Pollen is neither spore (microspore) nor gamete(s). It is an immature male
gametophyte formed by the germination of the microspore. In angiosperms,
germination consists of one round of mitosis to produce the generative cell
and the tube cell. In gymnosperms, germination involves more than one round
of mitosis to form a 4-celled immature male gametophyte. The male
gametophyte is mature, i.e. sexually mature, once it has formed the two
sperm, within the pollen grain in some (corn) and within the tube in others
(petunia). I'm not sure if the initiation of pollen tube growth is
necessarily a criterion for maturity.
Peggy E. Pollak
Department of Biological Sciences
Northern Arizona University
Flagstaff, AZ 86011-5640
