A Closer Look
Among organisms which display an alternation of generations
as part of their life cycle (such as plants and certain algae), the haploid organism that produces gametes. Each of its cells has only one, unpaired set of chromosomes, as opposed to the corresponding diploid form of the organism, called the sporophyte
. A gametophyte develops from spores produced by the sporophyte. The gametophytes of homosporous
plants are bisexual (produce both eggs and sperm), while the gametophytes of heterosporous
plants, such as all seeds plants, are unisexual (produce only eggs or only sperm). See more at alternation of generations, sporophyte.
The evolution of plants has seen a steady reduction in the independence and size of the gametophyte
generation when compared to the sporophyte
generation. In the more primitive nonvascular plants, such as the mosses and liverworts, the gametophyte is the main plant form. It is relatively large and not dependent on the sporophyte for nutrition. In some species, the gametophytes are bisexual; in others, the sexes are separate. Female gametes remain in the gametophyte, where they are fertilized by sperm, usually from another gametophyte. The resultant embryos develop into sporophytes. These grow as smaller plants in or upon the gametophytes. The diploid sporophytes then produce haploid spores by meiosis, and these are dispersed and grow into new gametophytes. Among the vascular plants, however, the sporophyte is the larger plant form. For example, the familiar pine tree represents the diploid sporophyte generation. By meiosis, the pine eventually produces male and female gametophytes. The male gametophytes of the pine are small—they are the pollen grains released by male cones to be dispersed to female cones. The larger female gametophytes remain within the ovules of the female cones of the pine and are dependent on the surrounding sporophyte tissues for their nutrition. Each female gametophyte develops two egg cells, both of which are fertilized by a male gamete from a pollen grain. However, usually only one embryo will complete its development. This embryo is a new diploid sporophyte, housed within the developing pine seed. A pine seed thus contains three generations of the pine plant: the seed coat that arises from diploid tissue belonging to the ovule, the haploid gametophyte tissue that will serve as a food reserve for the embryo after germination, and the diploid embryo itself. In angiosperms, the female gametophyte has evolved into an even more reduced form. It consists of only the seven cells of the embryo sac.