ULOTHRIX 



311 



Ulothrix. Ulothrix (Fig. 267) is one of the simpler forms of 

 the group, and its filaments, an inch or two in length, form bright 

 green fringes about stones and other objects in lakes, ponds, 

 streams, and troughs. There is some differentiation within the 

 filament, for the basal cell is modified into a holdfast by which 

 the filament is attached to a support. The other cells are alike 

 and each contains one nucleus and a large encircling chloroplast. 



FIG. 267. Ulothrix zonata. A, portion of a filament, showing the hold- 

 fast and a number of vegetative cells; B, portion of a filament, show- 

 ing three cells containing gametes; C, a portion of a filament, showing 

 gametes escaping at 6, and zoospores formed at a and escaping at c; D, a new 

 filament developing from a zoospore, the character of which is shown at z. 

 At g gametes are shown fusing to form zygospores. At zy a zygospore, just 

 after the fusion of the gametes and when fully mature, is shown. A zygo- 

 spore which has germinated and produced four zoospores is shown at y. 

 X 200-300. Redrawn with modification from Coulter and from Strasburger. 



The plant reproduces asexually by four-ciliate zoospores, and 

 sexually by two-ciliate isogametes. The zoospores are formed usu- 

 ally two or more in a cell. They escape together from the mother 

 cell enclosed in a membrane, but soon separate and after swim- 

 ming about for a short time become attached to some object by 

 the ciliated end and by growth and cell division become new 

 filaments. Some cells produce gametes, which, besides having 

 only two cilia, are much smaller and more numerous than zo- 

 ospores. After escaping, the gametes fuse in pairs to form resting 

 zygospores. Upon germination, the zygospore does not pro- 



