical because if pollen loses viability before reaching the stigma, fertilisation cannot take place.
Two factors affecting viability: temperature and humidity — low temperature and low humidity (as in liquid nitrogen storage) prolong viability.

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Case Set 3

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A botany student was studying the pistil of a typical flowering plant. She observed that the pistil consists of three main parts. The basal swollen part, when cut transversely, revealed one or more ovules attached to a cushion-like tissue. Each ovule appeared as a small structure attached by a stalk. The ovule had two protective layers, a central nutritive tissue, and a large cell with a prominent nucleus at the micropylar end. The student further studied the development of the female gametophyte and noticed that one particular cell in the ovule undergoes meiosis to produce four cells, but only one survives to develop further.

Question 1: Name the three main parts of a pistil and identify the tissue to which ovules are attached inside the ovary.

The three main parts of a pistil are: stigma, style, and ovary.
The ovules are attached to the placenta inside the ovary.

Question 2: Name the large cell at the micropylar end of the ovule with a prominent nucleus. What is the stalk of the ovule called?

The large cell at the micropylar end is called the megaspore mother cell (MMC).
The stalk of the ovule is called the funicle (funiculus).

Question 3: Describe megasporogenesis. How does the megaspore develop into the female gametophyte (embryo sac)? Name all the cells of a mature embryo sac.

The megaspore mother cell (2n) undergoes meiosis to produce a tetrad of four megaspores; three degenerate and only one (functional megaspore) survives — this is megasporogenesis.
The functional megaspore undergoes three successive mitotic divisions to produce 8 nuclei arranged into 7 cells: 3 antipodal cells, 2 synergids, 1 egg cell (at micropylar end), and 1 central cell with 2 polar nuclei — forming the Polygonum type embryo sac.
The mature embryo sac cells are: egg cell, 2 synergids (with filiform apparatus), 2 polar nuclei (in central cell), and 3 antipodal cells.

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Pollination is a vital event in the sexual reproduction of flowering plants. During a nature walk, a group of students observed bees hovering around brightly coloured, fragrant flowers, while other nearby plants had inconspicuous, greenish flowers releasing large amounts of fine, dry pollen. Some aquatic plants they saw appeared to release pollen at or below the water surface. Back in class, their teacher explained that each pollination strategy is an evolutionary adaptation. Flowers pollinated by specific agents show particular structural and chemical features that attract or suit those agents. The teacher also discussed that pollination within the same flower or between flowers of the same plant is different from pollination between flowers of different plants.

Question 1: Distinguish between autogamy and geitonogamy with one example each.

Autogamy is the transfer of pollen from the anther to the stigma of the same flower (e.g., in wheat).
Geitonogamy is the transfer of pollen from the anther of one flower to the stigma of another flower on the same plant (e.g., in maize); it is genetically equivalent to autogamy but functionally cross-pollination.

Question 2: List two characteristic features of wind-pollinated (anemophilous) flowers.

Wind-pollinated flowers produce large quantities of light, dry, non-sticky pollen grains.
They have well-exposed stamens and large feathery or sticky stigmas to trap airborne pollen; flowers are usually small, inconspicuous, and without bright colours or fragrance.

Question 3: Describe hydrophily with suitable examples. How do aquatic plants like Vallisneria ensure pollination? What is the significance of water-impermeable pollen in such plants?

Hydrophily is pollination by water, occurring in aquatic plants; it is relatively rare (only about 30 genera).
In Vallisneria, the female flowers reach the water surface by long stalks; male flowers are released and float on the surface, reach female flowers, and pollination occurs at the water surface.
In Zostera (a marine angiosperm), pollen is released underwater; pollen grains are long, ribbon-like, and have a specific gravity equal to water, allowing them to be carried to the stigma.
Water-impermeable (mucilage-covered) pollen is protected from wetting and dissolution, ensuring viability during water transport.

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Case Set 5

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A research student was working on a cross-breeding experiment in rice. She used the bagging technique to prevent contamination from unwanted pollen. First, she removed the anthers from the bisexual flowers before they matured using fine forceps — a process called emasculation. The emasculated flowers were then covered with butter-paper bags. When the stigmas were mature and receptive, she collected pollen from the desired male parent and dusted it on the bagged stigmas, then rebagged the flowers. She also observed that some plants in the field produce only unisexual flowers, making emasculation unnecessary. She noted that flowers preventing self-pollination had different strategies: in some, anthers and stigmas matured at different times; in others, pollen did not germinate on the same plant's stigma.

Question 1: What is emasculation and why is it performed before bagging in plant breeding?

Emasculation is the removal of anthers from bisexual (hermaphrodite) flowers before they shed pollen.
It is done to prevent self-pollination and ensure that only the desired pollen is used for cross-pollination in a plant breeding programme.

Question 2: What is self-incompatibility? How does it prevent self-pollination?

Self-incompatibility is a genetic mechanism that prevents self-pollen from germinating on the stigma of the same flower or the same plant.
In self-incompatible species, pollen recognises the stigma as 'self' and its germination is inhibited, so fertilisation does not occur even if pollen lands on the stigma.

Question 3: Name and explain any three outbreeding devices evolved by flowering plants to prevent self-pollination.

Dichogamy: Pollen release and stigma receptivity occur at different times (e.g., protandry — anthers mature before stigma; protogyny — stigma matures before anthers).
Herkogamy: The anther and stigma are placed at different positions in the flower, making self-pollination mechanically difficult (e.g., Gloriosa).
Self-incompatibility: A genetic mechanism where pollen from the same plant is rejected by the stigma, preventing germination and fertilisation.
(Also acceptable) Unisexuality (monoecious/dioecious): Having separate male and female flowers on the same or different plants eliminates self-pollination.

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Chapter 1: Sexual Reproduction in Flowering Plants | Case Study Questions

Passage

Question 1: Name the innermost wall layer of the microsporangium and state its primary function.

Question 2: What is the name of the central homogenous cells Riya observed, and what process do they undergo to form microspores?

Question 3: How do tapetal cells become bi-nucleate? Describe the structure of a mature pollen grain, mentioning the role of each wall layer.

Passage

Question 1: Why are pollen grains well-preserved as fossils? Name the substance responsible.

Question 2: What is a pollen bank? At what temperature is pollen stored, and what is its significance?

Question 3: What are germ pores? Explain the significance of pollen viability in the context of fertilisation, and mention two factors affecting viability.

Passage

Question 1: Name the three main parts of a pistil and identify the tissue to which ovules are attached inside the ovary.

Question 2: Name the large cell at the micropylar end of the ovule with a prominent nucleus. What is the stalk of the ovule called?

Question 3: Describe megasporogenesis. How does the megaspore develop into the female gametophyte (embryo sac)? Name all the cells of a mature embryo sac.

Passage

Question 1: Distinguish between autogamy and geitonogamy with one example each.

Question 2: List two characteristic features of wind-pollinated (anemophilous) flowers.

Question 3: Describe hydrophily with suitable examples. How do aquatic plants like Vallisneria ensure pollination? What is the significance of water-impermeable pollen in such plants?

Passage

Question 1: What is emasculation and why is it performed before bagging in plant breeding?

Question 2: What is self-incompatibility? How does it prevent self-pollination?

Question 3: Name and explain any three outbreeding devices evolved by flowering plants to prevent self-pollination.

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