Alternation of Generations
All plants undergo a life cycle that takes them through both haploid and diploid
generations.
1. The multicellular diploid plant structure is called the sporophyte (regular plants and
trees), which produces spores (2n) through mitosis.
2. The multicellular haploid plant structure is called the gametophyte (n), which is
formed from the spore and give rise to the haploid gametes.
The fluctuation between these diploid and haploid stages that occurs in plants is called
the alternation of generations. The way in which the alternation of generations occurs in
plants depends on the type of plant. In bryophytes (mosses and liverworts), the dominant
generation is haploid, so that the gametophyte comprises what we think of as the main
plant.
The opposite is true for tracheophytes (vascular plants), in which the diploid generation
is dominant and the sporophyte (2n) comprises the main plant.
• Mosses etc. called:
Bryophytes are nonvascularized plants that are still dependent on a moist environment
for survival. Like all plants, the bryophyte life cycle goes through both haploid
(gametophyte) and diploid (sporophyte) stages. The gametophyte comprises the main
plant (the green moss or liverwort), while the diploid sporophyte is much smaller and is
attached to the gametophyte. The haploid stage, in which a multicellular haploid
gametophyte develops from a spore and produces haploid gametes, is the dominant stage
in the bryophyte life cycle. The mature gametophyte produces both male and female
gametes, which join to form a diploid zygote. The zygote develops into the diploid
sporophyte, which extends from the gametophyte and produces haploid spores through
meiosis. Once the spores germinate, they produce new gametophyte plants and the cycle
continues.
• Regular plants and trees called:
Tracheophytes are plants that contain vascular tissue; two of the major classes of
tracheophytes are gymnosperms (conifers, pine trees) and angiosperms (flowering
plants). Tracheophytes have developed seeds that encase and protect their embryos. The
dominant phase in the tracheophyte life cycle is the diploid (sporophyte) stage. The
gametophytes are very small and cannot exist independent of the parent plant.
The reproductive structures of the sporophyte (cones in gymnosperms and flowers in
angiosperms), produce two different kinds of haploid spores by meiosis: microspores
(male) and megaspores (female).
These spores give rise to similarly sexually differentiated gametophytes (male, stamen or
anther in a flower) (female, pistol or ovary in a flower), which in turn produce gametes
(pollen in the male, and unfertilized seed core in the female). Fertilization occurs when a
male and female gamete join to form a zygote (fertilized seed core).
The resulting embryo, encased in a seed coating (i.e. the seed), will eventually become a
new sporophyte plant.