378 University of California Publications in Botany [Vol. 6 



receive a spirally twisted filamentous form whose transverse cell walls 

 are plainly visible. Thus the only apparent morphological difference 

 between the two genera is the stated difference regarding the trans- 

 verse walls. This distinction is made use of by Gomont (1892, pp. 

 95-96), by Forti (1907, p. 145), and by Kirchner (1900, p. 63). By 

 emplojung methods of differential staining, Professor W. A. Setchell 

 and I have determined repeatedly that this distinction can no longer 

 maintain. The small, tightly coiled forms are really multicellular, 

 and hence no fundamental difference exists between the two genera. 

 However, since the described species are few and there is not likely 

 to be a very large number of new species added in the future, and 

 since the forms thus far discovered fall rather naturally into two 

 groups, those very small tightly coiled species with obscure cross- 

 walls, and those rather loosel}^ but regularly coiled species with con- 

 spicuous cross-walls, all of which are fairly well known, it seems wise, 

 for the sake of convenience, to retain both genera, and emend the 

 diagnosis of SjnruUna. Certain species of Phormidiuni and Oscilla- 

 toria can scarcely be distinguished as to the principal generic char- 

 acter, the presence or absence of a sheath. All species of Phormidiuni 

 are at times of active growth after formation of hormogonia without 

 a sheath, and some well-recognized species of Oscillatoria may, under 

 certain conditions, develop a sheath. I have observed this in 0. 

 sancta. The two genera are thus not absolutely distinct, but for 

 convenience they are both kept up. Again, some of the small species 

 of both genera have very obscure cross-walls. In this respect they 

 bear the same relation to the larger species with definite cross-walls 

 that Spirulina bears to Arthrospira; but in neither Phormidiuni nor 

 Oscillatoria has this been considered sufficient to warrant generic 

 distinction. The genus Arthrospira would doubtless never have been 

 established if the real character of Spirulina had been known at the 

 time. 



A very close relationship exists between Spirulina and Arthro- 

 spira, on one hand, and a group of species of Lynghya on the other, 

 as regards the spirally twisting character. Moebius (1889, p. 312) 

 described a new genus from Brazil which he called Spirocoleus Lager- 

 heimii, concerning which he say "Diese Alge, fiir welche ich eine neue 

 Gattung aufstellen muss, steht zu Spirulina in demselben Verhiiltniss 

 wie Lynghya Ag. zu Oscillaria Bosc, indem die schraubenformig 

 gedrehten Fiiden mit einer deutlichen Scheide versehen sind. " 

 Gomont transferred Moebius' species to Lynghya, and Forti (1907, 



