6.2 - Development of Pollen Grains and Embryo Sac

Development of Pollen Grains in the Anther

During anther development, a group of tissues grows inside each lobule to form four pollen sacs.
Each pollen sac contains hundreds of diploid ( $2 n$ ) pollen mother cells, also called microspore mother cells.
Each microspore mother cell divides by meiosis to produce four haploid ( $n$ ) microspore cells, collectively called a tetrad.
Each cell in the tetrad develops into a pollen grain.
The nucleus in each pollen grain divides by mitosis to produce two nuclei: the generative nucleus and the tube nucleus.
The thick, waterproof wall of the pollen sac breaks when the pollen grains mature, releasing the pollen grains.

The rough surface of pollen helps it adhere easily to the stigma and pollinating agents.

An ovule develops from a layer of tissue inside the ovary; one ovary may contain one or more ovules.
The ovule attaches to the ovary wall through the funicle.
The placenta is the point where the funicle attaches to the ovary and supplies nutrients to the ovule through the funicle.
The nucellus is a mass of parenchyma tissue inside the ovary.
Nucellus tissue develops into two layers called the integument.
The micropyle is a small opening at the end of the integument that allows air and water to enter the seed during germination.
A nucellus cell becomes the diploid ( $2 n$ ) megaspore mother cell, also called the embryo sac mother cell.

The megaspore mother cell divides by meiosis to produce four haploid ( $n$ ) megaspore cells.
Three megaspore cells degenerate; one develops.
The nucleus of the developing megaspore divides by mitosis three times, producing one cell with eight nuclei.
The eight nuclei form:
- Three antipodal cells at one end of the embryo sac.
- Two synergid cells and one egg cell at the opposite end.
- Two polar nuclei at the centre of the embryo sac.