142 INTRODUCTION TO CRYPTOGAMIC BOTANY. 



of Nostocldnece are for the most part more or less shapeless 

 and irregular, where they do not assume a globular or elliptic 

 form. But I have a species with a linear dichotomous flagelli- 

 form frond, Avhich creeps over naked albuminous soil of the 

 San Pedro, in Texas, where it was found by Mr. Charles 

 Wright. It is Nostoc flacjelliforme, Berk, and Curt., No. 3809. 

 The substance is firm, but there is no external envelope. One 

 of the most curious genera is Monormia, which forms floating 

 masses of jelly on the top of brackish water. The necklaces 

 are of immense length, and, together with the jelly which 

 surrounds them, form branched masses, which wave with the 

 slightest motion of the fluid. In general, the species prefer 

 fresh or brackish water, a few only flourish when fully exposed 

 to the action of the sea. Floating masses are likewise formed 

 occasionally in large ponds or lakes, which assume in conse- 

 quence a delicate green tinge. Mr. Thomson has described 

 a curious species which he calls Sphcerozyga sjyiralis, as pro- 

 ducing such an effect, together with an allied Alga which 

 assumes almost a membranous form, to which Morren has given 

 the name of A]ylianizomenon incurvum* The lake at 

 Glaslough (sea-green lake) in Monaghan owes its colour, 

 according to Drummond, to Oscillatoria cerugescens.f 



8. Oscillatoria, Ag. 



Threads articulated, simple, or branched by the diversion 

 of a metamorphosed cell ; more rarely by the protrusion of 

 the central cord, consequent on the rupture of its outer coat. 

 Cells generally very narrow. Propagation by minute zoospores, 

 where the mode of fructification has been ascertained. 



116. This division is strictly parallel with Confervacece and 

 Nostochinece, and the species rise gi-adually in each, from simple 

 to compound forms. Conferva leading to Chcetophora, Ana- 

 baina to Nostoc, and Calothrix to Rivularia. In deference 

 to established usage, I had determined to keep the two latter 

 distinct, but on mature reflection, it did not appear that I 



* Ann. of Nat. Hist. vol. i„ p. 83. 



t Annals of Nat. Hist. vol. i. p. 1. The connection of the words glas, 

 glass, glastum, and vitrum, is very curious. Vitrum means glass, sea- 

 green, and woad, and we have the same meanings in the three first-men- 

 tioned words. Perhaps the word glastiim was invented by Caesar, 



