CYANOPHYCE& 



439 



to a lateral branch ; the branches therefore spring in pairs at a right 

 angle from the main axis. Thick-walled heterocysts may be formed 

 at any spot in the filament. In Tolypothrix Ktz. the false branches 

 spring singly from beneath heterocysts. In the Sirosiphoneae, including 

 Stigonema (Ag.), Sirosiphon (Ktz.), Fischera (Schw.), Capsosira (Ktz.), 

 Hapalosiphon (Nag.), Mastigocladus (Cohn), and Mastigocoleus 

 (Lagerh.), the formation of a false branch is preceded by a change in 



FIG. 367. Hapalo siphon byssoideus Kirch, (x 200). (From nature.) 



the direction of cell-division. Two or three contiguous pseudocysts in 

 an older portion of the filament divide in a direction parallel to the 

 axis of growth of the filament, and one of the new pseudocysts thus 

 formed now becomes the basal pseudocyst of a lateral branch, which 

 generally consists of a single row of pseudocysts at right angles to the 

 axis. In Plectonema Thur. the branches protrude outside the mucila- 

 ginous sheath. Heterocysts are formed in all parts of the filament, but 

 their function is unknown. 



