44 CONJUGATE 



sometimes bears thorn-like projections, which in some species 

 are simple (Fig. 37 /), in others branched or variously marked ; 

 in some, however, it remains always smooth (e.g. Tetmemorus, 

 Desmidium). Deviation from this mode of conjugation may occur 

 within certain genera (e.g. Glosterium, Penium). Upon germina- 

 tion the contents of the zygote emerge, surrounded by the inner- 

 most layers of the wall (Fig. 37 g, h), and generally divide into 

 two parts which develope into two new individuals, placed trans- 

 versely to each other (Fig. 37 i) ; these may have a somewhat more 

 simple marking than is generally possessed by the species. 



FIG. 38. Desmidiacese. A Closterium moniliferum ; B Penium crassiusculnm ; C 

 Micr aster ias Lruncata (front and end view) ; D Euastrum elegans ; E Staura>>trum muticum, 

 (end view). 



The most frequent genera are : 



A. Solitary cells : MESOT^ENIUM, PENIUM (Fig. 38 B), CYLINDBOCYSTIS, 

 EUASTBUM (Fig. 38 D), MICEASTEBIAS (Fig. 38 C), COSMABIUM (Fig. 36, 37), 

 XANTHIDIUM, STAUBASTBUM (Fig. 38 E), PLEUROT^NIUM, DOCIDIUM, TETMEMOBUS, 

 CLOSTEBIUM (Fig. 38 A), SPIBOT^NIA. 



B. Cells united into filaments : SPH^BOZOSMA, DESMIDIUM, HYALOTHECA, 

 GYMNOZYGA, ANCYLONEMA, GONATOZYGON. 



Order 2. Zygnemaceae. Cell-wall without markings. The 

 cells are cylindrical, not constricted in the centre, and (generally) 

 united into simple, unbranched filaments. The whole contents of 

 the conjugating cells take part in the formation of the zygote, 

 which on germina.tion grows out directly into a new filament. 



Spirogyra is easily recognised by its spiral chlorophyll band ; 

 Zyynema has two star-like chromatophores in each cell (Fig. 40) ; 

 both these genera are very common Algae in ponds and ditches. 



