During sexual reproduction two gametes, each of which is haploid, unite to form a single-celled zygote, which is diploid. As a consequence of the chromosome doubling that occurs during fertilization, at some point in the organism's reproductive cycle meiosis, or reductive cell division, must also occur to restore the haploid condition. In many organisms, including most animals, the zygote develops into a multicellular individual, and meiosis occurs during gamete production. In such organisms, gametes are the only haploid cells in the life cycle. In many algae and fungi, in contrast, the diploid zygote undergoes meiosis immediately to form haploid cells, called spores. Spores subsequently grow into multicellular haploid individuals. In both of these life cycles there is only one multicellular phase. In some algae and in all plants, however, there are actually two multicellular phases, one haploid and one diploid...