Chapter 31 Notes

Within the angiosperms, the alternation of generations occurs ON the adult plant. The adult plant is the sporophyte (diploid) and the sporophyte generation goes through meiosis to produce haploid gametophytes (eggs and sperm).

A typical flower consists of four whorls emanating from a common receptacle:

1. Sepals: The first whorl consists of sepals, nonfertile modified leaves that usually protect the developing flower (some also function much like petals).

2. Petals: The next whorl consists of petals, nonfertile modified leaves that often produce pigments and fragrant oils.

3. Stamen: The male reproductive structures produce pollen. The stamen may consist of a filament and an anther. Within the anther, pollen is produced while the filament holds the anther.

4. Carpel/Pistil: The female reproductive structure is called the carpel. The carpel consists of three parts: the stigma (often sticky as this is the portion that pollen grains adhere to), the style (long shaft that elevates the stigma), and the ovary (often found within the receptacle, at the base of the flower).

The four parts of the flower listed above are not found in all flowers. Perfect flowers have male and female parts, while imperfect flowers only have the male or female parts. Also, as we observe and dissect flowers keep in mind the flower parts are often fused together; a monocot should have three carpels, but often the three are fused into a single carpel with three distinct lobes.

II. Alternation of Generations in the Angiosperms:

Development of Pollen: The interior of an anther is divided into divisions, each known as a pollen sac. The cells within each sac will undergo meiosis, producing haploid microspores. These spores divide again to produce two nuclei within each pollen grain. Each pollen grain thus contains two nuclei within elaborate cell walls. When the pollen grain lands on a stigma, the nuclei (haploid) will be released from the cell wall.

Development of an Ovule: On the surface of the ovary, an ovule forms. The ovule consists of two layers of tissue surrounding a cell that goes through meiosis, producing megaspores (female gametophyte). Usually, only one megaspore will survive and this one megaspore will go through mitotic divisions. The result is a gametophyte with more than one cell: one egg, and multiple smaller cells.

Fertilization: Fertilization occurs when the pollen lands on the stigma and a pollen tube grows down the style. The pollen tube is formed when as the two nuclei migrate towards the ovary of the carpel. When the pollen nuclei reach the ovule, double fertilization occurs. One nuclei fertilizes the egg (2n), while another fertilizes the endosperm (3n). The endosperm becomes food for the developing sporophyte (zygote).