162 WISCONSIN PHYTOPLANETON 



COELASTRUM PROBOSCIDEUM Bohlin. PL 42, Figs. 7-8; PI. 43, Figs. 1-2. 

 Bih. t. Kgl. Sv. Vet.-Ak. Handl. 23, Afd. 3, No. 7: 33, pi. 2, figs. 19-22. 1897. 



Coenobia pyramidal or cubical, rarely polygonal; of 4r-8-16-32 (gen- 

 erally 4 or 8) cells; frequently joined in compound coenobia of irregu- 

 lar shape. Cells truncate cones, at times with concave sides ; with base 

 of cone towards the center of coenobe. Interstices between cells large, 

 polygonal. Chloroplast single, parietal, with one pyrenoid. (Faculta- 

 tive Planktont) . 



Diam. cells 6.5-17 /*; diam. 4-celled coenobia 12-35 /*; compound 

 coenobia up to 350 ^ in diam. 



Bear Trap (rrr), Cochrane (rr), Durchmans (rrr), Pine Tree (aa), Round 

 (rrr). 



The appearance of this alga as the dominant organism of the plankton 

 of Pine Tree lake is of particular interest since Coelastrum species have 

 not been reported as occurring in abundance by any student of the 

 phytoplankton. Compound coenobia were very common and it was not 

 at all unusual to find 50-80 four- or eight-celled coenobia joined in one 

 irregular mass. The cells of four-celled coenobia are tetrahedrically 

 arranged in the manner that G. S. West has described them for Coelas- 

 trum compositum (Jour. Linn. Soc. Bot. 38: 136, pi. 5, figs. 8-9. 1907) 

 and I believe that C. compositum should be considered a synonym of 

 C. proboscideum. The cellular arrangment of eight-celled coenobia is 

 exactly as Bohlin and Senn describe. In the sixteen-celled coenobia, 

 which are of very rare occurrence, the cells form a hollow sphere so that 

 the general appearance of the colony resembles the other species of the 

 genus. This difference in cell number is of minor significance, the 

 number of cells in a colony being dependent upon the vegetative vigor 

 of the mother cell at the time colony formation takes place. 



SORASTRUM Kiitzing. 1845. 



Cells pyriform, semilunar, or reniform; united to form spherical 

 colonies of 8-16-32-64-128 cells. Center of colony a polyhedral body 

 from which gelatinous strands radiate to each cell. Distal side of cells 

 with 1, 2, or 4 spines. Chloroplast diffuse, with one pyrenoid. 



Reproduction unknown ; presumably by autocolonies as in Coelastrum, 

 possibly by zoospores as in Pediastrum. 



