Sexual Reproduction in Flowering Plants: Class 12 Biology Practice Questions

Dr. Meera, a plant biologist, was studying pollination and fertilisation in a flowering plant species. She observed that when pollen grains from the same plant were placed on the stigma, the pollen tubes failed to penetrate the style and degenerated within minutes. However, when pollen from a different plant of the same species was used, successful pollen tube growth occurred, leading to seed formation. She also noted that the central cell of the embryo sac contained two nuclei that participated in one of the two fertilisation events. After successful fertilisation, she tracked the development of various structures within the ovule.

Read the following passage and answer the questions that follow:

Dr. Meera, a plant biologist, was studying pollination and fertilisation in a flowering plant species. She observed that when pollen grains from the same plant were placed on the stigma, the pollen tubes failed to penetrate the style and degenerated within minutes. However, when pollen from a different plant of the same species was used, successful pollen tube growth occurred, leading to seed formation. She also noted that the central cell of the embryo sac contained two nuclei that participated in one of the two fertilisation events. After successful fertilisation, she tracked the development of various structures within the ovule.

(i) Name the phenomenon Dr. Meera observed when pollen from the same plant failed to germinate on the stigma. What is the biological significance of this phenomenon? (1 mark)

(ii) The two nuclei present in the central cell of the embryo sac participated in triple fusion. Name the product formed and state its ploidy. How does this product develop into a nutritive tissue? (1 mark)

(iii) After successful double fertilisation, Dr. Meera tracked two simultaneous developmental events. Complete the following table showing the fate of different structures post-fertilisation: (2 marks)

| Structure Before Fertilisation | Structure/Product After Fertilisation |
|---|---|
| Zygote (2n) | ? |
| Primary Endosperm Nucleus (3n) | ? |
| Integuments | ? |
| Ovule | ? |

A botanist studying pollination biology in a greenhouse observed two flowering plant species — Species X and Species Y. In Species X, the pollen grains are large, sticky, and brightly coloured, while the flowers produce nectar. In Species Y, the pollen grains are small, light, and produced in enormous quantities, but the flowers are inconspicuous and lack nectar. When the botanist examined the pistil of Species X under a microscope after pollination, she observed that pollen grains had germinated on the stigma, but only pollen from the same plant had failed to form a functional pollen tube, whereas cross-pollen tubes grew normally and reached the ovule.

Read the following case and answer the questions that follow:

A botanist studying pollination biology in a greenhouse observed two flowering plant species — Species X and Species Y. In Species X, the pollen grains are large, sticky, and brightly coloured, while the flowers produce nectar. In Species Y, the pollen grains are small, light, and produced in enormous quantities, but the flowers are inconspicuous and lack nectar. When the botanist examined the pistil of Species X under a microscope after pollination, she observed that pollen grains had germinated on the stigma, but only pollen from the same plant had failed to form a functional pollen tube, whereas cross-pollen tubes grew normally and reached the ovule.

(i) Identify the likely pollinating agent for Species X and Species Y respectively. Give one structural feature from the passage to justify each identification. (1 + 1 = 2 marks)

(ii) Name the phenomenon observed in Species X where self-pollen fails to germinate/form a functional pollen tube on the stigma of the same plant. State one advantage this phenomenon provides to the species. (1 + 1 = 2 marks)

A botany student was examining a mature angiosperm ovule under the microscope. She observed a flask-shaped structure at the micropylar end containing three cells, a large central cell with two nuclei, and three cells grouped at the opposite end. She drew the structure and labelled it as the 'female gametophyte'. Her teacher told her that this structure is central to the process of double fertilisation — a unique and defining feature of angiosperms.

Read the following passage and answer the questions that follow:

A botany student was examining a mature angiosperm ovule under the microscope. She observed a flask-shaped structure at the micropylar end containing three cells, a large central cell with two nuclei, and three cells grouped at the opposite end. She drew the structure and labelled it as the 'female gametophyte'. Her teacher told her that this structure is central to the process of double fertilisation — a unique and defining feature of angiosperms.

(i) Name the structure the student observed. State the total number of cells and nuclei present in it at maturity. (1)
(ii) Draw a neat, labelled diagram of the mature female gametophyte as described above, showing the correct position of all its constituent cells/nuclei. (2)
(iii) Explain how double fertilisation occurs in this structure. Name the two products formed and state the ploidy of each. (1)

A researcher studying pollen-pistil interactions observes that when pollen from Plant X is dusted onto the stigma of Plant Y (a different but closely related species), the pollen germinates successfully and the pollen tube begins to grow through the style. However, fertilisation never occurs — the pollen tube invariably stops growing before it reaches the ovule. Which of the following is the MOST likely explanation for this observation?

In a typical angiosperm embryo sac, the two polar nuclei are located in the:

A botanist observes that in a particular flowering plant species, pollen grains germinate readily on the stigma of the same flower but the pollen tube growth arrests and degenerates within the style before reaching the ovule. Which of the following best explains this observation?

In the embryo sac of a typical angiosperm, which of the following correctly describes the position and number of nuclei in the central cell?

Assertion (A): In a developing angiosperm seed, the endosperm of an albuminous seed is partially consumed by the time the seed reaches maturity, while in an ex-albuminous seed the endosperm is completely consumed and food is stored in the cotyledons.
Reason (R): Endosperm develops from the Primary Endosperm Nucleus (PEN) formed by triple fusion and is triploid (3n); it provides nutrition to the developing embryo during seed development.

In a typical angiosperm embryo sac, which of the following correctly describes the position and ploidy of the central cell?

Assertion (A): In a mature embryo sac of a flowering plant, the central cell contains two polar nuclei and is the largest cell.
Reason (R): After double fertilisation, the central cell (with two polar nuclei) fuses with one male gamete to form the primary endosperm nucleus (PEN), which is triploid (3n).

A fruit orchard owner notices that mango trees grown from seeds show great variation in fruit quality, but trees propagated vegetatively (by grafting) produce fruits identical to the parent. He wants to develop mango seedlings that ALWAYS breed true AND can be grown from seeds (without grafting). Which of the following biological strategies, if incorporated into mango, would best fulfill BOTH his requirements simultaneously?

In an angiosperm embryo sac, the correct positional arrangement of cells from the micropylar end to the chalazal end is:

A botanist observes that in a flowering plant species, the pollen grains deposited on the stigma germinate readily and pollen tubes begin to grow. However, the pollen tubes stop growing midway through the style and never reach the ovule. Which of the following best explains this observation?

The embryo sac of a typical flowering plant is described as '7-celled but 8-nucleate'. (a) Which cell of the embryo sac contains two nuclei, and what is it called? (b) What happens to this cell after double fertilisation?

Write the names of the two types of cells that together constitute the 'egg apparatus' in a mature embryo sac. State the position (end) at which the egg apparatus is located.

A fruit orchard manager notices that mango trees grown from seeds show great variation in fruit quality, while trees propagated by grafting produce fruits identical to the parent tree. He decides to use apomixis as an alternative strategy for mango cultivation. (i) What cellular process is bypassed in apomixis that leads to identical offspring? (ii) State one agricultural advantage this gives the orchard manager over seed-grown trees.

A botanist observes that in a certain flowering plant, the pollen tube always enters the ovule through the chalazal end rather than the micropyle. (i) Name this type of pollen tube entry. (ii) In such a plant, the male gametes are released near the antipodal cells. Will fertilisation still be successful? Give a reason.

A botanist studying pollen-pistil interaction in a self-incompatible species noticed the following: When pollen from the same plant was dusted on the stigma, pollen tubes failed to grow beyond the style. However, when cross-pollen (from a different plant of the same species) was used, pollen tubes successfully reached the ovules and fertilisation occurred. Based on this observation, answer the following:

(i) Identify the type of incompatibility being described. Name the genetic basis responsible for this recognition mechanism. (1 mark)

(ii) Trace the complete path of a compatible pollen tube from stigma to the embryo sac, naming each structure it passes through. State what is released from the pollen tube at the end of this journey and where exactly it enters the embryo sac. (1 mark)

(iii) Once both male gametes are released inside the embryo sac, describe what happens to EACH of them. Name the two products formed, state the ploidy of each, and explain the significance of one of these products for the developing embryo. (1 mark)

Fill in the blanks A, B, C and D in the table given below, which relates to the structures formed after double fertilisation and their origins in a flowering plant:

Structure | Develops from
A | Zygote (2n)
Endosperm | B
Seed coat (Testa + Tegmen) | C
D | Remains of the nucellus

Attempt either option (A) or (B).

(A) (i) Draw a neat, labelled diagram of a mature embryo sac (female gametophyte) of an angiosperm. (Any four labels)
(ii) State the ploidy of any two of the labelled structures.

OR

(B) (i) Arrange the following events of double fertilisation in the correct sequence:
Pollen tube reaches embryo sac → Two male gametes released → Syngamy → Triple fusion → Formation of endosperm → Formation of zygote
(ii) State the ploidy of the products formed in syngamy and triple fusion respectively.

A botany student observes that when pollen grains of Species A are placed on the stigma of Species B, the pollen grains fail to germinate, even though both species belong to the same family. However, when pollen grains of Species A are placed on the stigma of Species A, germination occurs readily and pollen tubes grow successfully.

(i) Identify the phenomenon responsible for the failure of pollen germination in the first case. Name the molecules involved in this recognition.
(ii) Once a compatible pollen grain successfully germinates on the stigma of Species A, describe the pathway of the pollen tube from stigma to the egg cell, naming the structures it passes through.
(iii) How many male gametes are released by the pollen tube inside the embryo sac, and what is the fate of each? Name the phenomenon.

A botanist is studying two mango varieties in an orchard. She observes that Variety A produces plump, fully developed seeds with abundant fleshy endosperm, while Variety B consistently produces seedless fruits with no endosperm, even though both varieties flower normally and are visited by pollinators. She also notices that in Variety A, the ovule has a persistent, starchy tissue surrounding the embryo sac even after fertilisation.

(a) Identify the process by which Variety B produces fruits without fertilisation. Name ONE other example of such a fruit. (1 mark)

(b) Name the 'persistent starchy tissue surrounding the embryo sac' seen in Variety A. From which part of the ovule does it develop, and what is its ploidy level? (1 mark)

(c) The endosperm in Variety A is rich and persistent. State the ploidy of endosperm and explain the sequence of events — from triple fusion to endosperm formation — that accounts for its nutritive function for the embryo. (1 mark)

During post-fertilisation development in a flowering plant, the following events were observed:
(i) The zygote divides to form a proembryo, and subsequently a globular, heart-shaped, and then a torpedo-shaped embryo.
(ii) The primary endosperm nucleus divides repeatedly before the zygote begins to divide.
(iii) The integuments of the ovule harden and become the seed coat.

Answer the following:
(a) Name the two components that constitute the 'embryo axis' in a dicot embryo, and state what each gives rise to in the germinating seedling. (1 mark)
(b) Identify the correct ploidy of (i) zygote, (ii) primary endosperm nucleus, and (iii) cells of the seed coat. Justify why the endosperm develops before the embryo in most angiosperms. (2 marks)

A botanist is studying post-pollination events in two flowering plant species: Species A (self-incompatible) and Species B (self-compatible). In Species A, when self-pollen lands on the stigma, the pollen grain germinates but the pollen tube fails to reach the ovule. In Species B, cross-pollination results in successful fertilisation. The botanist observes the mature female gametophyte of Species B and notes 7 cells and 8 nuclei before fertilisation.

(i) Draw a neat, labelled diagram of the mature female gametophyte (embryo sac) of Species B, showing the correct position of all 7 cells and 8 nuclei. [2 marks]
(ii) Describe the events of double fertilisation that follow, clearly stating the chromosome number (ploidy) of each product formed. [2 marks]
(iii) With reference to Species A, explain the biological significance of self-incompatibility and name the molecular basis by which the pistil distinguishes self-pollen from cross-pollen. [1 mark]

With reference to the post-fertilisation events in a flowering plant, answer the following:
(i) Draw a neat, labelled diagram of a mature embryo sac (female gametophyte) of a typical angiosperm. (2 marks)
(ii) Describe the events of double fertilisation, clearly stating the chromosome number of each cell/nucleus involved. (2 marks)
(iii) What is the ploidy of the endosperm formed after double fertilisation? Name any one type of endosperm on the basis of development and give an example. (1 mark)

A botanist studying a flowering plant notices that each flower produces a large number of pollen grains but only a few seeds are formed per fruit. She collects a mature anther and observes its cross-section under a microscope, then traces the journey of a single pollen grain from its formation to the completion of double fertilisation inside the ovule.

(i) Draw a neat, labelled diagram of a transverse section (T.S.) of a young anther showing all four wall layers. Name the wall layer most important for pollen nourishment and state its function. (3 marks)

(ii) Describe the events of double fertilisation, clearly stating the ploidy of all nuclei involved and the structures formed. (2 marks)

A botanist is studying the reproductive cycle of a flowering plant (Hibiscus rosa-sinensis) in the laboratory. She carefully isolates a mature embryo sac from the ovule and also collects pollen grains from mature anthers of the same plant. She observes that after pollination and pollen tube entry, two male gametes are discharged and participate in two separate fusion events. Subsequently, the ovule undergoes a series of developmental changes to finally form a seed.

(i) Draw a neat, labelled diagram of the mature embryo sac, clearly showing the position of all the cells/nuclei. [2 marks]
(ii) Describe, step by step, the two fusion events that occur inside the embryo sac. Mention the ploidy of the products formed. [2 marks]
(iii) The botanist notices that the mature seed she obtains has a hard seed coat, fleshy endosperm, and a well-differentiated embryo. Identify the embryonic structures that give rise to: (a) seed coat, (b) endosperm, (c) embryo, and also state the ploidy of the endosperm nucleus before it begins to divide. [1 mark]

A botanist collected a mature angiosperm flower and observed its anther under a microscope. She noticed that the anther had a distinct wall with multiple layers and contained structures that would eventually give rise to pollen grains.

(a) Draw a neat, labelled diagram of a T.S. of a mature anther (microsporangium), showing all four wall layers and the pollen grains inside.
(b) Name the four wall layers of the microsporangium from outermost to innermost and state the function of each layer.
(c) The botanist also noticed that the innermost wall layer was nutritive in nature. Name this layer and explain how it helps in pollen grain development.

A botanist is studying sexual reproduction in a flowering plant species. She observes the following structures and events during her investigation:

(i) A structure with 7 cells and 8 nuclei is found inside the ovule.
(ii) Two male gametes are released from the pollen tube into this structure.
(iii) One male gamete fuses with the egg cell; the other fuses with the central cell.
(iv) The ovule eventually develops into a seed, and the ovary develops into a fruit.

Based on the above observations, answer the following questions:

(a) Name the 7-celled, 8-nucleate structure mentioned in observation (i). Where exactly is it located within the ovule? (1 mark)

(b) Identify the two fusion events described in observation (iii). Name the products formed and state the ploidy (chromosome number) of each product. (2 marks)

(c) What is the significance of the event described in observation (iii) that makes it unique to angiosperms? (1 mark)

(d) Name any ONE structure in the embryo sac, other than the egg cell and central cell, and state its position within the embryo sac. (1 mark)