Sexual Reproduction in Plants, Practice Problems, and FAQs

Reproduction is considered as the process by which living organisms are able to produce their young ones or offsprings. You have studied about the different types of reproduction in lower classes. So you know that there are two types of reproduction namely asexual and sexual reproduction. But do you know how they are different from each other? Yes, in lower classes, we have studied about the major difference between these two processes. In asexual reproduction only a single parent is involved, hence it is uniparental. In sexual reproduction two parents (male and female) are involved, so it is biparental.

You know that reproduction occurs in both plants and animals. Some reproduce by both asexual and sexual means while some reproduce only by one of these two processes. Plants reproduce by both asexual and sexual means. 

The fusion of male and female gametes occur during the process of sexual reproduction. 

Fig: Fertilisation

Now you must be thinking how does this take place in plants? Do plants have reproductive organs? Let’s find out the answer for all these questions. In this article we will take a deep dive into the details of sexual reproduction in the angiosperms or the flowering plants. 

Table of contents

• Sexual reproduction in plants
• Sexual diversity in flower 
• Events in sexual reproduction
• Advantages of sexual reproduction in plants
• Practice Problems
• FAQs

Sexual reproduction in plants

To achieve a given degree of development and maturity, plants must complete the vegetative phase. They can only reproduce sexually once this time has passed. The reproductive period of the plant is marked by flowering in higher order plants. The flowering period of plants differs from one plant to the next. The flower is the reproductive organ of a plant in sexual reproduction. The male and female reproductive organs are present in the flower. Flowers generate male and female gametes.

Fig: Structure of a typical flower

Sexual diversity in flower 

Petals, sepals, stamen, and pistil are the primary components of flowers. The male reproductive organ is the stamen, whereas the female reproductive organ is the pistil or carpel. Flowers are categorised as follows based on the presence of male and female reproductive organs:

Unisexual flowers

In unisexual or dioecious flowers, the stamen and pistil are present in separate flowers. The unisexual male flower of flowering plants is called staminate flower, meaning it has stamens, whereas the female flower is called pistillate flower, meaning it has pistils. Examples include cucumber, pumpkin, maize, papaya etc.

Fig: Unisexual flowers in papaya

Bisexual flowers

In bisexual or monoecoius flowers, the stamen and pistil are located in the same flower. Examples include Hibiscus, lily, tulip etc.

Fig: Bisexual flower - Hibiscus

Events in sexual reproduction

Even though sexual reproduction is complicated, it follows a predictable pattern. The fusion (or fertilisation) of male and female gametes, the formation of zygote, and embryogenesis are all part of sexual reproduction. These sequential events can be divided into three stages for ease of understanding as follows:

(i) Pre-fertilisation events

(ii) Fertilisation events

(iii) Post-fertilisation events

Pre-fertilisation events

The events which occur prior to the fusion of gametes are called the pre-fertilisation events. It includes gametogenesis and the transfer of gametes.

Gametogenesis

The production of the two types of gametes, that is male and female is referred to as gametogenesis. Gametes are the haploid cells. These male and female gametes can be similar or different from each other.

Homogametes (Isogametes)

In certain algae, the male and female gametes are so identical in appearance that it is difficult to differentiate them. Examples include the green algae Ulothrix and Spirogyra. 

Fig: Isogamy

Heterogametes 

The gametes generated by the majority of sexually reproducing organisms including flowering plants are of two physically dissimilar types. The female gamete is known as egg or ovum here.

Fig: Heterogamy

Division of cells during gamete formation

All heterogametic organisms have two types of gametes: male and female. Though the parent plant body from which gametes are produced might be either haploid or diploid, gametes are haploid. The plant body of bryophytes is haploid. Mitotic division yields gametes from a haploid parent. The parental body, on the other hand, is diploid in pteridophytes, gymnosperms and angiosperms. Meiotic division creates gametes from a diploid parent.

Microsporogenesis 

In angiosperms the male gametes are formed by microsporogenesis. In this process the pollen mother cell or microspore mother cell (MMC) undergoes meiosis and produces microspores. The microspores develop into pollen grains. Each pollen grain possesses two male gametes. 

Fig: Male gametes

Megasporogenesis

In angiosperms the female gametes are formed by megasporogenesis. In this process the megaspore mother cell undergoes meiosis and produces four megaspores. Out of the four megaspores one develops into an embryo sac whereas the three degenerates. It possesses only one egg or ovum. 

 Fig: Egg

Transfer of gametes

Male and female gametes must be physically brought together after production to enable fusion (fertilisation). Pollen grains carry male gametes, whereas embryo sacs contain the egg. Pollen grains formed in anthers must consequently be delivered to the stigma before fertilisation can occur. Pollen grains are easily transferred to the stigma in bisexual, self-fertilising plants, such as peas. Since the anthers and stigma are close together, pollen grains come into touch with the stigma shortly after they are shed. Pollination is the specialised event occurs in cross pollinating plants (including unisexual plants). It is considered as the process of transfer of pollen grains from the anther of a flower to the stigma of the flower. 

Fig: Pollination in flowers

Germination of pollen grain

Pollen grains germinate on the stigma. Pollen tubes containing male gametes make their way to the ovule, where they discharge male gametes near the egg. 

Fig: Transfer of male gametes to the ovule

Fertilisation events

Fertilisation is the process by which two haploid gametes join to produce a zygote, which is the predecessor to an embryo. The genetic material from both gametes are combined in this zygote. Syngamy is the other name of this process. Syngamy occurs inside the body of the organism in the majority of plants (bryophytes, pteridophytes, gymnosperms, and angiosperms), this type of fertilisation is called internal fertilisation.

Fig: Fertilisation 

Double fertilisation

In higher plants, one of the two male gametes once released combines with the egg nucleus and forms a diploid zygote. This process is called syngamy, true fertilisation or generative fertilisation. The second male gamete moves in the central cell and fuses with the two haploid polar nuclei or the diploid secondary nucleus to form a triploid primary endosperm nucleus (PEN). This process is called vegetative fertilisation. This process involves the fusion of three nuclei (one male gamete and two polar nuclei). Hence it is also called triple fusion. 

Parthenocarpy

Parthenocarpic fruits, such as bananas, develop without fertilisation. Growth hormones can be used to induce parthenocarpy, which results in seedless fruits. Other examples include seedless watermelon and seedless grapes.

Fig: Parthenocarpic watermelons (seedless)

Post-fertilisation events

The events that occur after zygote formation are called post-fertilisation events. They are as follows:

• Endosperm development
• Embryogenesis
• Fruit development 
• Seed development

Endosperm development

The development of the endosperm occurs before the embryo formation. The primary endosperm cell splits many times to generate a triploid endosperm tissue. This tissue is loaded with reserve food supplies and are utilised to nourish the growing embryo.

Fig: Endosperm development

Embryogenesis

The zygote is formed when one of the male nuclei fuses with the female gamete in the embryo sac. To give rise to the embryo, the zygote goes through mitotic cell divisions. The embryo now has two different ends, the micropylar end at the base and the chalazal end at the opposite side. The embryo cell located towards the chalazal end undergoes divisions and produces the embryo. 

Fig: Embryogenesis

Seed development

In angiosperms, double fertilisation causes the ovule to mature into a seed. The seeds develop inside the fruits. The seed coat is made up of the integument of the ovules. Outer integuments form the outer seed coat called testa. Inner integument form the inner seed coat allied tegmen.

Fig: Seed

Fruit development

The sepals of the calyx and the petals of the corolla fall off after fertilisation. To begin its transition into a fruit, the ovary undergoes cell division. 

Fig: Development of fruit and seeds

The ovary wall forms the pericarp, which is the wall of the fruit. Epicarp, mesocarp, and endocarp are the three layers that make up the pericarp. The pericarp provides protection to the seeds and aids in their dissemination.

Fig: Fruit - Mango

Advantages of sexual reproduction in plants

• The enhanced genetic variety that sexual reproduction produces is a huge benefit. It causes variation in the population of plants.
• Genetic diversity may improve a plant's fitness in a variety of ways. One way this may happen if it develops features that help it to live and reproduce better in its present environment. For example, through improved drought tolerance in a dry area.
• Another benefit is the enormous quantity of seeds that sexually reproducing plants frequently generate. This enhances the likelihood of some of a plant's progeny surviving and reproducing.
• It results in high yield in agriculture in many cases. 

Practice Problems

Q1. Choose the correct statement from below regarding fertilisation.

A. It is also known as syngamy
B. Transport pollen grains to stigma
C. Nourishes the embryo
D. Flowering period of plants

Solution: Angiosperms show double fertilisation. Each pollen tube brings two male gametes, which fertilise with two cells in the ovary. The egg cell is fertilised by one male gamete, which produces a diploid (2n) zygote from which the embryo is formed. This process of fusion is termed syngamy, or fertilisation. Second male gamete fuses with the two polar nuclei and forms the primary endosperm nucleus. This process is called vegetative fertilisation. Hence, option a is correct.

Q2. Select the correct set of pre-fertilisation events.

A. Endosperm and seed development
B. Seed and fruit development
C. Gametogenesis and transfer of gametes
D. Gametogenesis and seed development

Solution: Gametogenesis and gamete transfer (pollination) are the two major pre-fertilisation events. Gametogenesis is a biological process in which diploid or haploid precursor cells divide and mature into adult haploid gametes (n). Gamete transfer is the transfer of male gametes to the female reproductive organ. To enable fusion, male and female gametes must be physically brought together after their production (fertilisation). Pollen grains germinate on the stigma, and pollen tubes with male gametes reach the ovule and the male gametes are discharged near the egg. Hence, option c is correct.

Q3. Explain the formation of endosperms in plants?
Answer: In angiosperms, egg (n) fuses with one of the male gametes (n) to produce zygote, while the secondary nucleus (2n) combines with another male gamete (n) to produce primary endosperm nucleus (3n). PEN undergoes division and forms the endosperm. The cells of this tissue are stocked with food reserves and are used to feed the developing embryo.

Q4. What are the products of sexual reproduction in plants?
Answer: The process of sexual reproduction involves the union of haploid female and male gametes, which results in the formation of a diploid zygote (2n), which eventually develops into the embryo. Fertilisation or sexual reproduction causes plants to produce seeds and fruits. In angiosperms, the ovary forms the fruit, while the ovules develop into the seeds. Seeds are seen encased within the fruit normally. 

FAQs

Question 1. Can parthenogenesis be considered as a form of asexual reproduction? Describe.
Answer: The formation of an embryo from a female gamete without any genetic contribution from a male gamete, with or without eventual growth into an adult, is known as parthenogenesis. Asexual reproduction is a method of reproduction that does not include the fusion of male and female gametes and results in offspring that are genetically identical to their parents. Plants do not produce gametes during asexual reproduction. But parthenogenesis involves the synthesis of egg cells, hence parthenogenesis differs from asexual reproduction.

Question 2. Is it possible for a plant to reproduce without pollination? Explain your answer.
Answer: Pollination is a fundamental aspect of sexual reproduction in plants. It is the transfer of pollen grains to the stigma of a flower on the same plant or another genetically similar plant. For the fusion of male gamete and female gamete, male gamete must reach the stigma. There will be no fusion of gametes and no fertilisation if pollination does not take place.

Question 3. Do all plants have endosperm in their seeds?
Answer: The growing embryo is nourished by endosperm, a nutritious substance found in the seed. Endosperm is not present in some seeds like non endospermous seeds. Examples include orchids.

Question 4. How does sexual reproduction takes place in unisexual flowers?
Answer: Flowers that only possess male or female reproductive organs are known as unisexual flowers. They are termed as incomplete flowers. Papaya, pumpkin, cucumber are examples of plants with unisexual flowers. Only the process of cross pollination allows them to reproduce. Pollen is transmitted from anther of one flower (staminate) to the stigma of another flower (pistillate) in pollination.

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