164 WISCONSIN PHYTOPLANKTON 



ACTINASTRUM Lagerheim 1882. 



Coenobia free-floating, of 4-8-16 (generally 8), ovoid, cylindrical, 

 elub-shaped, or drum stick shaped cells radiating from, a common cen- 

 ter. Coenobia sometimes united to form multiple coenobia of irregular 

 shape. Each cell with a single, laminate, parietal chloroplast; with 

 or without pyrenoids. 



Reproduction by longitudinal and transverse division of the contents 

 of any cell, the young autocolony being liberated by a rupture of the 

 mother cell wall. 



KEY TO THE SPECIES. 



Center of cell twice as broad as poles A. HANTZSOHI 



Center of cells about the same width as the poles A, GRACILLIMUM 



ACTINASTRUM GRACILUMUM G. M. Smith. PI. 43, Figs. 3-5. 

 Bull. Torr. Bot. Cl. 43: 480, pi. 26, fig. 23. 1916. 



Coenobia of 4 or 8 cells radiating from a common center. Cells elon- 

 gate-cylindrical, tapering but slightly to a truncate end, 7-10 times as 

 long as broad. Chloroplast single, parietal, laminate, with or without 

 a pyrenoid. (Euplanktont). 



Cells 1.75-3 p. broad, 14-21 ju, long; colonies 30-45 p. in diarn. 



Horse (rr), Kegonsa (rrr), Mendota (rr). 



The species differs from the following in the very slight dilation of 

 the median portion of the cell and the greater proportional length. 



ACTINASTRUM HANTZSCHI Lagerheim. PI. 43, Figs. 6-7. 

 Ofvers. Kgl. Vet.-Ak. Forh. 39, No. 2: 70, pi. 3, figs. 25-26. 1882. 



Coenobia of 4 or 8 cells radiating from a common center. Coenobia 

 sometimes united to form multiple coenobia. Cells 3-6 times as long 

 as broad, with truncate poles, ovoid-cylindrical. Diameter of middle 

 of cells twice that of poles. Chloroplast single, parietal, laminate ; with 

 one pyrenoid. (Euplanktont). 



Cells 3-6 p broad, 10-26 ^ long ; coenobia up to 50 /* in diam ; com- 

 pound coenobia up to 200 /x. in diam. 



Chetek (rr), Kegonsa (r), Lower Turtle (rrr), Mendota (rr), Prairie (rr), 

 Upper Turtle (rr), Waubesa (rrr). 



Multiple coenobia are of rare occurrence and have only been found 

 in the plankton of Chetek lake. This may be due to the fact that such 

 colonies readily fragment into the individual coenobia and the handling 

 incident to the collection of samples is sufficient to break up multiple 

 coenobia. 



