THE GREEN ALGAE OF NORTH AMERICA 363 



ing usually sterile ; the germinating akinete dividing into two 

 parts, of which one develops a short rhizoid, the other the 

 initial cauloid filament of the future plant. 



A genus of chiefly tropical and subtropical fresh water plants, 

 closely allied to Cladophora, but the reproduction by akinetes 

 appears to take the place of the reproduction by zoospores and 

 gametes. Sterile plants are practically indistinguishable from 

 Cladophora, but a fertile frond of Pithophora is unmistakable 

 for anything else. Specific distinction, however, is not always 

 easy. The species being naturally tropical, are often found in 

 greenhouses where tropical plants are cultivated. 



KEY TO THE SPECIES OF PITHOPHORA. 



i. Intercalary akinetes all of about the same shape. 2. 



i. Intercalary akinetes varying in the same plant; cylindrical, cask- 

 shaped, obovoid or irregular. 4. 



2. Main filament seldom under 150 n diam. i. P. aequalis. 



2. Main filament less than 100 M diam. 3. 



3. Special helicoidal cells frequent. 3. P. Cleveana. 



3. Special helicoidal cells wanting or very rare. 2. P. oedogonia. 



4. Main filaments about 165 M diam. 5. P. Roettleri. 



4. Main filaments seldom reaching 100 n diam. 4. P. varia. 



1. P. AEQUALIS var. FLORIDENS1S Wolle, 1887, p. 131, PI. 



CXIV, figs. 1-5. Main filaments in fertile plant 150-175/4 diam., 

 with a few elongate branches, and many short branches ; cells 

 more or less swollen ; akinetes solitary or rarely two together, 

 intercalary in the main filaments or the long branches, rarely 

 terminal ; intercalary akinetes cask-shaped, with somewhat 

 rounded top, about 215 X 120/4 ; terminal akinetes cask-shaped, 

 conical or sometimes rounded above, about 270X90 /*. Fla. 



The type is found in So. America, and has main filaments 

 seldom exceeding 100 /x diam., and akinetes slightly larger than 

 in the variety. 



2. P. OEDOGONIA (Mont.) Wittrock, 1877, p.' 55, PI. VI, 

 figs. 1-6; including var. vaucherioidcs Wolle, 1887, p. 130, PL 

 CXIII, figs. 1-7. Main filaments about 70 p. diam., branches of 

 three orders scattered or opposite ; branches occasionally issu- 

 ing from the short cell below the akinete ; akinetes solitary, 

 rarely in twos, intercalary or terminal ; intercalary akinetes cask- 

 shaped, about 230X115 /"; terminal akinetes cask-shaped, 

 above shortly acuminate, with rounded apex, about 215X95^-* 

 Fig. 113. Pa., N. J., Neb. So. America. 



*P.affinis Nordstedt, 1878, p. 19; Saunders, 1894, p. 66, is reported 

 from Neb., but no specimens have been seen. It was described from 



