SIROSIPHON. 267 



Genus 90, PETALONEMA, Berk. 



Trichomes enclosed in a very broad, striate membranous 

 sheath, which forms a transparent layer, resembling a hyaline 

 wing. Internal thread as in Scytonema. 



PETALONEMA ALATUM, Berk. 



Forming a thin brown stratum. Trichomes small, a few 

 lines only in length, winged, obtuse, with numerous striae ; 

 when taken in conjunction with the wings or membranous 

 expansions, they are linear and plane. Each wing is about 

 thrice the breadth of the proper filament, bright yellow 

 near the trichome, gradually fading to the colorless margins ; 

 under favorable light numerous series of transverse lines 

 or folds are observable. 



Diameter of trichomes, about 10 f-i ; with sheath, 50-120 //. 



Syn. Arihrosiplion alatus, Bab. ; Arth. GreviUei, Kg.; Oscilla- 

 toria alata, Grev. 



The only locality hitherto discovered for this plant, is on 

 the high cliff, near the Cave of the Winds, Niagara Falls. 

 'Twas found there twenty-five years since and it may be 

 gathered there to-day. 



Plate CLXXXVIII, figs. 15, 16, from specimens of recent 

 date. 



SECTION III. SIROSIPHONE^E. 



Filaments destitute of a hair-point ; trichomes enclosed in a 

 sheath, profusely branched. Division of cells takes place in a 

 line parallel with the sides as well as transversely ; filaments 

 often contain three, four, or more series of cells. 



Genus 91, SIBOSIPHON, Kg. 



Cells of the filaments one, two to many seriate, in consequence 

 of their lateral division or multiplication. Younger forms have 

 one or two series, older ones often have six to ten series. Sheaths 

 large. Cells surrounded with a distinct membrane, very promi- 

 nent in the older filaments. 



"Hormogons originating in lateral branchlets formed of a 

 single row of cells." -Codke. 



Propagation by means of microgonidia, originating in the 

 single cells of the lateral branchlets after their sliding out of the 

 sheath. The microgonidia develop into macrogonidia and these 



