Alternation of generations is a common component of all land plants, both vascular and non-vascular. The life cycle of bryophytes consists of two distinct phases, one of which is the green thalloid or leafy plant body, and the other is the sporogonium. In a life cycle, when the two generations, sexual (haploid) and asexual (diploid) generations, alternate with each other by the production of gametes and spores respectively, it is called alternation of generation.
In bryophytes, the gametophytic or gamete-producing generation is distinct in its morphology and is haploid having a single set of chromosomes. It is green, thalloid (liverworts) or leafy (mosses) and bears sex organs, i.e. antheridium (male) and archegonium (female). They produce male and female gametes i.e. antherozoids and eggs respectively, that unite sexually to form diploid zygote.
The zygote is the first cell of the sporophytic generation. The zygote then divides to form an embryo which further differentiates to form the sporogonium. The sporophyte or spore-producing generation is morphologically different from the gametophyte and is diploid in nature. It is usually differentiated into foot, seta, and capsule. The diploid spore mother cells produced inside the capsule undergoes meiosis to form haploid spores which germinate to produce haploid gametophyte.
In this way the life cycle is completed with an alternation of generations. This type of alternation of generations are termed as diplohaplontic life cycles.
In bryophytes, the alternating individuals in the life cycle differs morphologically as well as physiologically. The gametophyte is independent while the sporophyte is attached and generally dependent fully or partially on the gametophyte. This kind of alternation of generation is known as heterologous or heteromorphic.
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