Algae are mostly unicellular. Algae bear mostly chlorophyll and they are autotrophic thalloid plants. They are present in a variety of habitats but majority of them are aquatic. Plant body does not show differentiation into various tissue systems. Reproduction is vegetative and asexual and sometimes sexual.
REPRODUCTION IN ORDOGONUIM:
It takes place by three methods i.e. vegetative, asexual and sexual.
1. Vegetative reproduction in ordogonium: It is simple type of reproduction in which the filament of ordogonuim breaks up into many small portions, each of which divides into new filaments.
2. Asexual reproduction in ordogonuim:
(i) By Zoospore: All cells of the filaments are capable of forming zoospore except basal and apical cells. Cytoplasm of zoospore forming cell
Zoosporangium contacts and a hyaline receptive spot is formed. Ring appears around the margin of hyaline area. Each zoosporangium forms a single zoospore liberated by transverse split near the apex of zoosporangium. Zoospore splits out of gelatinous membrane and swims freely in water. The zoospore is oval multi-ciliate structure with a single nucleus on germination the zoospore gives rise to new filament.
(ii) By Akinetes: During unfavourable condition a chain of 10 to 40 thick walled akinetes are formed. Each cell of the filament develops a thick walled akinete by rounding off cytoplasmic contents. They germinate directly into new filament.
(iii) Sexual reproduction: Sexual reproduction is oogamous type and is common in plants growing in standing water than following water. Sex organs are antheridia and oogonia. Plants may be monoecious or dioecious. It is of two kinds:
(a) Macraudrous Type: Antheridia are reproduced in filaments of normal size either separately or on the same filament which bears oogonia.
(b) Nanandeous Type: Here antheridia are not formed to the filaments of the normal size but on special dwarf male filaments called nanandria produced by special types of motile swarm spores called androspores.
Macrandrous Type:
Antheriduim: Antheridia are terminal or intercalary and are produced by division of antheridial mother cells. Any vegetative cell may function as antheridial mother cell. It divides into two unequal cells on upper and to form antheriduim lower sister cell divides again and again and a chain of 2 to 40 antheridia is produced. Protoplasm of each antheriduim gets metamorphosed in a single antherizoid. Two antherizooids may also be produced which swim freely in water.
Oogonuim:
Oogonia develop by terminal or intercalary oogonial mother cell which divides transversely and the upper daughter cell develops into an oogonuim. Each of oogonuim possesses one or two caps at its upper end. Each oogonium is rounded or avoid in structure with a hyaline spot at upper end. It consists of a large spherical egg cell.
Nanandrous Type: Oogonia are formed as in mancrandrous type. Antheridia are formed on special dwarf filaments or mannandria and hence manandrous species are dioecious. These dwarf males or nanandria originate by germination of special type of swormers called androspores which are produced singly in flat cells androsporangia formed by repeated transverse divisions of ordinary vegetative cells. Androspores may be produced on same filament which bears oogonia or on a separate filament. Androspores get liberated like zoospores and settle down on oogonium or on a supporting cell.
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