PROTOCOCCOIDEjE. 



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 (19, 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, Apioci/stis, Dactylococcus, Dictyuspharium, Chloranyium. 



Order 3. Chlorosphaeraceae. Chlurosphara. 



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



O 



and sexual reproduction are entirely absent. Vegetative repro- 

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

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

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

 the most common Algae throughout the world, occurring as o'reen 

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

 common lichen-gomdia. 



B 



FIG. -17. Pleurococcus vulgaris. 



FIG. 48. ScciictlcsiiiHS quadricauda. 



Order 5. Protococcaceae. 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 

 nearly always wanting. Reproduction takes 

 place by swarmspores, which have 1 or 2 cilia, 

 and sexual reproduction in some by gamete- 

 conjugation. The principal genera are : Chlo- 

 rococcum, Ghlorochytfium, Chlorocystis, Scotino- 

 sphc&ra, Endosplicera, PhylJobium, Characiurn, 

 Ophwcytium, Sciadium. 



Order 6. Hydrodictyacese. The indi- 

 viduals are unicellular but several unite after 

 the zoospore-stage into definitely formed 

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



FIG. 49. Characiwm 



strictum. A The cell-son- 

 tents have divided into 

 many swarrnspores. B 

 Swarmspores escaping. 



