CBSE Class 12 Biology NCERT Solutions: Sexual Reproduction in Flowering Plants

The four primary floral whorls of a flower are:

1. Calyx: The outermost whorl, typically consisting of sepals.
2. Corolla: The second whorl, composed of petals.
3. Androecium: The third whorl, made up of stamens (male reproductive organs).
4. Gynoecium: The innermost whorl, consisting of pistils or carpels (female reproductive organs).

While 'carpel' is a component of the gynoecium and 'sepal' is a component of the calyx, neither 'carpel' nor 'sepal' are technically names for the entire floral whorl itself. The terms for the whorls are calyx, corolla, androecium, and gynoecium.

This question highlights an analogy in plant reproduction. The ovule contains the female gametophyte, which is the embryo sac. Similarly, the anther contains the male gametophyte, which is the pollen grain.

Here's the breakdown:

• Within the ovule, a megaspore mother cell differentiates and undergoes meiosis to form megaspores. One functional megaspore develops into the embryo sac (female gametophyte).
• Within the anther, microspore mother cells undergo meiosis to form microspores. Each microspore develops into a pollen grain (male gametophyte).

Therefore, the relationship of the embryo sac to the ovule is analogous to the relationship of the pollen grain to the anther.

In a typical flower, the floral organs are arranged in whorls on the thalamus (the part of the flower stalk where the parts of the flower are attached). In a hypogynous flower, the ovary is superior, meaning it is positioned above the attachment points of other floral parts. The arrangement from the outermost to the innermost is as follows:

1. Calyx: The outermost whorl, consisting of sepals.
2. Corolla: Located inside the calyx, consisting of petals.
3. Androecium: Inside the corolla, consisting of stamens (the male reproductive organs).
4. Gynoecium: The innermost whorl, located in the center of the flower, consisting of pistils or carpels (the female reproductive organs).

This order ensures the protective outer layers are followed by the reproductive structures.

The production of fruits and seeds is a result of successful fertilization, which requires the fusion of male and female gametes. Let's analyze the options:

• Dioecious plants have separate male and female individuals. Male individuals bear only staminate (male) flowers, and female individuals bear only pistillate (female) flowers.
• Monoecious plants have both male and female reproductive organs or flowers on the same individual.

Analyzing the options for a plant that never produces fruits and seeds:

• (a) If a dioecious plant bears only pistillate flowers, it can still produce fruits and seeds if pollination occurs from a separate male plant.
• (b) If a dioecious plant bears both types of flowers, it implies it's not strictly dioecious in the sense of separate individuals, or it's a misunderstanding of the term. If it means separate male and female plants exist, then fruit/seed production is possible.
• (c) If the plant is monoecious, it typically has the capacity for self-pollination and fertilization, leading to fruit and seed production.
• (d) If a dioecious plant bears only staminate (male) flowers, it lacks the female reproductive parts (pistils/carpels) necessary to produce ovules and subsequently fruits and seeds. Even if pollination occurs, fertilization cannot happen without female gametes.

Therefore, a dioecious plant bearing only staminate flowers is the most probable cause for the absence of fruit and seed production.

A typical microsporangium, which is the structure within the anther where pollen develops, is surrounded by four distinct wall layers. These layers provide protection and play a role in the dehiscence of the anther and the nourishment of the developing microspores.

The layers, from outermost to innermost, are:

1. Epidermis: This is the outermost protective layer.
2. Endothecium: Located beneath the epidermis, it often consists of fibrous thickenings that aid in anther dehiscence.
3. Middle Layer(s): Typically one to three layers situated between the endothecium and the tapetum. These layers are ephemeral and degenerate during development.
4. Tapetum: This is the innermost layer, directly surrounding the sporogenous tissue (which forms microspores). The tapetum is crucial as it provides nourishment to the developing microspores and pollen grains.

Therefore, the outermost layer is the epidermis, and the innermost layer is the tapetum.