http://en.wikipedia.org/wiki/Alternation_of_generations
why does life cycle of moss involve alternation of generation?
The life cycle of moss involves alternation of generations because moss, along with other plants such as ferns, exhibit a reproductive cycle known as alternation of generations. This reproductive cycle involves two distinct phases: the gametophyte phase and the sporophyte phase.
The gametophyte phase is the dominant phase in the life cycle of moss. It begins when a haploid spore germinates and grows into a multicellular structure called a gametophyte. The gametophyte is a small leafy structure that is capable of producing haploid gametes (sperm and eggs) through mitosis. These gametes are produced in specialized structures called archegonia (for eggs) and antheridia (for sperm). Fertilization occurs when a sperm swims to an egg, resulting in the formation of a diploid zygote.
The zygote then develops into the sporophyte phase. The sporophyte is dependent on the gametophyte for nutrients and is typically attached to it. It grows as a slender stalk with a capsule at the top. Within the capsule, specialized cells called sporocytes undergo meiosis, resulting in the production of haploid spores. These spores are then released from the capsule and can be dispersed by wind or water. If conditions are favorable, the spores can germinate and give rise to new gametophytes, completing the cycle.
The alternation of generations in moss allows for the production of genetically diverse spores and offspring. The gametophyte phase produces haploid gametes that can fuse to form genetically diverse diploid zygotes. The sporophyte phase produces haploid spores that can give rise to new gametophytes with different genetic combinations. This genetic diversity is important for the adaptation and survival of moss in various environments.
To read more about alternation of generations in moss and other plants, you can visit the Wikipedia page you provided: http://en.wikipedia.org/wiki/Alternation_of_generations