TflALLOPIIYTES 



25 



cell of the Volvocales, even to the " eye spot" and contractile vacuoles. 

 The spores escape through an opening in the wall of the mother cell, 

 swim about, come to rest, settle upon some support, ciliate end foremost, 

 develop a holdfast, begin cell division, and new filaments are started 

 (fig. 57). This differentiation between zoospores and ordinary vegetative 

 cells, and the behavior of the zoospores in escaping individually from 

 the mother cell 

 and starting inde- 

 pendent individu- 

 als, are features in 

 contrast with the 

 ordinary situation 

 among the Vol- 

 vocales and Proto- 

 coccales. In any 

 event, it should be 

 kept in mind that 

 the zoospore type 

 of cell is probably 

 to be regarded as 

 the most primitive 

 type among the 

 green algae. 



The sexual re- 

 production is isog- 

 amous, the ga- 

 metes being pro- 

 duced just as are the zoospores, but differing in usually being smaller, and 

 of course more numerous, and in being biciliate (fig. 60). The freed 

 gametes conjugate in pairs and form zygospores (fig. 61), which after 

 the resting period do not give rise to new filaments directly, but produce 

 several zoospores (fig. 62) that give rise to filaments. This intercalation 

 of zoospores in the life history between the zygospores and the new 

 filaments is an important fact to note. 



Ulothrix is very often used to illustrate the origin of sex among the 

 green algae. In many forms the vegetative cells are found producing 

 swimming cells of all sizes, varying from the large zoospores to the 

 small gametes, so that it is often impossible to distinguish between zoo- 

 spores and gametes by their appearance. Some of the swimming cells 



Figs. 55-62. — Ulothrix: 55, base of filament with holdfast; 

 56, filament producing zoospores or gametes; 57, young fila- 

 ment developed from zoospore; 58, filament discharging zoo- 

 spores and gametes; 59, an escaped zoospore; 60, escaped and 

 pairing gametes; 61, zygospores; 62, zygospore producing 

 zoospores. — 55-61, after Coulter; 62, after Dodel-Port. 



