98 



Bacillariese 



Some diatoms are stipitate, secreting gelatinous stalks, either simple 

 or branched, by which they are attached to larger aquatic plants or to 

 some other substratum (Gomphonema, Achnanthes, some species of Cocconeis); 

 arid there are yet others which secrete a copious mucus so that a large 

 number of individuals become embedded in a common gelatinous mass 

 (Epithemia alpestris, Cocconema spp., Vanheurckia rhomboides var. saxonica). 

 Less often the cells are enclosed in simple or branched mucous tubes 



Fig. 71. A and B, Meridian circularc (Grev.) Ag. ; A, part of spirally disposed colony; B, valve 

 view of individual cell. C E, Mclosira varians Ag. ; C, part of a long filament of cells ; 

 Z), valve view of individual cell ; E, cell of filament showing formation of auxospore. F and 

 G, Tabellaria flocculosa (Both) Kiitz. ; F, part of a zigzag colony seen from the girdle view ; 

 G, longitudinal section of cell showing partial septum. 



[Cocconema ( Encyonema) spp., Navicula ( Scldzonema) spp.], a condition 

 which is more frequently met with in marine than in freshwater diatoms. 



The mucus is often secreted through special pores, with definite 

 localized positions in the valves of different genera. In Tabellaria such 

 secretory pores occur near the middle of the valve (fig. 72 F), in Diatoma 

 there is always one pore towards the end of the valve in a somewhat asym- 

 metrical position (fig. 72 A). In Synedra and Fragilaria the pores are also 

 near the extremities of the valves (fig. 72 D and E). The different forms 



