BLUE-GREEN ALGAE 



III 



50 (49, 51) Filaments short, straight, densely aggregated in parallel bundles to 

 form small, feathery, plate-like masses. 



Aphanizomenon Morren. 



Filaments without sheath, 

 straight or somewhat taper- 

 ing at the end; united in 

 small, spindle - shaped or 

 plate-like, free-floating bun- 

 dles; spores much elongated, 

 cylindrical, solitary, not bor- 

 dering on the intercalary 

 heterocyst. A. flos-aquae 



Ralfs is sometimes found floating in great abundance in the still waters of 



ponds and lakes. 



FlG. 55. Aphanizomenon flos-aquae Ralfs. X 465- (Original.) 



51 (49, 50) Filaments free; cells shorter than their diameter. 



Nodular ia Mertens. 



Filaments enclosed in a thin, often evanescent sheath; 

 free-floating as single filaments or united into colonies of 

 indefinite form; heterocysts intercalary, depressed; spores 

 almost spherical, in rows, not bordering on the hetero- 

 cysts. 





FIG. 56. Nodularia sp. X 465. (Original.) 



52 (48) 



FIG. 57. 



Heterocysts terminal and the spores always contiguous to them. 



Cylindrospermum Kiitzing. 



Filaments without sheath 

 relatively short, aggregated to 

 form an expanded film or 

 colony of indefinite shape; 

 vegetative cells cylindrical, 

 longer than the diameter; het- 

 erocyst terminal; spores gen- 

 erally solitary, borne next to 

 Common on damp earth and stones. 



Cylindrospermum stagnate Bornet and Flahault. 

 X 465. (Original.) 



the heterocyst. 



53 (27) Filaments with true or false branches 54 



54 (60) Filaments bearing false branches; sheaths firm, of more or less equal 



thickness; filaments consisting of a single row of cells, with 

 heterocysts (except Plectonema). 



Family SCYTONEMACEAE . . 55 



55 (56, 59) Without heterocysts or spores Plectonema Thuret. 



Filaments consisting 

 only of vegetative cells, 

 without heterocysts; false- 

 ly branched, branches 

 single or in pairs; borne 

 singly in a firm, colorless 

 or yellowish sheath. P. 

 wollei Farl. occurs in 

 some abundance in 

 ponds, attached to stones, 

 etc. 



FIG. 58. Plectonema wollei 

 Farlow. X 260. (After 

 Kirchner.) 



56 (55, 59) With intercalary heterocysts. One filament in each sheath. . 57 



