So A TEXTBOOK OF THEORETICAL BOTANY 



a relationship is postulated. It will be realized, however, that Pleiirococcus is 

 highly reduced, as is indicated by the loss of sexual reproduction and motile 

 zoospores. 



Ulotrichales 



The Ulotrichales are Chlorophyceae in which the cells are usually uni- 

 nucleate 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 



it 



is; 



Nucleus 



Chloroplast 

 with pyrenoids 





A B 



Fig. 62. — Ulothrix zonata. A, Portion of a vegetative 

 filament showing banded chloroplasts. B, Portion 

 of a filament showing zoospores. 



exceeding one thousandth of an inch (0-025 mm.) in diameter (Fig. 62). 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 



