ULOTHRIX 



185 



cells. Each cell contains a single chromatophore with pyrenoids, 

 which has the form of a wide band, or girdle, just under the 

 cell wall, and generally surrounds the nucleus in the middle 

 region of the cell (Fig. 180, A, B). The filaments are attached 

 at one end (Fig. 180, A), and the growth by cell division takes 

 place throughout the entire length and is not confined to the 



FIG. 180. Ulothrix zonata 



A, base of filament, showing its attachment and cells containing band-shaped 

 chromatophores with pyrenoids ; B, portion of a filament about fifty cells 

 above the base, showing a vegetative cell below and two cells which have 

 formed 4 and 8 zoospores respectively; C, the zoospores, each with a. pigment 

 spot and four cilia; D, stages in the germination of the zoospore; E, portion 

 of a filament illustrating the formation of gametes, 64 in each cell; F, the 

 gametes, showing pigment spot and two cilia, and stages in their fusion to form a 

 four-ciliate zygospore with two pigment spots ; G, germination of the zygospore, 

 which develops a number of zoospores. G, after Dodel 



tip as in some algae. The cells in the upper portions of older 

 filaments (Fig. 180, B) develop a type of reproductive cell very 

 common among the algse, called the zoo spore (meaning animal 

 spore) because of its animal-like habit of swimming about. 



Zoospores are naked ciliated protoplasts formed within parent 

 cells called sporangia. They swim rapidly through the water, 

 and each generally contains a red pigment spot. Zoospores are 



