Alternation of Generations
Figure 1: Alteration of generations in ferns. In the blue box on the top, the diploid generation, you see a mature sporophyte (which is the leafy appearance we think of when we talk about ferns). Following the diagram clockwise the sporophyte goes through meiosis producing spores, the small green dots, which enters the haploid generation in the lower pink box. Then through mitosis the spores divide and multiple to form a prothallus or the gametophyte. These can then form the male (light blue dots) or female (yellow dots) gametes. Then when the male and female gametes come together during fertilization a new sporophyte will develop completing the cycle and moving back into the diploid generation.
CC BY-SA: Jeffrey Finkelstein
Alternation of generations is an important adaptation. In this chapter, we will examine how this cycle works in different kinds of plants. It is important that you recall how mitosis and meiosis work because these are key processes in the life cycle of plants. Mitosis is the process that plants use for growth. In a mitotic division, one cell divides and produces two daughter cells that are genetically identical to the original cell. Meiosis is the process that plants use to produce haploid cells.
The haploid generation of the plant life cycle is called the "gametophyte generation." The gametophyte generation produces the gametes, sperm and egg, which plants use to reproduce sexually. The diploid generation is called the "sporophyte generation." The sporophyte generation produces spores by the process of meiosis. The spores are haploid and develop into the gametophyte generation. When a haploid sperm fertilizes a haploid egg, the resulting cell (zygote) is diploid. This diploid zygote undergoes mitotic cellular division and develops into the sporophyte.