THALLOPHYTES 



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 



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

 green algae. ^ filament producing zoospores or gametes; 57, young fila- 



The sexual re- rnent developed from zoospore; 58, filament discharging zoo- 

 production is isoe- s P res and S ametes ; 59, an escaped zoospore; 60, escaped and 

 ' pairing gametes; 61, zygospores; 62, zygospore producing 

 amous, the ga- zoospores. S5-6i, after COULTER; 62, after DODEL-PORT. 

 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 



