In the diplontic life cycle, the diploid sporophyte is the dominant photosynthetic independent phase of the plant body. The haploid phase is represented by gametes only. In gametic meiosis, instead of immediately dividing meiotically to produce haploid cells, the zygote divides mitotically to produce a multicellular diploid individual. Cells from the diploid individuals then undergo meiosis to produce haploid cells or gametes. Haploid cells may divide again (by mitosis) to form more haploid cells, as in many yeasts, but the haploid phase is not the predominant life cycle phase. In most diplonts, mitosis occurs only in the diploid phase, i.e., gametes usually form quickly and fuse to produce diploid zygotes. The main plant body is the diploid (sporophyte 2n). Example All seed-bearing plants, gymnosperms, and angiosperms.
In the haplontic life cycle, the haploid phase of the cells is predominant. The zygotic meiosis produces haploid spores. These haplonts will then divide mitotically to produce more haplonts. These haplonts will give rise to male and female gametes. In haplontic life cycle, 2n state of nuclei is present only in the zygote and this state is very short-lived. The organism ends its diploid phase and produces several haploid cells. These cells divide mitotically to form either larger, multicellular individuals, or more haploid cells. Two opposite types of gametes (e.g., male and female) from these individuals or cells fuse to become a zygote. The main vegetative plant body is a haploid. Example - fungi, green algae, and many protozoa.
In the haplodiplontic cycle, both the diploid (2n) and haploid (1n) phases are multicellular. The 2n sporophyte produces sporocytes in a sporangium, each of which goes through meiosis to create four (4) spores. These spores are dispersed and germinate into gametophytes. Gametophytes mature and produce sex cells: male (e.g. sperm) and female (e.g. eggs) in gametangia. Sex cells fuse, during fertilization, to create a young sporophyte or zygote. This cycle is also called the diplo-haplontic life cycle or alternation of generations or sporic meiosis. Examples: All land plants, chytridiomycetes, red algae, some brown algae, and some green algae. Most land plants (except bryophytes) have a dominant sporophyte phase, with a reduced gametophyte. This can make land plants seem like they are diplontic, but they are not. Every land plant group has a multicellular gametophyte (although hidden), which makes them haplo-diplontic.