All plants exhibit a phenomenon
called alternation of generations.
During the course of their life cycles, plants alternate between two phases. Depending on
the type of plant, these two phases may be similar or very different.
As we talk about this phenomenon, remember the following:
generation is the phase that produces spores.
generation is the phase that produces gametes, or reproductive cells (egg and sperm).
Spores germinate to grow into the gametophyte generation.
The gametophyte generation produces gametes. Egg and sperm unite to form a zygote, which
then develops into the sporophyte generation, and the cycle continues.
Among more primitive plantsthose believed to be
lower on the evolutionary scale, and therefore olderthe sporophyte and gametophyte
generations of a plant exist somewhat independently. For example, in the ferns (considered
to be lower plants), the two generations exist as separate plant bodies, and both
generations are able to photosynthesize. (Well talk more about this in the sections
on ferns, below.)
As you move from primitive plants to more and more complex
higher plants, the more dependent one phase is upon the other. In flowering plants,
evolutionarily the youngest and most complex group, the gametophyte generation is reduced
to a microscopic size and is wholly dependent upon the sporophyte generation. The
flowering plants you see in your garden all represent the sporophyte generation.
Now lets use this information to explore plant
groups familiar to gardeners.