Problem 157 - Objective Biology for NEET

Question

In gymnosperms (a) Pollen grains germinate inside the ovary. (b) The development of pollen grain takes place within the microsporangia. (c) The cones barring megasporophylls are called male cones or male strobili. (d) All are correct

Step-by-Step Solution

Answer

The correct answer is statement (b) - The development of pollen grain takes place within the microsporangia.

Step 1: Analyze statement (a)

In gymnosperms, unlike in angiosperms, pollen grains do not germinate inside the ovary. Instead, they are transferred from the male cone to the female cone where they germinate. So, statement (a) is incorrect.

Step 2: Analyze statement (b)

In gymnosperms, the development of a pollen grain indeed takes place within the microsporangia. Microsporangia, located on the male cones, produce microspores which develop into pollen grains. Therefore, statement (b) is correct.

Step 3: Analyze statement (c)

Cones bearing megasporophylls are not male cones, but female ones, as megasporophylls contain ovules which develop into seeds after fertilization. Hence, statement (c) is incorrect.

Step 4: Analyze statement (d)

Statement (d) suggests that all the provided statements are correct, which is not true as we have found (a) and (c) to be incorrect

Key Concepts

Pollen Grain Development

The process of pollen grain development in gymnosperms is a fascinating journey from spore to seed. It begins within the microsporangia, the pollen factories of male cones. Here, cells undergo meiosis to produce haploid microspores. These microspores then undergo several stages of transformation, ultimately becoming mature pollen grains.

Every pollen grain contains a generative cell that will divide to form sperm cells and a tube cell that will grow into the pollen tube during fertilization. This process is not only crucial for the reproduction of gymnosperms but also for the diversity of plant life as it allows for genetic variation through sexual reproduction.

Microsporangia in Gymnosperms

Function and Location

The microsporangia are integral components of the male cones, also known as strobili, in gymnosperms. Located on the lower surfaces of the scales, microsporangia are often clustered together, giving them a bumpy appearance. Each microsporangium is a pollen-producing structure where meiosis happens, leading to the formation of microspores.

Structure

Microsporangia are typically enveloped by a protective layer and connected to a nutrient source, which sustains them during development. Once the microspores are mature, they become pollen grains, ready to embark on the next stage of the reproductive cycle upon release into the air.

Gymnosperm Fertilization

In gymnosperms, fertilization is a unique process characterized by the absence of an ovary. Pollen grains, once released from the male cone, are carried by wind to the female cone. Upon landing on the receptive surface of the ovule, which sits on a megasporophyll within the female cone, the pollen grain begins to germinate, developing a pollen tube.

The sperm cells travel down the pollen tube to reach the female gametophyte lodged within the ovule. Here, fertilization occurs, and a zygote forms, which will eventually develop into the embryo of the seed. This mechanism ensures genetic exchange between individuals and is pivotal for the perpetuation of gymnosperm species.

Megasporophylls

Megasporophylls are fundamental to the reproductive system of gymnosperms and are found in female cones. These leaf-like structures hold one or more ovules, which are the sites where megaspores are produced. Following meiosis, a megaspore develops into a female gametophyte, creating the egg cells necessary for fertilization.

The strategic layout of megasporophylls within the cone facilitates effective pollination by capturing wind-borne pollen grains. After fertilization, the megasporophylls play another vital role: they aid in protecting and nourishing the developing seeds until they are mature and ready to be dispersed, ensuring the continuation of the plant lineage.