Development of pollen grains and embryo sac
Before getting into the development of pollen grains and embryo sac, it is useful to have some basic knowledge of the reproductive structures of the flower. You can read all about it here.
If you do not want to go through the whole post, you can refer to the diagram of the labelled parts of the flower and identify the ovule and anther. These are the two structures involved in the development of pollen grains and embryo sac.
Suggested pre-reading : Structure of a Flower
Watch the video for a detailed explanation.
Development of Pollen Grains
Pollen grains are tiny structures that carry the male gametes. They can be found in the pollen sac. The diagram shows the cross-section of an anther. Anthers are the top part of the stamen.
Recall from the previous topic that the stamen is the male reproductive organ in the flower. Each anther contains four pollen sacs (refer to the diagram). Hundreds of pollen grains can be found in each pollen sac.
The development of the pollen grain begins with the microspore mother cell, which is a diploid (2n) cell. This cell is located in the pollen sac.
Three processes lead to the formation of the pollen grain – meiosis, development and mitosis.
- Process 1 – Meiosis
In sexual reproduction, gametes need to be formed via meiosis to maintain the diploid (2n) number of chromosomes in the organism. Each of the numerous microspore mother cells, which are diploid (2n) cells, undergo nuclear division by meiosis to form four haploid (n) microspore cells. Initially, the four cells are bound together in a structure known as the tetrad (refer to diagram).
- Process 2 – Development
Each microspore cell then grows and develops into a pollen grain. One microspore mother cell produces four pollen grains.
- Process 3 – Mitosis
The nucleus in each pollen grain undergoes mitosis to form two haploid (n) nuclei – tube nucleus (pollen tube nucleus) and generative nucleus.
Development of Embryo Sac
The development of the embryo sac occurs in the ovule. The nucellus is the tissue that surrounds the embryo sac. Encasing the embryo sac and the nucellus are two layers of the integuments. At the bottom part of the integuments, there is a small opening called the micropyle.
The funicle is the part that connects the ovule to the ovary. However, the point of attachment of the ovule to the ovary is called the placenta.
The development of the embryo sac involves similar processes to the development of pollen grains.
It begins with the megaspore mother cell, which is a diploid (2n) cell. This cell is located in the ovule. Three processes lead to the formation of the embryo sac – meiosis, development and mitosis.
Process 1 – Meiosis
Unlike the numerous microspore mother cells in the pollen sac, each ovule only contains one megaspore mother cell. The megaspore mother cell, which is diploid (2n), undergoes nuclear division by meiosis to form four haploid (n) megaspore cells. Out of the four megaspore cells, three degenerate (refer to diagram).
Process 2 – Development
Only one megaspore cell grows and develops into an embryo sac.
Process 3 – Mitosis x 3
The nucleus in each embryo sac undergoes mitosis three times to form eight haploid (n) nuclei :
- three antipodal cells,
- two polar nuclei,
- two synergid cells and
- one egg cell/ovum.
(refer to diagram)