Double Fertilization in Angiosperms: Definition & Process

Double Fertilization in Angiosperms: Definition & Process
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  • 0:00 Reproductive…
  • 0:50 Pollination
  • 1:24 The Process of Double…
  • 2:22 After Double Fertilization
  • 2:56 Lesson Summary
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Lesson Transcript
Instructor: Angela Lynn Swafford

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

You may already know that fertilization is when an egg and sperm cell fuse to create a new individual, but do you know what double fertilization is? It is a process that is unique to angiosperms, and you can learn about it in this lesson.

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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