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



however, been observed in only a few species ; other species 

 have been passed back and forth between the two genera ; the 

 chief difficulty is not in knowing whether an alga under exam- 

 ination belongs to the Chlorophyceae or to the Cyanophyceae, 

 but in knowing whether the long lost original of a scanty specific 

 description belonged to one or the other. Fresh water and 

 marine. 



It is quite probable that supposed species of this genus are 

 merely stages of other algae, like the " Palmella-stage " ; others 

 seem to be permanent, and not connected with any other organ- 

 ism. The only sure test would be prolonged cultures under 

 varying conditions ; but observations for a series of years of G. 

 vesiculosa and G. rupestris, in a state of nature, have shown 

 great uniformity. Other species are here included which seem 

 fairly well marked. 



KEY TO THE SPECIES OF GLOEOCYSTIS. 



i. Marine. 6. 



i. Fresh water. 2. 



2. Cells not over 6 M diam. 3. 



2. Cells 6-12 /u diam. 4. 



3. Forming a soft coating, or scattered among other algae. 5. 



3. Forming a firm, cartilaginous layer. 4. G. Paroliniana. 



4. Cells 6-8 /u. diam., bright green. i. G. vesiculosa. 



4. Cells 9-12 fj. diam., more or less brownish. 3. G. gigas. 



5. On pottery and glass in greenhouses. 6. G.fenestralis. 



5. On wet rocks. 2. G. rupestris. 



6. Cells averaging over 15 p. diam. 5. G. zostericola. 



6. Cells averaging 4-6 /* diam. 7. G, scopulorum. 



1. G. VESCICULOSA Nageli, 1848, p. 66, PI. IV. F. ; Wolle, 

 1887, p. 196, PI. CLXVI, figs. 9-15 ; P. B.-A., No. 609. Cells 

 bright green, globose or flattened, solitary or 2-8 forming a 

 family 16-35 f- diam.; membrane soft, hyaline, lamellate; 

 among other algae, on wet rocks, etc., rarely forming a distinct 

 stratum. Me., Mass. Europe. 



2. G. RUPESTRIS (Lyngb.) Rabenhorst, 1863, p. 128 ; Wolle, 

 1887, p. 196, PI. CLXVI, figs. 19-21 ; P. B.-A., No. 608. Cells 

 green, globose, 3-5 //. diam., solitary or 4-12 forming a family 

 12-60 p. diam. ; membrane soft, hyaline, lamellate, quite wide. 

 Greenland, Mass. Europe. 



In the same stations as G. vesiculosa , and often in company 

 with the latter ; distinguished by the smaller cells, more in a 

 family, with relatively wider membrane. 



