328 



Myxophycece 



himself. Why, under these circumstances, should the genus Nostoc 

 be made more confusing than it is already ? 



There are about 10 British species of the genus, some of which are 

 relatively abundant in the waters of ponds and lakes. .1. Flos-aqiux Breb. 

 and A. drcinalis Rabenb. are constituents of the freshwater plankton. A. 

 oscillarioides Bory and A. incequalis (Kiitz.) Born. & Flah. (fig. 150 A D) are 

 found among other Algae in still water. Several species occur among Sphag- 

 num in bogs, but spores seem to be rarely developed in these situations. The 

 trichomes vary in thickness in the different species from 4 to 10 /*, and the 

 cylindrical type of spore may reach a length of 60 p.. 



Genus Aphanizomenon Morren, 1838. The trichomes are 



straight and without a defi- 

 nite sheath, being aggluti- 

 nated to form spindle-shaped 

 or small plate-like bundles, 

 which float freely in the water. 

 Each trichome is slightly at- 

 tenuated towards the extremi- 

 ties. The spores are solitary, 

 cylindrical, much elongated, 

 and only developed sparingly 

 between the heterocysts. 



A. Flos-aquce (L.) Ralfs is 

 found floating in the still waters 

 of ponds and lakes, sometimes in 

 abundance. The trichomes are 

 rigid and 5 6 p. in diameter. 

 The cells are subquadrate and 

 the heterocysts are cylindrical. 

 The spores are 6080 /* in length 

 and only 7 8 /x in breadth. 



Fig. 150. A D, Anab<e,na incequalis 

 (Kiitz.) Born. & Flah., from Epping Forest, 

 Essex. E G, Cylindrospermum stannalc 

 (Kiitz.) Born. & Flah., from Eibblehead, 

 W. Yorks. H, Nodular ia spluerocarpa Born. 

 & Flah., from near Ely, Cambridgeshire. 

 (All x480.) 



Genus Nodularia Mer- 

 tens, 1822. The filaments 

 are free-floating and are 

 generally furnished with a 

 distinct sheath. The latter is hyaline and mucous, closely en- 

 veloping the trichome, and sometimes becomes diffluent. The 

 trichomes are more or less straight, with short, depressed, often 

 discoidal cells. The heterocysts are also depressed. The spores 

 are spherical or discoidal, and are developed in series- between the 

 heterocysts. 



