WISCONSIN PHYTOPLANKTON 87 



Sexual reproduction by the fusion of isogamous zoogametes. 

 There is but one family in the order, the Tribonemaceae, whose char- 

 acters are the same as those of the order. 



TRIBONEMA Derbes and Solier 1856. 



Cells in simple unbranched filaments destitute of an enveloping gelat- 

 inous sheath. Cells cylindrical or barrel-shaped, with flattened end 

 walls. Cell wall frequently of considerable thickness and laminate in 

 structure. Chromatophores two to several, yellowish-green, disciform, 

 parietal, without starch and with oil as the assimilation product. 



Asexual reproduction by aplanospores and biciliate zoospores that 

 have cilia of unequal length. The cell wall pulling apart and breaking 

 down into H-shaped pieces at the time of reproduction. 



Sexual reproduction by the fusion of isogamous zoogametes. 



TRIBONEMA MINUS (Wille) Hazen. PL 15, Figs. 17-18. 

 Mem. Torr. Bot. Cl. 11: 185, pi. 25, figs. 7-8. 1902. 



Filaments generally forming a floccose yellowish to greenish mass. 

 Cells cylindrical or slightly swollen in the median portion, generally 

 2-4, rarely 6, times as long as broad. Chromatophores 2-4, yellowish- 

 green, disciform, parietal, generally in pairs. (Facultative planktont). 



Cells 5-6 p. broad, 10-35 p. long. 



Green (1) (rrr), Mendota (s). 



Members of this genus are frequently found in ditches but seldom in 

 lakes. During the past season T. minus has appeared in considerable 

 quantity in the plankton of Lake Mendota, although previous to this 

 time the alga has not been found in the plankton. A similar sudden 

 appearance of T. bombycina forma depauperata Wille, a form closely 

 related to T. minus, has been noted in the plankton of Danish lakes by 

 Wesenberg-Lund (Studier over de danske soers Plankton p. 100. 

 Kjobenhavn, 1904). 







\0' 



-7! 



. 



V/4 



