166 WISCONSIN PHTTOPLANKTON 



zoospores but smaller, the zoogametes fusing in pairs to form a spheri- 

 cal zygote after their liberation through a pore in the cell wall. Zygote 

 germinating, after a short period of rest, into 2-5 large uni- or biciliate 

 zoospores that form irregular polyhedral cells on coming to rest. Poly- 

 eders ultimately germinating into 200-300 zoospores that are liberated 

 in a vesicle and form a net as in asexual reproduction. 



HYDRODICTYON RETICULATUM (L) Lagerheim. PI. 44, Fig. 6; PI. 45, 



Fig. 1. 



Ofvers. Kgl. Vet.-Ak. Forh. 40, No. 2: 71. 1883; G. S. West, Brit. Freshw. Algae 

 208, figs. 84A-84F. 1904 



Cells elongate cylindrical, forming a definite closed net. (Tycho- 

 pJanktont). 



Cells 5-250 p broad, up to 1.5 cm. long. Coenobia up to 30 cm. long. 



Little Bass (2) (rr), Mendota (rr), Monona (sss). 



An alga that is in no sense a plankton organism but one which some- 

 times grows in abundance in shallow, well protected bays and so may, 

 at times, be found free-floating in large patches a considerable distance 

 from the shore. 



PEDIASTRUM Meyen 1829. 



Coenobia disciform to stellate, free-floating, of 2-4-8-16-32-64-128 

 cells arranged in a layer one cell in thickness. Coenobe entire, perfor- 

 ate, or clathrate. Marginal cells polygonal, with one, two, or four proc- 

 esses that sometimes terminate in a tuft of long hyaline setae. Interior 

 cells polygonal, without processes. Cell wall smooth, granulate, or 

 covered with a meshwork of fine ridges; without a gelatinous sheath. 

 Chloroplasts parietal and disciform at first, later filling the entire cell ; 

 with 1-4 pyrenoids. Cells coenocytic. 



Asexual reproduction by the division of contents of any cell into 

 2-4-8-16-32-64-128 biciliate zoospores that are extruded in a gelatinous 

 vesicle and, after a short period of swarming, on becoming quiescent 

 appose themselves in the same position as the mature cells of the 

 coenobe. 



A fusion of biciliate zoogametes into polyhedral resting cells has been 

 observed. On germination these zygospores form 8-16-32 zoospores 

 that behave as in asexual reproduction. 



