38 WISCONSIN PHYTOPLANKTON 



The cells of the Wisconsin individuals are somewhat longer than 

 Wolle found them, his measurements for the length of the cell varying 

 from 9-16 u,. 



MICROCYSTIS Kiitzing 1833. 



Colonies free-floating; spherical, elongate, irregular, or perforate to 

 form reticulate or clathrate masses of microscopic or macroscopic size. 

 Frequently occurring in sufficient quantities to give the water a verdi- 

 gris-green color. Envelope of colonies generally copious, delicate to 

 firm. Cells small, spherical, distributed in a solid mass throughout the 

 colony and densely aggregated. Individual cell sheath confluent with 

 colonial envelope. Cell contents homogeneous or with numerous pseudo- 

 vacuoles ; light to dark blue-green, olive-green, or rose colored. 



Reproduction by vegetative division of cells in three directions. 



Certain species of the genus occur in prodigious quantities and cause 

 the familiar blooming of lakes. These species contain pseudovacuoles, 

 while species without pseudovacuoles rarely occur in quantity. 



KEY TO THE SPECIES. 



Cells with numerous pseudovacuoles. 



Several colonies enclosed by the same envelope .......... M. ICHTHYOBLABE 



Colonies not differentiated into several within the same envelope. 



Colony entire, not perforate .......................... M. FLOS-AQUAE 



Colony saccate and perforate ......................... M. AERUGINOSA 



Cells without pseudovacuoles. 



Diameter of cells less than 1.5 /* .............................. M. INCEBTA 



Diameter of cells 2-3 //, ...................................... M. PurmiA 



MICROCYSTIS ICHTHYOBLABE Kiitzing. 



Phycol. generalis 170. 1843. 



Polycystis ichthyoblabe Kiitzing, Tabulae Phycol. 1: 7, pi. 8. 1845. 



Colonies spherical or irregular, fragmented into several distinct sub- 

 colonies, each Avith its own gelatinous envelope, and with the sub-col- 

 onies enclosed by a common gelatinous envelope. Cells spherical, close 

 together. Individual cell sheaths confluent with colonial envelope. Cell 

 contents pale to bright blue-green. Generally containing numerous 

 pseudovacuoles. (Facultative Planktont). 



Diam. cells 2-4 ju,. 



Catfish (rr), Clear (rr), Kawaguesaga (ss), Minocqua (r). 



