Sexual Reproduction in Flowering Plants - Pre-fertilization: Structures and Events
Review the key concepts, formulae, and examples before starting your quiz.
🔑Concepts
Stamen, Microsporangium and Pollen Grain: The male reproductive unit consists of a filament and an anther. A typical angiosperm anther is bilobed and dithecous. The microsporangium consists of four wall layers: epidermis, endothecium, middle layers, and the tapetum (which provides nourishment to the developing pollen).
Microsporogenesis: The process of formation of microspores from a Pollen Mother Cell () through meiosis. Each () undergoes meiosis to produce a cluster of four cells called the microspore tetrad ().
Pollen Grain Structure: Represents the male gametophyte. It has a two-layered wall: the outer hard exine made of (highly resistant organic material) and the inner thin intine made of cellulose and pectin. It contains two cells: a large vegetative cell and a small generative cell.
The Pistil, Megasporangium (Ovule) and Embryo Sac: The female reproductive unit consists of the stigma, style, and ovary. The ovule is attached to the placenta by a funicle. The main parts of the ovule include the integuments, micropyle, chalaza, and the nucellus ().
Megasporogenesis: The process of formation of megaspores from the Megaspore Mother Cell (). The () undergoes meiosis to form four megaspores (). Usually, only one megaspore remains functional (monosporic development) while the other three degenerate.
Female Gametophyte (Embryo Sac): The functional megaspore undergoes three sequential mitotic divisions of the nucleus to form an 8-nucleate, 7-celled embryo sac. It consists of an egg apparatus (one egg cell and two synergids), three antipodals, and a large central cell containing two polar nuclei.
Pollination: The transfer of pollen grains from the anther to the stigma. Types include Autogamy (same flower), Geitonogamy (different flower, same plant), and Xenogamy (different plant). Outbreeding devices like dichogamy and self-incompatibility prevent self-pollination and promote genetic variation.
📐Formulae
💡Examples
Problem 1:
How many meiotic divisions are required to produce seeds in a typical angiosperm plant?
Solution:
Explanation:
To produce seeds, we need female gametes and male gametes. (1) One meiotic division in the produces functional megaspore, so divisions are needed for egg cells. (2) One meiotic division in produces pollen grains, so divisions are needed for pollen grains. Total divisions = .
Problem 2:
If the diploid () number of a flowering plant is , what will be the chromosome number in the (a) Tapetum, (b) Synergids, and (c) Nucellus?
Solution:
(a) or more (due to endopolyploidy), (b) , (c) .
Explanation:
The Tapetum is part of the anther wall and is initially (), though it can become polyploid. Synergids are part of the haploid gametophyte (), so . The Nucellus is diploid sporophytic tissue ().