HORMOGONALES 



15 



parent sheath (fig. 9 E). More commonly, false branching is 

 initiated by degeneration of a vegetative cell or heterocyst and sub- 

 sequent growth of the two filaments on either side. 



Fig. 9, Cyanophyceae. A-C, geminate branching in Scytonema pseudoguyanense 

 (A, X 470, B, C, X 340). D, false branching in Calothrix ramosa ( x 570). E, false 

 branching in Scytonema pseudoguyanense showing branch sheath (bs) terminating 

 at heterocyst. ^5 = parent sheath, d = dead cell ( x 590). F, hormogones emerging 

 from parent sheath in S. guyanense ( x 750). (After Bharadwaja.) 



*RivuLARiACEAE : Rtvulana (rivuluSy small brook). Fig. 10. 



The colonies form spherical, hemispherical, or irregular gela- 

 tinous masses that are attached to plants or stones, those of R. atra 

 being especially frequent on salt marshes. They contain numerous 

 radiating filaments with repeated false branching, each branch 

 terminating in a colourless hair. The individual sheaths can be seen 

 near the base of the trichomes, but they are diffluent farther up. 

 The heterocysts are basal, and in one section of the genus spores are 

 produced next to them. The genus is also interesting because it 

 has been shown to contain xanthophyll. 



*NosTOCACEAE : Nostoc (used by Paracelsus). Fig. 11. 



The gelatinous thallus is solid or hollow, floating or attached, 

 and varies much in size and shape. There is a dense limiting layer 



