THE GREEN ALGAE OF NORTH AMERICA 131 



The authority for the occurrence of this species in North 

 America is Wille, Natiirlichen Pflanzenfamilien, p. 40. 



Family 2. VOLVOCACEAK. 



Motile, biciliate vegetative cells as in Chlamydomonadaceae, 

 but united in families of definite form, mostly spherical or ovoid, 

 continuing motile. Asexual reproduction by the formation of 

 daughter cells, similar to the parent ; sexual reproduction vary- 

 ing much in the different genera, from union of similar cells, to 

 fully specialized oogonia and antheridia. Fresh water plants. 



KEY TO THE GENERA OF VOI/VOCACEAE. 

 i. Cells arranged in the form of a plate with cilia on one face. 



i. GONIUM. 

 i. Cells arranged in spherical, ellipsoidal, or flattened colonies; cilia 



not confined to one face. 3. 



2. Colony ellipsoidal or spherical, cells crowded together, conical, 



reaching toward the center. 2. PANDORINA. 



2. Cells not crowded nor reaching towards the center. of the colony. 



3- 



3. Colony ellipsoidal or flattened, cells uniform in size. 4. 



3. Colony spherical or sphaeroidal, or if ellipsoidal, with small vege- 

 tative and large gonidial cells. 5. 

 4. Colony ellipsoidal or spherical, poles not differentiated. 



5. EUDORINA. 

 4. Colony flattened, horse-shoe-shape, poles differentiated. 



4. PLATYDORINA. 



5. Cells not connected by protoplasmic processes ; of two sizes, smaller 

 vegetative at one pole and larger gonidial at the other. 



3. PI.EODORINA. 



5. Cells connected by protoplasmic processes, not markedly different. 



6. VOLVOX. 



i. GONIUM Miiller, 1773, p. 60. 



Colony in form of a quadrangular disk, with rounded 

 angles, composed of 4-16 biciliate cells in a single layer, en- 

 closed in a wide, colorless, gelatinous coating, the cilia all on 

 one side of the disk; cells globose, or polygonal by mutual 

 pressure ; membrane thin ; chromatophore with one large pyre- 

 noid and red stigma. Asexual reproduction by successive 

 division of individual cells in the colony, also by akinetes ; sex- 

 ual reproduction not observed. 



KEY TO THE SPECIES OF GONIUM. 



i. Colony 4-celled. T - G.sociale. 



i. Colony i6-celled. 2 - G. perforate. 



