Flower structure and gametophytes
Stamen, microsporangium, pollen grain, pistil, megasporangium and embryo sac; microsporogenesis and megasporogenesis.
The essentials
A flower is a modified shoot that carries the reproductive organs. The androecium is made of stamens; each stamen has a long filament and a terminal anther. A typical angiosperm anther is bilobed and dithecous, and in cross-section it shows four microsporangia (pollen sacs). The wall of a microsporangium has four layers: epidermis, endothecium, middle layers and the innermost tapetum, which nourishes the developing pollen. The central sporogenous tissue gives rise to pollen mother cells (microspore mother cells), each of which divides by meiosis to form a tetrad of microspores. This process is microsporogenesis.
A mature pollen grain has a tough outer exine made of sporopollenin, one of the most resistant organic materials known, and an inner intine of cellulose and pectin. Germ pores are gaps in the exine where sporopollenin is absent. At shedding, the pollen grain is usually 2-celled: a large vegetative cell and a small generative cell; the generative cell later divides to form two male gametes.
The gynoecium is made of one or more pistils. Each ovule (megasporangium) is attached to the placenta by a funicle and has a body called the nucellus surrounded by integuments, with an opening called the micropyle. A megaspore mother cell undergoes meiosis to form four megaspores (megasporogenesis); usually one remains functional. Its nucleus divides three times mitotically to form the 7-celled, 8-nucleate embryo sac (monosporic development).
Key points
- Embryo sac: egg apparatus (1 egg + 2 synergids) at the micropylar end, 3 antipodals at the chalazal end, 1 central cell with 2 polar nuclei.
- Synergids have filiform apparatus that guides the pollen tube.
- Pollen viability: about 30 minutes in rice and wheat, months in some members of Rosaceae, Leguminosae and Solanaceae.
- Pollen can be stored in liquid nitrogen at −196 °C in pollen banks.
Common mistakes
- The tapetum is the innermost wall layer, not the outermost.
- The nucellus is diploid, while cells of the embryo sac are haploid.