THALLOPHYTA: ALGAE 



29 



together within a mucilaginous matrix and surrounding a small central 

 cavity (Fig. 18A). Sometimes the colony consists of only 8 cells or, less 

 frequently, of 32 cells. Each cell resembles that of Chlamydomonas. In 

 asexual reproduction each cell divides simultaneously to produce a group 

 of as many daughter cells as were in the parent colony (Fig. ISB). Each 

 group then escapes as a new colony. In sexual reproduction each vegeta- 

 tive cell similarly produces a group of daughter cells as numerous as the 

 cells in the colony, the groups separate, and the daughter cells escape 

 individually as biciliate gametes. Although Pandorina is isogamous, one 



Fig. 18. Pandorina morum, X750. A, free-swimming vegetative colony of 16 cells, 

 those lying below not shown; B, colony undergoing ase.xual reproduction; C, a large and a 

 small gamete; D, gametic union; E, a zygote. 



of the fusing gametes is slightly larger and less active than the other, thus 

 showing a tendency toward heterogamy (Fig. 18C, D). The zygote 

 remains motile for a while, finally settling down and secreting a cell wall 

 (Fig. 18E). Upon germination the zygote divides internally into four 

 protoplasts, but generally only one becomes a zoospore. The zoospore 

 produces a new colony. 



Eudorina. Eudorina is a spherical colony usually consisting of 16, 32, 

 or 64 biciliate cells, each like a cell of Chlamydo?nonas. The cells are 

 loosely arranged in a single layer near the surface of a mucilaginous 

 matrix. As in the preceding genera, any cell may give rise to a new 

 colony by internal division of its protoplast, but an advance is seen in 

 sexual reproduction (Fig. 19). Some of the cells divide to form groups 

 of sperms, as many as 64 usually arising from a single vegetative cell. 

 The other cells enlarge slightly by the accumulation of food and become 

 eggs. Although both male and female gametes are biciliate, only the 

 sperms escape from the colony and become free-swimming. At first the 



