TAXONOMY 
l8 7 
Family Ulothricaceae. — Ulothrix zonata, a typical representative 
of this family, is a filamentous organism found growing on stones 
around ponds, on rocks along the shores of lakes, in slow-moving 
streams, etc. Each filament is unbranched and consists of a row of 
short cells, one of the terminal cells, called the rhizoid cell, being 
elongated and serving as an attachment structure. Each cell con- 
sists of a cell wall of cellulose enclosing cytoplasm, a nucleus and a 
wide band-shaped green chromatophore, more or less cylindrical in 
shape. The chromatophore lies next to the cell wall and contains 
pyrenoids or starch-forming centers. The filament grows in length 
by the fission of its various component cells. After attaining a 
Fig. 76. — Vaucheria lerreslris. anth, antheridium (empty) ; o, oogonia. (Gager.) 
certain size it reproduces either asexually or sexually. Asexual 
reproduction takes place by certain cells becoming altered in their 
protoplasmic contents through division to form rounded or pear- 
shaped zoospores. Each zoospore contains a red pigment spot and 
bears four cilia (protoplasmic outgrowths). The zoospores escape 
into the water by lateral openings in the walls of cells containing 
them. They swim rapidly about, propelled by their cilia, and ere 
long attach themselves to various objects and grow into Ulothrix 
filaments. The sexual method of reproduction is effected through 
the production of many gametes, in cells of the filament, which re- 
semble the zoospores in shape but differ from them in being smaller 
and possessing but two cilia. These escape into the water, and, 
after swimming about for a short time come together in pairs and 
