GENERA OF ALGiE. 3! 



D. In a sheath or a jelly mass (K). 



D. Not sheathed nor in jelly (P). 



E. Branched at almost right angles to the stem; cells in a single se- 



ries .... .... ... Hapalosiphon, 92 



E. Branched irregularly or with twin branches (F). 



F. Cells in single series in young plants, several series when older, 



Sirosiphon, 91 



F. Cells in a single series only (G). 



G. Sheath broad, striate, hyaline, wing-like . . Petalonema, 90 

 G. Sheath not wing-like (H). 



H. Sheaths distinct, not agglutinated to one another (I). 



H. Sheaths agglutinated in erect, wick-like bundles, 



Symphyosiphon, 87 



I. Heterocysts present (J). 



I. " not present; sheath with a single bluish filament, 



Plectonema, 8g 



J. Heterocysts disconnected from the twin branches, Scytonema^ 86 



J. Heterocysts near the usually single branches . Tolypothrix, 88 



K. In a sheath; filaments moniliform, curved . . Nostoc, 93 



K. In a sheath; filaments not moniliform (Q). 



K. In mucilage or jelly mass (L). 



L. Jelly mass spherical; filaments transversely plicate, radiate, 



Glxotrichia 84 



L. Jelly mass hemispherical, filaments radiate . Rivularia, 85 



L. Jelly mass a stratum or indefinite (M). 



M. Filaments erect, attached at base, jelly stratum flat, often 

 incrusted . . . Isactis, 83 



M. Filaments not attached at base, not moniliform, heterocysts ter- 

 minal, single . . ... . Cylindrospermum, 98 



M. Filaments not attached, not moniliform, colorless, heterocysts 

 none . . ... Crenothrix, 99 



M. Filaments moniliform, cells globose or elliptical (N). 



M. " " cells compressed disciform (O). 



N. Heterocysts intercalated in the filaments, 



Anabcsna, 94, or Spharozyga, 95 



O. Heterocysts intercalated at regular intervals, yellow, 



Modularia, 97 



P. Bluish, bluish-green, blackish (T). 



P. Whitish or silvery; in sulphur waters . . . Beggiatoa, 105 



Q. Two or more filaments in each sheath . . Microcoleus, 103 



Q. -One filament only in each sheath (R). 



