Body of ulothrix has un-branched filaments. Filaments contain short cylindrical cells joined end to end. Cells are as broad as long. The filament remains attached to substratum by modified based cell called hold fast. Upper cell or tip cell is sub spherical in outline. Each cell except hold fast cell has got an outer wall composed of cellulose plus pectic substances.
Within the cell wall is cytoplasmic layer in which nucleus is embedded. Chloroplast has two more pyrenods. Filament is autotrophic in nutrition and grows in length.
Reproduction: It takes place by vegetation, Asexual and Sexual method.
(1) Vegetative Reproduction: It takes place by chance but not by regular method of multiplication. In this case the filament breaks up into two or more parts and each part is capable of converting itself into a new filament.
(2) Asexual Reproduction:
(A) By zoospore formation: The cell of filament under favourable conditions produces the zoospores. The zoospores are produced by ordinary cells of the filament whose contents divide into 2, 4, 8 and 16 parts. Zoospores are (i) Macro zoospores which are slightly flattened and have four flagella and (ii) Micro zoospores which are ovoid and have four or two flagella. Each zoospore is uni nucleate and has a chloroplast in broader part of spore and the apical part consists protoplasm and flagella. After liberation the spores swim for some time and come to rest, attach themselves and grow out directly into new filament.
(B) By Aplanospores: Occasionally the development zoospores stop just before development of cilia. In such cases non motile thin walled aplanospores are produced instead of motile zoospores. These aplanospores may germinate inside the parent cell or may be liberated outside. They then develop into new filament.
(C) By Akinetes: Sometimes in unfavourable conditions cell of the filament forms a single rounded thick walled structure called akinete. Akinetes are double walled with exospore and endospore. On return of favourable conditions the exospore ruptures and it develops into new filament.
(D) Palmella stage: Sometimes the contents of cell divide and their walls become mucilaginous. These cells either directly form new filaments or they give rise to zoospores which form new filaments.
(3) Sexual Reproduction: It is oogamous. Sexual reproduction consists simply fusion of similar two gametes. In unfavourable conditions each cell of filament produces 16, 32 or 64 gametes by divisions on the same manner as zoospores are produced. Isogametes are avoid and biflagellate. Each has chloroplast and a single pyrenoid. Isogametes come out of parent cell in membranous vesicle. Very soon the vesicle disappears and gametes move freely in the water.
Gametes fuse in pairs interiorly and quadric-flagellate zygospores or zygotes are developed. The cilia of zygotes are withdrawn, it becomes round and thick wall is secreted around it. Zygote after a definite resting period increases in size and its nucleus divides by reduction division. Protoplast of zygote divides and re-divides and 4 to 16 aplonospores or zoospores formation takes place. Each one of them develops into new filament of ulothrix.
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