Problem 160 - Objective Biology for NEET

Question

In gymnosperms, the male and female gametophyte I. Do not have independent free living existence. II. Remain within the sporangia retained on parent sporophyte. III. May be born on same or different tree. IV. Perform meiosis to produce gametes. (a) I and II (b) II and III (c) III and IV (d) All

Step-by-Step Solution

Answer

The correct option is (b) II and III. In gymnosperms, both male and female gametophytes remain within the sporangia retained on the parent sporophyte and they may be born on the same or different tree, but they do not perform meiosis to produce gametes. Instead, gametes are created through mitosis.

Step 1: Understanding Gametophytes in Gymnosperms

In terms of gymnosperms, both male and female gametophytes do not have an independent free-living existence. Instead, they remain confined in the sporangia and are retained on the parent sporophyte. This covers points I and II.

Step 2: Geographical Distribution of Gymnosperms

The gametophytes -be it male or female- of gymnosperms may be born on the same tree or a different one. This can occur due to the wind dispersal of their male gametophytes or pollen grains, which can get dispersed to the cones of the same tree or another. This validates point III.

Step 3: Gametes Production in Gymnosperms

The production of gametes in gymnosperms occurs without the process of meiosis. It is the sporophyte that undergoes meiosis to produce spores, which then develop into gametophytes. The gametophytes in turn produce gametes by means of mitosis, not meiosis. Thus, point IV is incorrect.

Key Concepts

Gymnosperms Reproduction

Gymnosperms, a group of seed-producing plants that includes conifers, cycads, Ginkgo, and gnetophytes, exhibit a unique mode of reproduction compared to other vascular plants. Unlike angiosperms, gymnosperms do not produce flowers or fruits. Instead, their reproductive structures are cones or strobili.

The reproductive process begins when the male cones, or microstrobili, release pollen grains into the air. These pollen grains, which contain the male gametophytes, are carried by wind to female cones, the megastrobili. The key aspect here is that the gymnosperms' gametophytes do not live independently; they develop within the protected environment of the sporangia on the parent sporophyte.

• The male gametophyte, encapsulated within a pollen grain, develops from a microspore after it has undergone a division known as mitosis.
• The female gametophyte, which develops within an ovule's megasporangium, also arises from mitotic divisions after meiosis has occurred.
• Fertilization occurs when the male gametophyte successfully reaches and penetrates the ovule harboring the female gametophyte.

Overall, gymnosperms exhibit a complex life cycle where gametophytes are nurtured within sporangia, ensuring that gametes are produced in a protected environment, enhancing reproductive success.

Sporangia in Plants

Sporangia are essential structures in the life cycle of plants, functioning as the site where spores are produced. In the context of gymnosperms, sporangia are pivotal for housing the developing gametophytes.

In gymnosperms, sporangia are found within cones. Male sporangia, known as microsporangia, produce microspores which eventually give rise to male gametophytes. Female sporangia, called megasporangia, are located within the ovules and give rise to megaspores. These megaspores develop into female gametophytes.

• Microsporangia release pollen grains that are dispersed by the wind to reach megasporangia.
• Megasporangia remain attached to the parent plant, bearing the female gametophytes until fertilization.
• The sporangia not only protect the developing gametophytes but also facilitate their nourishment throughout development.

Understanding sporangia's role is critical in comprehending plant reproduction, as they ensure the continuity of generations by fostering the development of the next sexually reproductive phase.

Gamete Production in Plants

Plants produce gametes through a phase known as the gametophyte generation. The gametophyte, which is haploid, gives rise to gametes through mitotic divisions. This contrasts with the sporophyte phase, which is diploid and produces spores through meiosis.

For gymnosperms, gamete production occurs as follows:

• The sporophyte undergoes meiosis to produce haploid spores. These spores then grow into the gametophyte.
• The male gametophyte, within the pollen grain, and the female gametophyte, inside the ovule, both mature through successive mitotic divisions.
• Eventually, the male and female gametophytes form gametes—sperm and eggs, respectively—by mitosis.
• The process culminates in the union of these gametes through fertilization, leading to the formation of a diploid zygote, which will develop into the next generation's sporophyte.

When studying gamete production in gymnosperms, it is crucial to recognize that gametophytes produce gametes without completing meiosis. Instead, meiosis is restricted to the sporophyte phase. This distinct separation of roles ensures genetic diversity and stability throughout plant reproduction.