FROTOCOCCOIDE.E. 



51 



zoids (Fig. 46 a). After the oosphere has been fertilised by these, 

 the oospore surrounds itself by a thick, sometimes thorny cell- 

 wall, and on germination becomes a new individual of few cells. 

 A few cells conspicuous by their larger size may be found (1-9, but 

 generally 8) in certain individuals, and these provide the vegeta- 

 tive reproduction, each forming by division a new individual. 



Order 2. Tetrasporaceae reproduce both by vegetative divisions and 

 swarmspores, some have also gamete-conjugation. The principal genera are : 

 Tetraspora, Apiocystis, Dactylococcus, Dictyosphcerium, Chloranyium. 



Order 3. Chlorosphaeracese. Chlorosphcera. 



Order 4. Pleurococcaceae. In this order the swarm-stages 

 and sexual reproduction are entirely absent. Vegetative repro- 

 duction by division. The principal genera are : Pleurococcus (Fig. 

 47), Scenedesmus (Fig. 48), Raphidium, Oocystis, Schizoclilamys, 

 Crucigenia, Selenastrum. Pleurococcus vulgaris (Fig. 47) is one of 

 the most common Algae throughout the world, occurring as green 

 coverings on tree-stems, and damp walls, and it is one of the most 

 common lichen-gonidia. 



FIG. 47. Pieurococcas vulgaris. 



FIG. 48 S. 



uiiadricauda. 



Order 5. Protococcacese. The cells are motionless, free or 

 affixed on a stalk (e.g. Characium, Fig. 49), either separate or 

 loosely bound to one another ; they never form multicellular 

 individuals. Multiplication by division is 

 learly always wanting. Reproduction takes 

 )lace by swarmspores, which have 1 or 2 cilia, 

 ind sexual reproduction in some by gamete- 

 mjugation. The principal genera are : Ghlo- 

 coccum, Chlorochytrium, Ghlorocystis, Scotino- 

 sphcera, Endosphcera, Phyllobitim, Characium, 

 Ophiocytium, Sciadium. 



Order 6. Hydrodictyaceae. The indi- 

 viduals are unicellular but several unite after 

 the zoospore-stage into definitely formed 

 families (coenobia). Ordinary vegetative division is wanting, but 



Characium 

 strictum. A The cell-con- 

 tents have divided into 

 many swarmspores. B 

 Swarmspores escaping. 



