The alternation of generations in plants is an expression chiefly used in describing the life cycle of plants – meaning the Archaeplastida. Alternation of generations also goes by the names "metagenesis" or "heterogenesis."
Simply explained, the alternation of generations in plants is a reproductive cycle of those plants, fungi, as well as protists in which a sexual reproductive phase interchanges or "alternates” with an asexual reproductive phase. For example, some protists undergo an alternation of generations. These include the slime molds, foraminifera, and numerous marine algae.
In principal, the aforementioned phases (generations), are often morphologically (form and structure of an organism or one of its parts), and occasionally chromosomally, different.
The life cycle of organisms with "alternation of generations" is characterized by each phase comprising one of two distinct organisms. These are a gametophyte (thallus (tissue) or plant) that is genetically haploid, and a sporophyte (thallus or plant) that is genetically diploid.
The gametophyte phase is the sexual phase. The sporophyte phase is the asexual phase.
Therefore, in essence, here is what happens during alternation of generations in plants. The sexual phase (gametophyte) creates gametes (sex cells). The asexual phase (sporophyte) creates spores asexually. Alternation of generations is a common element of all land plants.
A haploid plant of the gametophyte generation makes gametes by mitosis. Mitosis is a process of cell reproduction – one cell develops into two genetically identical daughter cells. Two gametes combine to produce a zygote. The zygote develops into a diploid plant of the sporophyte generation. This sporophyte makes spores through meiosis. These spores germinate and develop into a gametophyte of the next generation.
The cycle from gametophyte to gametophyte is the manner in which all land plants and numerous algae undergo sexual reproduction. Fundamentally, generations alternate from diploid to haploid to diploid to haploid, and so on.