Ncert Books Class 12 Biology Chapter 2 Free Download

For the CBSE board exam, mastering diagrams from this chapter is essential for scoring well. The most frequently asked diagrams are:

• Anatropous Ovule: Key labels include the micropyle, chalazal pole, integuments, nucellus, funicle, and the embryo sac with its component cells (egg apparatus, central cell, antipodals).
• Mature Embryo Sac (Female Gametophyte): Ensure you correctly label the filiform apparatus, synergids, egg cell, polar nuclei within the central cell, and the three antipodal cells.
• Dicot Embryo: Key labels are the cotyledons, plumule, radicle, and embryonal axis.
• L.S. of a Grass Embryo (Monocot): Important labels include the scutellum, coleoptile, plumule, coleorhiza, and radicle.

Always draw with a sharp pencil and ensure all labels are clear and correctly pointed.

To secure full marks for a question on double fertilisation, your answer should be structured logically with all key events included. Follow these steps:

• Introduction: Start by defining double fertilisation as a unique characteristic of angiosperms.
• Pollen Tube Entry: Mention the journey of the pollen tube through the style and its entry into the ovule, typically through the micropyle, guided by the filiform apparatus of synergids.
• Discharge of Male Gametes: State that the pollen tube releases two male gametes into the cytoplasm of a synergid.
• Event 1 - Syngamy: Describe how one male gamete fuses with the egg cell nucleus to form the diploid zygote (2n).
• Event 2 - Triple Fusion: Describe how the second male gamete fuses with the two polar nuclei in the central cell to form the triploid Primary Endosperm Nucleus (PEN) (3n).
• Conclusion: Conclude by stating that since two fusion events (syngamy and triple fusion) occur, the phenomenon is called double fertilisation. A simple labelled diagram showing the process will enhance your score.

Outbreeding devices are crucial evolutionary mechanisms in flowering plants. Their significance lies in promoting cross-pollination (xenogamy) and preventing continuous self-pollination (autogamy). This is biologically important because:

• Prevents Inbreeding Depression: Continuous self-pollination can lead to a loss of genetic variation and vigour in the offspring, known as inbreeding depression.
• Promotes Genetic Variation: Cross-pollination brings together different genetic combinations, leading to healthier offspring with better chances of survival and adaptability to changing environments.
• Evolution of New Species: The genetic recombination resulting from cross-pollination is a key driver for evolution.

Devices like dichogamy (pollen release and stigma receptivity are not synchronised) and self-incompatibility (genetic mechanism to prevent self-pollen from fertilising the ovules) ensure these advantages.

For 1-mark questions, a clear understanding of specific terminology is vital. Important terms include:

• Tapetum: The innermost nutritive layer of the microsporangium wall.
• Filiform apparatus: The cellular thickenings in synergids that guide pollen tube entry.
• Perisperm: The persistent remnant of nucellus in some seeds, like black pepper.
• Pericarp: The wall of the fruit, which develops from the wall of the ovary.
• Parthenocarpy: The development of fruit without fertilisation, resulting in seedless fruits.
• Apomixis: Production of seeds without fertilisation.
• Scutellum: The large, shield-shaped cotyledon of a grass embryo.

The development of the endosperm precedes embryo development because the primary function of the endosperm is to provide nourishment to the growing embryo. The zygote typically divides only after a certain amount of endosperm is formed. This is a biological adaptation to ensure that the developing embryo has a confirmed and readily available food supply for its growth and differentiation into plumule, radicle, and cotyledons. Without this nutritive tissue, the embryo would likely fail to develop, making the entire reproductive process futile.

Yes, a fruit can develop without the act of fertilisation. This phenomenon is called parthenocarpy. In parthenocarpy, the ovary is stimulated to develop into a fruit without the formation of a zygote and seeds. A common example is the banana.

The key differences are:

• Parthenocarpic Fruit: Develops from the ovary without fertilisation. These fruits are typically seedless.
• True Fruit: Develops from a mature, fertilised ovary. These fruits contain seeds, which themselves contain the embryo for the next generation.

Understanding the differences between these two processes is often tested in 3-mark questions. The key distinctions are:

• Location: Microsporogenesis occurs inside the anther (in the microsporangium), while megasporogenesis occurs inside the ovule (in the nucellus).
• Starting Cell: Microsporogenesis starts from a diploid microspore mother cell (MMC) or pollen mother cell (PMC). Megasporogenesis starts from a diploid megaspore mother cell (MMC).
• Product: In microsporogenesis, one MMC produces four functional, haploid microspores (pollen grains). In megasporogenesis, one MMC produces four haploid megaspores, but typically only one remains functional while the other three degenerate.
• Final Structure: The functional microspore develops into the male gametophyte (pollen grain). The functional megaspore develops into the female gametophyte (embryo sac).