The flower and the making of pollen grains and ovules
Flowering plants, the angiosperms, have seeds that develop inside a fruit wall. They range from small herbs to giant trees, and the flower is their organ of reproduction. A flower grows from the axil of a bract. It may sit on a stalk, the pedicel, or have none. A typical flower, such as that of the bean, apple, onion or petunia, has four whorls, one inside the other. The calyx is the outer whorl of green sepals, which protect the flower against drought, rain and wind. The corolla is made of petals, which protect the inner parts and attract insects for pollination. In most monocots, such as tulip and onion, the calyx and corolla look alike and are called the perianth. The androecium is made of stamens, each with a filament carrying an anther with four pollen sacs. The gynoecium is the central whorl, of one or more carpels. Each carpel has a swollen base, the ovary, which holds the ovules, a thin style, and a sticky or feathery stigma. Before fruit and seeds can form, the anther must make pollen and the ovary must make ovules. In the anther of a lily, the four sacs hold diploid spore mother cells (2N). Each divides by meiosis into four haploid microspores (N). The nucleus of each microspore divides by mitosis into a tube nucleus and a generative nucleus, and the wall thickens: this is a pollen grain. The anther opens and sets the grains free. In the ovary an ovule starts as a swelling on the wall. It gets a funicle that brings food, and two integuments that surround it, leaving a small hole, the micropyle. A spore mother cell (2N) in the ovule divides by meiosis into four haploid cells. Three degenerate and one grows into the embryo sac inside the nucellus. Its nucleus divides by mitosis three times, giving 8 nuclei, 4 at each pole. One nucleus from each pole moves to the centre; they are the 2 polar nuclei. The other 3 at each pole become cells. At the micropyle end the egg lies between two synergids, and at the far end lie three antipodal cells. The ovule is ready for fertilization.
| Whorl | Parts | Job |
|---|---|---|
| Calyx | sepals | protect the flower |
| Corolla | petals | protect, and attract insects for pollination |
| Androecium | stamens: filament + anther with 4 pollen sacs | makes pollen grains |
| Gynoecium | carpel(s): ovary with ovules, style, stigma | makes the ovules and receives the pollen |
| Where | Nuclei | What they become |
|---|---|---|
| micropyle end | 3 | egg + 2 synergids |
| centre | 2 | polar nuclei |
| far end | 3 | antipodal cells |
Look and name, with a grown-up. Look at a picture of a flower in a book or on a screen (do not pick, cut or taste real flowers). Make a table with four rows, one for each whorl, and write its parts and its job from memory. Then draw a simple sketch of an ovule with the words integuments, micropyle and embryo sac, and write how many nuclei the embryo sac has.