Re: what is alternation of generations?

Meiosis only occurs once in each cycle. Plant meiosis gives rise to spores. 
This is a major difference between animals and plants because animal meiosis 
gives rise directly to sperm and egg. However, plant meiosis gives rise to 
spores, which then divide by mitosis to produce gametophytes which produce 
sperm and egg by mitosis.

Maybe a way to help understand alternation of generations in plants is to first 
consider the situation in humans. The only cells in the human body that have 
just one set of chromosomes (called haploid or 1N) are the sperm and the eggs. 
All the other human cells that have chromosomes have two sets of chromosomes 
(called diploid or 2N). In plants, however, there are other haploid cells 
besides sperm and egg. Thus, we speak of an alternation of generations between 
a multicellular haploid stage (called gametophyte) and a multicellular diploid 
stage (called sporophyte).  Haploid gametophytes produce haploid gametes, the 
sperm and eggs. Diploid sporophytes produce haploid spores. Animals do not 
produce spores or a multicellular haploid stage.

The alternation of generations shows a clear progression from primitive to more 
advanced plant groups. In primitive mosses, the haploid organism is the 
dominant photosynthetic plant. The diploid stage of the moss is a transitory 
capsule dependent on the haploid moss for survival. The haploid moss or 
gametophyte produces gametes (sperm and egg). The sperm swim in external water 
to the egg, they then fuse to produce a zygote. The zygote grows into the 
diploid stage (sporophyte) which produces haploid spores. The spores released 
from the capsule germinate to produce the haploid gametophyte. 

In ferns, both the diploid and haploid stages are independent photosynthetic 
organisms. The haploid fern is a tiny green heart-shaped plant or prothallus 
that produces sperm and eggs. The sperm swim to the egg, they fuse and produce 
a zygote, which then grows into a large diploid plant which is what we normally 
think of as a fern plant. Again the diploid sporophyte produces spores that are 
released and germinate to give rise to the gametophyte.

In the most advanced plants, the flowering plants, the haploid stage is reduced 
to a few cells starting with spores retained within the sporophyte. The male 
microspores or pollen grains divide to produce the male gametophyte. The male 
gametophyte is inside the pollen grain. The female macrospore divides to 
produce the egg sac or female gametophyte. Unlike ferns and mosses where 
external water is required for sperm to swim to the egg, flowering plant sperm 
are retained with a pollen tube. Flowering plant zygotes develop within a seed 
into an embryo, which can remain dormant, or in suspended animation, for long 
periods. The seed protects the embryo and contains nutrients for its growth. 
The seed can be easily transported long distances. 


References


Cell Division and Mulicellular Life Cycles



Alternation of Generations



FLOWERING PLANT REPRODUCTION: Fertilization and Fruits