SCYTONEMA. 251 



brownish yellow. In older filaments the internal trichome 

 is often contracted to less than one-third of the diameter of 

 the sheath. Heterocysts somewhat oblong or cylindrical, 

 bright brownish yellow. 



The development of this species from JVostoc-forms has 

 often become evident by such conditions as are illustrated. 



Syn. Sc. cdlotrichoideSj Wood. 



Most frequent on dripping rocks ; also on wet or moist 

 earth on the banks of rivers, etc. 



Plate CLXXXII, fig. 4, a normal vegetative condition ; 

 fig. 5, an older filament with the internal trichome greatly 

 contracted ; figs. 6, 7, part of a filament with secondary 

 branches; fig. 8, a young filament just developed from a 

 JVosfoc-forni such as figs. 9, 10, corresponding with condi- 

 tions of N. rupestre. Kg., and N. verrucosum, Vauch. ; fig. 11, 

 a jVosfoc-form found mingled with the others, showing the 

 tendency to geminate branching before assuming the strictly 

 Scytonema character. 



SCYTONEMA CALOTRICHOIDES, Kg. 



Tomentose, aeruginous ; filaments more or less curved 

 and interwoven, sometimes distinctly, sometimes indistinctly 

 articulate ; articulations subequal or half as long as wide. 

 Ps<mcZo-branchlets usually geminate, thinner than the stem ; 

 aeruginous, cytioplasm more or less granular. Sheath close 

 and thin, rarely lamellate, colorless or light yellow ; hetero- 

 cysts oblong subcylindrical, hyaline. 



Diameter of filaments, 12-16 yw. 



Frequent 011 submerged sticks, ponds, New Jersey. In 

 size and color this is the typical plant; in habitat it is nearer 

 the Forma natans, Kab. 



Plate CLXXXIII, figs. 5-7, three parts of filaments with 

 geminate branchlets and heterocysts; from submerged 

 specimens. 



SCYTONEMA NATANS, Breb. 



Floccose, tomentose, usually brown or olive ; filaments 

 slender, smooth, shining, elongated, sometimes colorless, 

 but more frequently yellowish brown ; internal trichomes 

 aeruginous or brownish, one-third or less than the diameter 

 of the sheath ; articulations about as long as wide, not 

 always distinct. Pseudo-T&muli mostly in pairs, rather dis- 

 tant. Heterocysts interspersed, oblong or ovoid, pellucid. 



