102 TUFTS COLLEGE STUDIES, VOL. II, No. 3 



Family i. ZYGNEMACEAE. 



Filaments sometimes attached, more generally free, all cells 

 except the original basal cell equally capable of division ; fila- 

 ments simple or rarely with short rhizoidal branches ; chromat- 

 ophores of different shapes in the different genera ; zygospores 

 formed by the union of the contents of two cells, either of the 

 same filament (lateral conjugation), or of distinct filaments (sca- 

 lariform conjugation) ; formed either in one of the two cells, or 

 in the passage between them. Spore with membrane of three 

 layers, of which the middle layer, mesospore, is the thickest and 

 usually colored brown or yellow in the ripe spore, and often with 

 pits or other markings. In germination the two outer layers are 

 broken, the inner forming the membrane of the new plant, which 

 at once divides into an indivisible basal cell, and a continuously 

 divisible filament cell. 



KEY TO THE GENERA OF ZYGNEMACEAE. 



i. Two cells uniting to form one, before the formation of the spore. 2. 

 i. Each cell dividing; spore then formed by the union of the two sec- 

 ondary cells. 4. ZYGOGONIUM. 

 2. Chrouiatophores two, star-shaped. i. ZVGNEMA. 

 2. Chromatophores one or more, parietal, more or less spiral. 



2. SPIROGYRA. 

 2. Chromatophore an axial plate. 3. DEBARYA. 



i. ZYGNEMA Agardh, 1814, p. 33. 



Cells cylindrical, about as long as broad or somewhat longer ; 

 dissepiments smooth and even ; two axillary stellate chromato- 

 phores in each cell, each with a pyrenoid, the nucleus between 

 the two. Conjugation lateral or scalariform ; zygospore formed 

 either in the connecting tube or in one of the cells ; spore with 

 median membrane colored, either smooth or pitted ; outer mem- 

 brane colorless, smooth or with prominences. Aplanospores 

 formed in unchanged vegetative cells and similar in appearance 

 to zygospores. Akinetes formed from single cells taking on a 

 thicker membrane and richer contents ; both aplanospores and 

 akinetes exceptional. 



KEY TO THE SPECIES OF ZYGNEMA. 



i. Spore formed in the tube. Sect. PECTINATA. 2. 



i. Spore formed in one of the cells. 4. 



2. Spore with thick, lamellate membrane. 2. Z. pachydermuDi. 



2. Spore without thick lamellate membrane. 3. 



3. Spore bluish. 3. Z. cyanospermum. 



3. Spore brownish. i. Z. pectinatmn. 



4. Spore with smooth median membrane. Sect. LEIOSPERMA. 5. 



