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Double Fertilization in Angiosperms: Overview, Structures & Process

Sujata Kumari, Angela Lynn Swafford
  • Author
    Sujata Kumari

    Sujata Kumari has taught College level Biology and Biotechnology to undergraduate students for over 7 years. She has a doctorate (PhD) degree in the field of biosciences from Jawaharlal Nehru Centre for Advanced Scientific Research and Masters degree in Biotechnology from Madurai Kamaraj University. She also has certification from Government of India endorsing her eligibility for lectureship in colleges and universities.

  • Instructor
    Angela Lynn Swafford

    Lynn has a BS and MS in biology and has taught many college biology courses.

Understand what double fertilization is. Learn about the reproductive structures in angiosperms, pollination, and what happens after double fertilization. Updated: 01/15/2022

Reproductive Structures In Angiosperms

The angiosperms, which are popularly known as flowering plants, are the most abundant group of plants. Angiosperms have successfully occupied all the habitats on the earth. Angiosperms are the largest-known and most diverse group found in the plant kingdom. Other groups of plants which coexist are non-flowering plants like gymnosperms (such as pines), seedless vascular plants (such as ferns), and nonvascular plants (such as mosses). Angiosperms and gymnosperms are the seed-bearing plants and are known as phanerogams. Both groups of plants differ by the presence of flowers and fruits in the case of angiosperms which is lacking in the gymnosperms. Angiosperms represent the most evolved form of the plant groups and share some unique characteristics reasoning their success on the planet. The main characteristics of angiosperms are

  • The presence of flowers as the reproductive structures/organs.
  • The presence of fruits enclosing the seeds with the embryo.
  • Double fertilization - syngamy and triple fusion.
  • Well-differentiated plant body with fully developed root and shoot systems.
  • Capable of surviving in the various habitats.

The term angiosperm is derived from two Greek words angion meaning container and sperma meaning seed, which together describe it as the seed container. One of the main characteristics of angiosperms is the presence of fruits which serve as protective containers for the seeds. Vast diversity is found in the morphology of angiosperms, and their examples range from tiny herbs to large, woody oak trees. Reproduction in angiosperms includes many unique features as well, such as flowers being reproductive organs, the occurrence of double fertilization to give rise to zygote and endosperm, and the post-fertilization process converting the ovary into fruit that encloses ovules or seeds.

Apart from being an ornamental part of the plant, flowers are the reproductive organs in angiosperms. A flower consists of four major parts. From outside to inside, those parts are: sepals, petals, stamens, and carpels.

Sepals: outermost part of the flower surrounding the flower petals before it has opened

Petals: often attractive and brightly colored; surround the carpel and stamen

Stamen: male reproductive structure that produces sperm cells

Carpel or Pistil: female reproductive structure that makes eggs


Schematic diagram showing different parts and reproductive structures of flower

Reproductive structure of a flower


The flowers can be unisexual or bisexual. Unisexual flowers contain either the male reproductive structure (stamen) or the female reproductive structure (carpel). So, accordingly they are referred to as male and female flowers. Bisexual flowers contain both a stamen and a carpel in them.

Reproductive Structures in Angiosperms

Angiosperms are plants that produce flowers and fruits, and they are the most common types of plants on Earth. Flowers are the reproductive structures of angiosperms. They include four basic parts:

  1. Carpel: Female reproductive structure that makes eggs.
  2. Stamen: Male reproductive structure that makes sperm cells.
  3. Petals: Surround the carpel and stamen and are often brightly colored.
  4. Sepals: Surround the flower petals before it has opened.

During flowering plant reproduction, two structures are created: a zygote and the endosperm. A zygote will grow and develop into an embryo, or baby plant. The endosperm is the plant embryo's food source. This process of forming a zygote and endosperm is called double fertilization, and it is unique to angiosperms.

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Pollination

Pollen is a powdery or grainy substance produced by stamens and has the major purpose of transporting sperm cells to the egg in the ovary. The stamen is usually a slender, long, tube-like structure supporting the anther. The anther is the topmost part of the stamen which produces pollen which, in turn, makes sperm cells. The carpel includes an enlarged basal portion known as the ovary that supports a long, filamentous structure, called the style. The style is topped by a sticky knob called a stigma where pollen is deposited and germinates. The ovary houses ovules that produce egg cells.

Pollination is defined as the transfer of pollen onto the stigma of the carpel. Pollen exists in various shapes and modifications, aiding in the way the pollen is transmitted. There are several modes of pollen transmission, such as wind pollination, mechanical discharge, and pollination by animals or insects. Wind-transmitted pollens are very light in weight, are made in large numbers, and are seen as yellow dust when dispersed in air. Pollen grains, which are transmitted by insects such as bees, by butterflies, or by animals, often have attractive flowers with loads of nectar to lure their pollinators. The entire pollen grain represents a male gametophyte (male gamete-producing body) and consists of two types of cells:

a) Tube cell: develops into the pollen tube

b) Generative cell: gives rise to male gametes (sperm cells)

Once the pollen gets deposited onto the stigma, the tube cell initiates the formation of a tube-like structure called the pollen tube. The generative cell which is contained within the pollen tube will divide and produce two sperm cells. Meanwhile, the pollen tube digests its way down the style and descends with sperm nuclei toward the egg.

Pollination takes place in two forms, namely, self-pollination and cross-pollination. When the pollen from the anther of a flower is deposited on the stigma of the same flower or another flower on the same plant, the phenomenon is called self-pollination. For self-pollination to occur, it is necessary that the stamen and carpel in those plants mature at the same time. The stamen and carpel also need to be positioned adequately in the flowers so the pollen can land on the flower's stigma. The plants undergoing self-pollination need not invest for the production of nectar for their pollinators. Cross-pollination on the other hand, involves the transfer of pollen grains from the anther of one flower to the stigma of another flower on a different plant of the same species. Many times, plant breeders perform inter-species cross-pollination (pollination between two closely related species) to yield hybrid products of higher commercial value. Cross-pollination leads to greater genetic diversity as compared to self-pollination, where plants with limited genetic differences are produced.

Double Fertilization In Angiosperms

Double fertilization is the unique characteristic of angiosperms and is not found in any other group of plants. As the name suggests, double fertilization refers to two fertilization events occurring during the sexual reproduction process in angiosperms. The two fertilization events constituting double fertilization in angiosperms are

  • Syngamy: refers to the fusion of a sperm with the egg to form the zygote.
  • Triple fusion: refers to the fusion of another sperm with polar nuclei to form the endosperm.

The basic parts of a flower and pollination occurring when a pollen grain lands on the stigma of a carpel.
Parts of a Flower and Pollination

Pollination

Before double fertilization can occur, pollination must take place. This is when a pollen grain, produced by the stamen of a flower, lands on the carpel of another flower. Specifically, the pollen grain must land on the sticky platform, or stigma, of a carpel.

A pollen grain is a protective structure that contains sperm cells. Once it lands on the stigma, it grows a pollen tube. This structure will grow down the style of a carpel to its ovary. The ovary is where double fertilization will take place.

After pollination, a pollen tube grows inside of a carpel towards the micropyle of an ovule.
Carpel Internal Structure and Pollen Tube

The Process of Double Fertilization

An ovary contains at least one ovule. Inside an ovule, cells divide to produce an egg and two other cells called polar nuclei. These three cells are haploid, which means they have one set of chromosomes, and is designated by n. Most cells in angiosperms are diploid, or have two sets of chromosomes. In diploid (2n) cells, one set of chromosomes comes from the male parent and the other set comes from the female parent.

In addition to eggs and polar nuclei, sperm cells from a pollen grain are also haploid. Once the pollen tube reaches the micropyle, or the opening of an ovule, it releases two haploid sperm cells into the ovule. One sperm cell will fuse with the egg, resulting in a diploid zygote. The other sperm cell will fuse with the two polar nuclei, creating a triploid (3n) structure that will grow rapidly into the endosperm.

1: A pollen tube releases two sperm cells into an unfertilized ovule. 2: Double fertilization occurs when one sperm cell fuses with the egg to produce a zygote, and the other sperm cell fuses with the two polar nuclei to make the endosperm. 3: After double fertilization, a seed and fruit develop.
Double Fertilization Process

After Double Fertilization

Double fertilization produces one diploid cell and one triploid cell. The diploid zygote will divide and grow to produce an embryo. The triploid cell grows into the endosperm and will nourish the developing embryo. The rest of the surrounding ovule will develop into a protective seed coat. A seed consists of the external seed coat and two internal structures: the embryo and endosperm. A fruit will usually develop from the ovary tissue to provide additional protection.

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Video Transcript

Reproductive Structures in Angiosperms

Angiosperms are plants that produce flowers and fruits, and they are the most common types of plants on Earth. Flowers are the reproductive structures of angiosperms. They include four basic parts:

  1. Carpel: Female reproductive structure that makes eggs.
  2. Stamen: Male reproductive structure that makes sperm cells.
  3. Petals: Surround the carpel and stamen and are often brightly colored.
  4. Sepals: Surround the flower petals before it has opened.

During flowering plant reproduction, two structures are created: a zygote and the endosperm. A zygote will grow and develop into an embryo, or baby plant. The endosperm is the plant embryo's food source. This process of forming a zygote and endosperm is called double fertilization, and it is unique to angiosperms.

The basic parts of a flower and pollination occurring when a pollen grain lands on the stigma of a carpel.
Parts of a Flower and Pollination

Pollination

Before double fertilization can occur, pollination must take place. This is when a pollen grain, produced by the stamen of a flower, lands on the carpel of another flower. Specifically, the pollen grain must land on the sticky platform, or stigma, of a carpel.

A pollen grain is a protective structure that contains sperm cells. Once it lands on the stigma, it grows a pollen tube. This structure will grow down the style of a carpel to its ovary. The ovary is where double fertilization will take place.

After pollination, a pollen tube grows inside of a carpel towards the micropyle of an ovule.
Carpel Internal Structure and Pollen Tube

The Process of Double Fertilization

An ovary contains at least one ovule. Inside an ovule, cells divide to produce an egg and two other cells called polar nuclei. These three cells are haploid, which means they have one set of chromosomes, and is designated by n. Most cells in angiosperms are diploid, or have two sets of chromosomes. In diploid (2n) cells, one set of chromosomes comes from the male parent and the other set comes from the female parent.

In addition to eggs and polar nuclei, sperm cells from a pollen grain are also haploid. Once the pollen tube reaches the micropyle, or the opening of an ovule, it releases two haploid sperm cells into the ovule. One sperm cell will fuse with the egg, resulting in a diploid zygote. The other sperm cell will fuse with the two polar nuclei, creating a triploid (3n) structure that will grow rapidly into the endosperm.

1: A pollen tube releases two sperm cells into an unfertilized ovule. 2: Double fertilization occurs when one sperm cell fuses with the egg to produce a zygote, and the other sperm cell fuses with the two polar nuclei to make the endosperm. 3: After double fertilization, a seed and fruit develop.
Double Fertilization Process

After Double Fertilization

Double fertilization produces one diploid cell and one triploid cell. The diploid zygote will divide and grow to produce an embryo. The triploid cell grows into the endosperm and will nourish the developing embryo. The rest of the surrounding ovule will develop into a protective seed coat. A seed consists of the external seed coat and two internal structures: the embryo and endosperm. A fruit will usually develop from the ovary tissue to provide additional protection.

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