The following points highlight the three types of life cycle of plant groups. The types are: 1. Haplontic 2. Diplontic 3. Haplodiplontic.
Type # 1. Haplontic (Fig. 3.24):
There is a single vegetative individual or somatic phase. It is haploid and is often called gametophyte. The haploid plant body may be unicellular, colonial or multicellular. It can multiply vegetatively and by accessory spores or mitospores.
Ultimately it gives rise to haploid gametes. The gametes fuse and produce a diploid zygote. The zygote remains single-celled. It does not multiply itself, neither does it give rise to a multicellular diploid structure. Instead it may take some rest.
Meiosis occurs at the time of zygote germination. Four haploid nuclei are formed as a result. Three of them degenerate in some cases and the haploid protoplast of the zygote gives rise to new plant (e.g., Spirogyra, Zygnema, Vaucheria, etc.). In others the protoplast of the zygote cleaves into four meiospores (zoospores or aplanospores). The latter may divide further into 8-16 spores before liberation.
An alternation of generations is absent since the plant does not have two cytologically distinct somatic phases. Some authors consider that there is an incipient alternation of generations because the zygote behaves as an incipient sporophyte by producing 4—16 meiospores.
Examples are found in Chlamydomonas, Volvox Spirogyra, Ulothrix, Oedogonium, Chara, Coleochaete and several other chlorophyceae, xanthophyceae and some members of other groups.
Fritsch (1935, 1942) and Stebbins (1960) consider the haplontic life history to be the most primitive. However, Feldmann (1952) believes that the haplontic life history has been derived in nature by the elimination of diploid somatic phase in a diplohaplontic life history.
Type # 2. Diplontic (Fig. 3.25):
There is a single somatic phase or vegetative individual. It is diploid and is often called sporophyte though it produces gametes in its body or sex organs. The diploid plant body is elaborated by the growth of the diploid zygote.
It may multiply vegetatively and by producing accessory spores. Meiosis occurs in the plant body or its sex organs at the time of gamete formation. Therefore, the gametes are the only haploid structures in the life. They fuse during fertilisation and give rise to the diploid individual of the progeny. Alternation of generations is absent in diplontic life history.
Occur in Cladophora glomerata, Caulerapa, Bryopsis, Codium and many other siphonales, some chlorococcales and fucales like Fucus and Sargassum. The diplontic life history of fucales is clearly derived from a diplohaplontic life history where the gametophytic or haploid somatic phase gets eliminated through progressive evolution.
In gymnosperms and angiosperms, sporophytic generation is dominant and independent while gametogenetic generation is highly reduced and is dependent. Therefore, some authors call their life cycle to be diplontic though the same is reduced diplohaplontic.
Type # 3. Haplodiplontic (Fig. 3.26):
This type of life history involves the sequential recurrence of two well developed somatic phases or vegetative individuals, gametophyte and sporophyte. The sporophyte possesses diploid chromosome number (2n). Meiosis takes place in it at the time of formation of meiospores.
The haploid meiospores germinate to produce haploid gametophytes. The gametophytes produce gametes. The fusion product of gametes is a diploid zygote which develops into the sporophytic thallus of the progeny.
There is thus a clear alternation of generations between a haploid gamete producing gametophyte and a diploid spore producing sporophyte in diplohaplontic life history, e.g., Dictyota, bryophytes, pteridophytes.