ALGAL AND FUNGAL PROTISTS 23 



Ulothrix zonata is to be found in spring and summer 

 attached to stones and other objects near the surface of slow 

 streams or ponds ; if brought in the morning from a cool to 

 a warm place, it forms its reproductive elements. A fila- 

 ment mounted in water and covered may show both macro- 

 and microzoospores, Fig. 3, A ; ma, mi ; from two to eight 

 of the former, and from four to sixteen of the latter being 

 produced from a single cell. The macrozoospores, which 

 have a contractile vacuole, sink, and lose their flagella, in 

 place of which an organ of attachment or rhizoid is developed, 

 the element germinating directly as a new plant, Fig. 4, A. 

 The microzoospores conjugate by their anterior ends, then 

 lose their flagella, coalesce completely, and the resulting 

 zygospore secretes a cell- wall and matures slowly for germi- 

 nation next spring. In other terms the microzoospores 

 are gametes, their fusion is isogamy, and the zygospore is a 

 zygote. Both kinds of zoospore in Ulothrix are at first 

 enveloped in a thin capsule which is ruptured to allow the 

 active cell to escape. The same feature is observed in the 

 zoospores of the fungus Achlya, Fig. 4, C. 



The macrozoospores of Ulothrix correspond to the 

 parthenogonidia of Volvox, the sexual elements in the latter 

 being differentiated as oogonia and spermatozoids. 



Another alga, Vaucheria sessilis, is a species belonging 

 to a group called the Siphoneae. It grows on damp earth 

 and can often be found in greenhouse flowerpots, and a 

 plant is easily isolated by unaided vision. The thallus is a 



