250 FRESH-WATER ALG^E OF THE UNITED STATES. 



SECTION II. SCYTONEME^E. 



Filaments not terminating in a hair-point, enclosed in a sheath, 

 branched ; cells divide transversely only ; ramifications produced 

 by the deviation of the trichome and emerging through the 

 sheath ; usually furnished with heterocysts. 



Genus 86, SCYTONEMA, Ag. 



Sheath enclosing a single trichome. Eamification usually 

 twin, produced by a fold of the trichome which ruptures outside 

 of the sheath and gives origin, usually to two, but sometimes 

 only one branch, at right angles. Heterocysts scattered here 

 and there in the trichome, without any evident relation to the 

 ramifications. The branched filaments produce interwoven mats 

 of greater or less extent. 



Propagation by microgonidia, which arise from the cells of 

 the trichome after having passed out of the older sheaths. The 

 microgonidia increase in size after separation from the maternal 

 cell ; the contents divide and redivide producing larger forms 

 (Microcystis), which again divide producing others of like kind. 

 Finally the small cells of a cyst arrange themselves in series, 

 JTostoc-like, and these encysting in the maternal tegument 

 ( Hormosiphon-forms ) reproduce the original type of the species. 



In the process of the propagation of Scytonema species, perhaps 

 not always strictly in accord with the successive steps in the plan 

 briefly described, occur various forms of so-called unicellular 



t/ 



plants, heretofore known by such generic names as Microcystis, 

 Polycystis, Gloeocapsa, Gloeothece, Hormosiphon, Nostoc. and others ; 

 all the forms under these heads are doubtless merely intermedi- 

 ate polymorphic conditions of development in higher alga3 life. 

 Different observations made in the study of different species, 

 will be illustrated with the description of the species. Some of 

 these were given to the public in the Microscopic Journal, in 1878 

 and 1879. One article, U A Nostoc the matrix of Scytonema^' an- 

 other, "Dubious Character of some of the Genera of Fresh-water 

 Algce," etc. 



SCYTONEMA TOLYPOTRICHOIDES, Kg. 



Thallus turf-like ; filaments much interwoven, mucous, 

 dark olive or brown. Filaments long; ^semZo-branchlets 

 mostly geminate. Thinner, even to one-half that of the stem, 

 usually close at the base ; cells two or three to the diameter, 

 sometimes constricted at the joints, aeruginous when young ; 

 later, brown. Sheath primarily thin, but later, thick and 



