Worksheet with Solutions: Sexual Reproduction in Flowering Plants | Biology Class 12 - NEET PDF Download

Ans: Microsporogenesis

Ans: Embryo sac

Ans: Exine

Ans: Triple fusion

Ans: Apomixis

Ans: 1 - B, 2 - A, 3 - D, 4 - C, 5 - E

Ans: (c) Pollen-pistil interaction involves a chemical dialogue between pollen and pistil components, determining compatibility.

Ans: (b) Four

Ans: (c) Xenogamy

Ans: (c) Tapetum

Ans: (d) One

Ans: (a) Both assertion and reason are true, and the reason is the correct explanation of the assertion.
In angiosperms, the typical mature female gametophyte (embryo sac) is formed after megagametogenesis and consists of seven cells but eight nuclei. These include:

• Three antipodal cells at the chalazal end

• Two synergids and one egg cell at the micropylar end

• One central cell that contains two polar nuclei

Though there are only seven cells, the central cell's two nuclei make the total count of nuclei eight. Therefore, both the assertion and the reason are correct, and the reason accurately explains the assertion.

Answer: (c) Assertion is true, but the reason is false.
Cleistogamous flowers are closed flowers that do not open at all. As a result, self-pollination (autogamy) is ensured because the pollen is transferred to the stigma within the same flower. This process does not involve any external pollination agent like wind or insects.
The reason incorrectly states that cross-pollination occurs via wind, which is not applicable to cleistogamous flowers. Hence, the assertion is true, but the reason is false.

Solution:

Structure of a Microsporangium

A microsporangium is a four-sided structure within the anther, surrounded by four wall layers: epidermis, endothecium, middle layers, and tapetum. The tapetum nourishes developing pollen grains, while the outer layers protect and aid in anther dehiscence. The sporogenous tissue in the center undergoes meiosis to form microspores.

Structure of a Pollen Grain

Pollen grains are spherical, 25-50 micrometers in diameter, with a two-layered wall: the exine (made of sporopollenin, with germ pores) and the intine (made of cellulose and pectin). They contain a vegetative cell with abundant food reserves and a generative cell that forms two male gametes.

Solution:
Process of Double Fertilisation
After pollination, the pollen tube enters the ovule through the micropyle and a synergid, releasing two male gametes. One gamete fuses with the egg cell (syngamy) to form a diploid zygote, which develops into the embryo. The other gamete fuses with two polar nuclei in the central cell (triple fusion) to form a triploid primary endosperm nucleus, which develops into the endosperm.

Significance of Double Fertilisation
Double fertilisation ensures the formation of the zygote (embryo) and endosperm (nutrient source), providing essential nourishment for the developing embryo, enhancing reproductive success.

Ans: The tapetum, the innermost wall layer of the microsporangium, nourishes developing pollen grains by providing nutrients. Its cells have dense cytoplasm and often more than one nucleus.

Ans:

• Autogamy: Transfer of pollen grains from the anther to the stigma of the same flower, requiring synchrony in pollen release and stigma receptivity.

• Xenogamy: Transfer of pollen grains from the anther to the stigma of a different plant, bringing genetically different pollen grains to the stigma.

Ans: The filiform apparatus, present at the micropylar tip of the synergids, guides the pollen tube into the synergid, facilitating the delivery of male gametes for fertilisation.

Ans: Apomixis is the formation of seeds without fertilisation, mimicking sexual reproduction. It is important in hybrid seed production as it allows hybrids to produce seeds with consistent traits, reducing the need for costly annual hybrid seed production.

Ans: A typical angiosperm ovule consists of a funicle (stalk), hilum (junction with funicle), one or two integuments with a micropyle, a chalaza (basal part), and a nucellus containing the embryo sac.

Ans: The endosperm, formed after triple fusion, provides nutrients to the developing embryo. It may be consumed during embryo development or persist in the mature seed for use during germination.

Ans: Self-incompatibility is a genetic mechanism that prevents self-pollen from fertilising ovules by inhibiting pollen germination or pollen tube growth in the pistil, promoting cross-pollination.

Ans: The zygote remains dormant until sufficient endosperm is formed to provide assured nutrition for the developing embryo.

Ans: Flowering plants have evolved several mechanisms to promote cross-pollination and discourage self-pollination:

• Dichogamy: Pollen release and stigma receptivity are not synchronised, with pollen released before stigma receptivity (protandry) or vice versa (protogyny).

• Herkogamy: Anthers and stigmas are placed at different positions to prevent self-pollination.

• Self-Incompatibility: A genetic mechanism prevents self-pollen from germinating or growing pollen tubes, ensuring only compatible pollen fertilises ovules.

• Unisexual Flowers: Monoecious plants (e.g., maize) have separate male and female flowers, preventing autogamy. Dioecious plants (e.g., papaya) have male and female flowers on different plants, preventing both autogamy and geitonogamy.

• Biotic Pollination: Flowers are large, colourful, fragrant, and nectar-rich to attract pollinators like bees, butterflies, and birds, ensuring cross-pollination.

Ans: Artificial hybridisation is a controlled breeding technique to produce hybrids with desired traits. It involves:

• Emasculation: Removal of anthers from the female parent flower (bisexual flowers) to prevent self-pollination.

• Bagging: Covering the emasculated flower with a bag to protect it from unwanted pollen.

• Pollination: Dusting pollen from the desired male parent onto the stigma of the emasculated flower.

• Rebagging: Covering the pollinated flower to ensure controlled pollination.
  This technique ensures cross-pollination between selected parent plants, producing hybrids with improved traits like higher yield or disease resistance. It is significant in crop improvement as it allows precise combination of desirable genetic traits, enhancing agricultural productivity.

Ans:

• Pollen-pistil interaction is a dynamic pre-fertilisation process involving a chemical dialogue between pollen and pistil to determine compatibility.
• When pollen lands on the stigma, the pistil recognizes compatible pollen, allowing germination and pollen tube growth through the style, guided by stigma secretions.
• The pollen tube enters the ovule via the micropyle and a synergid, releasing two male gametes for double fertilisation.
• Incompatible pollen is rejected by inhibiting germination or tube growth, often due to self-incompatibility.
• This interaction ensures only compatible pollen leads to fertilisation, maintaining genetic diversity and preventing self-fertilisation in self-incompatible plants. 

Ans:

• Pre-Fertilisation Events:

  • Microsporogenesis and Pollen Formation: Pollen mother cells in the anther’s microsporangia undergo meiosis to form microspore tetrads, which develop into pollen grains (male gametophytes) with a vegetative and generative cell.

  • Megasporogenesis and Embryo Sac Formation: A megaspore mother cell in the ovule undergoes meiosis to form four megaspores, one of which develops into a 7-celled, 8-nucleate embryo sac (female gametophyte) .

  • Pollination: Transfer of pollen grains from anther to stigma, either by autogamy, geitonogamy, or xenogamy, facilitated by biotic or abiotic agents.

  • Pollen-Pistil Interaction: Compatible pollen germinates on the stigma, forming a pollen tube that grows through the style and enters the ovule, guided by the filiform apparatus.

• Post-Fertilisation Events:

  • Double Fertilisation: One male gamete fuses with the egg (syngamy) to form a diploid zygote, and the other fuses with two polar nuclei (triple fusion) to form a triploid primary endosperm nucleus.

  • Endosperm Development: The primary endosperm nucleus forms free-nuclear, then cellular endosperm, providing nutrients to the embryo.

  • Embryo Development: The zygote develops into a proembryo, then globular, heart-shaped, and mature embryo with cotyledons and embryonic axis.

  • Seed and Fruit Formation: Ovules become seeds with a seed coat, and the ovary develops into a fruit (true or false) with a pericarp.