Alternation of Generations During billions of years of evolution plants have developed a specific mode of living as autotrophic organisms distributed in different environments in both water and on land. They have changed their morphologies from unicellular algae to multicellular flowering organisms and modified their reproductive processes. Relatively early, they developed sexual reproduction, which is associated with meiosis, a process of two division cycles with accompanying recombination of genetic material. After meiosis four haploid daughter cells arise from one diploid cell. Each daughter cell exhibits little change in gene arrangement comparised to the previous generation. There is a difference between the sexual reproduction of plants and animals. In animals, meiosis occurs within a sexual gland producing gametes, so meiosis is directly connected with sexual reproduc-
tion in every generation. In plants, meiosis also occurs in diploid cells, but results in spores, which develop into a haploid generation. As a result of evolution, plants undergo alternation of generations in their life cycle with meiosis occurring between the diploid and haploid phases (Fig. 9.1A).