alternation of generations
Alternation of generations is a feature of the life cycle of most plants and many lower animals by which successive
generations reproduce alternately sexually and asexually.
|Many plants have two distinct phases in their life
cycles. The gametophyte which produces the haploid male and female
gametes; and the sporophyte which produces diploid spores. During
the course of evolution there has been a gradual shift from the gametophyte
being the dominant form, i.e., the planet we recognize, to the sporophyte
being the dominant form. This illustration shows the gametophytes
of various plant types below the diagonal line and the sporophytes
above the line. From left to right the examples are: a liverwort in which the green thallus is the gametophyte, while the sporophyte
is represented by the fertilized egg and
the capsule that develops from it;
a moss in which the upright leafy plant is the gametophyte and the
capsule the sporophyte that lives upon it; in the ferns the dominant plant is the sporophyte , while the gametophyte is a
small, free-living flattened prothallus that produces the gametes;
in conifers and flowering plants the sporophyte is the only conspicuous
phase, the male gametophyte being contained in the pollen grains and
the female gametophyte represented by the egg and surrounding tissues.
In animals the feature is exhibited by flukes, tapeworms, and other cnidaria,
including the common jellyfish (Aurelia
aurita) and the sea fir Obelia.
In plants the sexually reproducing or gametophyte generation gives rise to haploid male and
female sex cells (gametes) which, on fertilization,
become a diploid zygote which in turn germinates
into the asexually reproducing sporophyte generation. This reproduces by forming spores which germinate to give the gametophyte generation again. In lower plants
such as the liverworts and mosses, the gametophyte
generation is dominant but in flowering plants the gametophytes are reduced
to microscopic proportions, the plant itself being the sporophyte generation.