The Algae - Chlorophycae - Ulotrichales - Ulothrix zonata

Phylum Thallophyta - The Algae - Chlorophycae - Ulotrichales - Ulothrix zonata 

The Ulotrichales are Chlorophyceae in which the cells are usually uni­nucleate and form simple or branched filaments. Asexual reproduction takes place by zoospores, or more rarely by non-motile spores. In the simpler members sexual reproduction is by isogametes. There is generally a single chloroplast in each cell which lies in close proximity to the wall and is therefore said to be parietal. The members of the group occur chiefly in fresh water. 

Ulothrix zonata 

This Alga is extremely common in running water and also in rain puddles. It is bright green in colour and consists of extremely fine filaments rarely exceeding one thousandth of an inch (0.025 mm.) in diameter. These filaments consist of a single row of small cells, often as many as a thousand in a single filament. The threads are generally attached to stones, but they can live equally well when unattached. 

The cell has a cellulose wall, inside which is a lining layer of protoplasm in which the nucleus is embedded. The single chloroplast is in the form of a broad transverse band lying close against the wall and curved round the cell in the form of a ring. Situated in the chloroplast are several pyrenoids. The centre of the cell consists of a large vacuole containing cell sap. 

In cases where the filaments are attached at the end, the anchoring cells are devoid of chlorophyll and are either brown or nearly white in colour. There is no special growing region and any cell of the filament may divide as elongation takes place. Vegetative multiplication is often effected by fragmentation of the filaments. 

Asexual Reproduction in Ulothrix Zonata

This consists in the formation of zoospores. They arise from the ordinary cells of the filament, which divide up into a number of separate parts, generally from two to sixteen, though occasionally only a single zoospore may be produced from the mother cell. They are liberated into a delicate mucilage vesicle through a small aperture on one side of the cell. 

The zoospores are of two kinds, macrozoospores, which are slightly flattened and have four flagella, and microzoospores, which are ovoid and have either four or two flagella. The latter type form an intermediate group between the macrozoospores and the gametes, and are not clearly distinguish­able from the latter in appearance. 

Each zoospore contains a single nucleus; the chloroplast is situated in the broader part of the spore, while the apical end consists of clear protoplasm, attached to which are the flagella. There is a single contractile vacuole which pulsates about every ten seconds. Each zoospore is provided with an eye spot which is apparently sensitive to light. In light of moderate intensity they swim towards the source of light, but in very brilliant illumina­tion they swim in the opposite direction. Finally, after swimming about for several hours, they come to rest, attach themselves and grow out directly into new filaments. On germination the first cell produced is a colourless rhizoidal cell attached to the substratum. 

Sexual Reproduction in Ulothrix Zonata

The gametes are produced either in cells of the same filaments as the zoospores or in different ones. They resemble the microzoospores osely, except that each possesses only two flagella. Each mother cell indes to produce from eight to thirty-two gametes. Conjugation normally takes place only between gametes from different filaments. Fusion is similar that described in Chlamydomonas, forming a zygote with four flagella. After fusion movement does not continue for long and soon the flagella are retracted and the zygote comes to rest, develops a thick wall and is trans­formed into a zygospore. It divides into a number of non-flagellated cells (aplanospores) which are liberated and form new filaments. The first division of the zygote is the reduction division, so that the cells of the filaments are monoploid. 

We see, therefore, that in Ulothrix we have an example of perfect isogamy, in which the two gametes are of identical size and structure, though from the fact that gametes from the same mother cell will not fuse together we realize that there must be some physiological difference between them, which may explain why gametes of different origin are at­tracted to one another. 

Asexual reproduction in Ulothrix takes place chiefly in the winter, and it is at this time of the year that the growth of this Alga is most active. During the summer re­production is by sexual means, and it is interesting to note that after the forma­tion of the zygospore ger­mination may proceed very slowly, and it may rest for a considerable period whilst still unicell ular. I t is only when conditions become more favourable that the new filament is produced. We see, there­fore, that here, as in many other lower organ­isms, sexual reproduction unfavourable to vegetative may be associated with conditions which are growth. 

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