There are four types of life cycles in algae.
They are:
(a) Haplontic or Monogenetic Type:
The algae are always haploid (n) and gametophytes as they are produced sexually by gametes. After gametic union zygote is produced which is (2n). The zygote nucleus undergoes reduction division and gives rise to 4 haploid nuclei, of which three are disintegrated. From the single functioning nucleus a haploid plant is produced.
Examples:
Spirogyra, Zygnema, etc.
In the case of Oedogonium, Ulothrix, etc., the diploid zygote nucleus, by meiosis, gives rise to four zoospores, which germinate into four haploid plants. Thus, it is seen that in both the cases the haploid gametophytic generation is predominant, but the diploid or sporophytic generation is represented only by the zygote. No true alternation of generations is found in this case.
(b) Diplontic Type:
The plant is always diploid (2n). The nuclei in the gametangia divide by meiosis and form haploid gametes. The zygote formed after gametic union is diploid (2n), which gives rise to a diploid plant. Thus, it is seen that in such cases the diploid generation is predominant and it is multicellular. The haploid generation is represented only by the gametes.
Examples:
Fucus, Sargassum, etc.
(c) Haplodiplontic or Digenetic Type:
In such cases, the diploid zygote instead of germinating into a new plant gives rise to a new sporophytic generation. It bears sporangium and its nucleus undergoes reduction division and gives rise to spores. The spores germinate into new haploid gametophytic plants. Thus, it is seen that in such cases, the two generations are equally prominent and multicellular. The sporophyte bears spores and the gametophytes gametes. Distinct alteration of generations is not seen such cases. If the plants belonging to both the generations are structurally the same, they are isomorph (e.g., Dictyota) but if different, heteromorphic (e.g., Laminaria).
(d) Diplohaplontic Type:
In such cases both the generations are equally prominent, but the sporophyte is more complicated and extended.
The sporophyte consists of two parts:
i. The first part constitutes the independent heteromorphic plant, and
ii. The second part is cyctocorp, which bears the diploid carposporephylls. The second part is dependent on the gametophytic plant.
Distinct alternation of generations is seen and the life cycle consists of three plants:
(1) Independent gametophytic plant,
(2) The sporophytic cystocarp, which is dependent on the gametophytic plant and
(3) Independent tetrasporic plant.
Such types of life cycle are known as triphasis or diplohaplontic.
Example:
Polysiphonia.
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