BOT 360F - Families of Vascular Plants

The life cycle of green plants

Alternation of generations in green plants

How does this life cycle compare with your own? Where do meiosis and syngamy occur in the life cycle of vertebrates, or of some brown algae (e.g. Fucus)? Click HERE for an image of Fucus from the University of British Columbia. Images of Fucus male and female gametangia are also available. What components of the plant life cycle shown at the left are missing in the vertebrate life cycle? For more information on life cycles, as well as stimulating ideas about their evolution, see Corner (1964).

Plants are distinguished from the other kingdoms of living things by photosynthetic energy capture, multicellularity, as well as by their common life cycle with
  • alternation of spore- and gamete-producing generations (sporophyte, gametophyte, respectively), and

  • sporic meiosis, in contrast to gametic meiosis, as in life cycle of vertebrates and Fucus.

Evidence from biochemistry and ultrastructure strongly suggests that the life cycle shown here was inherited from green algae (such as Ulva, mentioned in class as an example of an isomorphic alternation of generations).

Life cycle terminology and related notes

  • The gametophyte is the gamete-producing plant, or stage of the life cycle.
  • The sporophyte is the spore-producing plant, or stage of the life cycle.
  • Spores and gametes are both single cells, but they are differ in how they function. A spore develops into a new individual following germination. A gamete functions only by fusing with another gamete of the opposite mating type to form a zygote (the single-celled product of gamete fusion).
  • Development of the sporophyte from a zygote involves production of an embryo (hence the name "Embryophyta"), i.e. an immature stage of the sporophyte that is nutritionally dependent on and (or) enclosed by gametophyte tissue.
  • Separation of plants into pigeonholes (Bryophyta, Pteridophyta, seed plants, etc.) is based to a considerable extent on the increasing degree to which the sporophyte is the dominant phase of the life cycle, with:
    • presence of water-conducting tissue;
    • increased stature, independence from the gametophyte;
    • more complex organization (stem, leaves, roots);
    • heterospory;
    • shift from dispersal via spores to dispersal of embryo sporophytes (seeds);
    • enclosure of embryos in increasingly complex sporophytic structures (fruits) so as to maximize dispersal and establishment.
  • So as to evoke the extent of the shift from a dominant gametophyte to a dominant sporophyte, a moss gametophyte may be compared with pollen grains of a flowering plant (e.g. ragweed or other Asteraceae); although the latter may consist of only two or three nuclei these are equivalent to the moss in having developed from a spore and in giving rise to (male) gametes.

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