What are the main events in the life cycle of an angiosperm?

Answer 1

The angiosperm life cycle begins with a diploid (2n) flower on the mature sporophyte plant. Within the anther, the microsporocytes develop and undergo meiosis (reduction of chromosomes) to produce haploid (n) microspores. Each of these undergoes one mitotic division to produce a generative cell and a tube cell. Together they make an immature microgametophyte, or pollen grain. The generative cell completes a second mitotic division to produce two sperm nuclei. Inside the ovule a single megasporocyte develops, undergoes meiosis, and produces four haploid (n) megaspores. Three of these die off, while the fourth undergoes three mitotic divisions to produce an eight-nucleate embryo sac, or mature megagametophyte. Upon pollination, the pollen grain germinates on the stigma, a pollen tube grows down the style and into the ovary through the micropyle. One sperm nucleus fuses with the egg to create a diploid (2n) zygote, while the other sperm nucleus fuses with the two polar nuclei to produce the nutritive, triploid endosperm. The embryo develops inside the embryo sac, integuments of the ovule form a protective seed coat around it that provides protection and nutrients. The seed is then shed from the fruit and the development of the embryo is temporarily halted until the proper conditions are established. Once the proper conditions are established seed germination takes place and the embryo grows into a mature diploid (2n) sporophyte, which produces flowers and completes one cycle of the alternation of generations.

Answer 2

1. The zygote and ovule develop into a seed, which grows into a new sporophyte. A flower produces male spores inside its anthers and female spores inside its pistil. Spores develop into male and female gametophytes. Sperm enter an ovule through a pollen tube, and fertilization occurs.