6.2 - Development of Pollen Grains and Embryo Sac

The Formation of Pollen Grains in an Anther

Anther is the part of the flower that produces pollen grains.

Figure 2 - The process of formation of pollen grains. 1. When the anther develops, a group of tissues grows within each of the lobe to form four pollen sacs. There are hundreds of pollen mother cells or microspore mother cells which are diploid (2n) in each of the sac. 2. The mother cells will divide via meiosis and form four microspore cells which are haploid (n). The four microspore cells are known as tetrad. Each cell in the tetrad will develop to form pollen grain. 3. The nucleus in the pollen grain will divide via mitosis to produce two nuclei, which are regenerative nucleus and tube nucleus. The wall of the pollen sac will burst to release the pollen grains upon maturation.

The Formation of Embryo Sac in an Ovule

Ovule is the structure that can be found inside the carpel. It is formed from a layer of tissues inside the ovary. One ovary can contain one or more ovules. Ovule is connected to the wall of the ovary through funicle. The area of attachment from the funicle to the ovary is called placenta. Placenta functions by providing nutrients to the ovules through the funicle. A group of tissues in the ovary will form nucellus which consists of parenchyma tissues. The nucellus tissues will develop to form double layers called the integument. Micropyle is a small opening which is located at the end of the integument. It allows the entry of air and water into the seed during germination. One of the nucellus cells which is called megaspore mother cell or embryo sac mother cell will develop to form the embryo sac.

Figure 3 - The structure of an ovule.

Figure 4 -

1. The megaspore mother cell (2n) divides by meiosis to produce four haploid megaspore cells (n).
2. Only one of the haploid megaspore cells will develop whereas the remaining three cells will degenerate.
3. The developed nucleus in the megaspore cells will divide three times by mitosis to form cells with eight nuclei.
4. Three nuclei will move to the end of the embryo sac to form three antipodal cells. Three other cells will move to the opposite end to form two synergid cells and one egg cell. The two nuclei in the middle of the embryo sac will form polar nuclei.