440 PROTOPHYTA 



Coleodesmium Bzi. appears to be one of the simplest forms of the 

 Scytonemaceae. No pseudo-ramuh are formed ; the filaments increase 

 by fission only, and a number remain united in a bundle within a 

 common envelope. Mazcea (Bornet, Bull. Soc. Bot. France, 1881, 

 p. 287) is, on the other hand, a genus in which the development is 

 carried to its highest point. The gelatinous ' fronds ' are about 25 mm. 

 in diameter ; the heterocysts are borne on pedicels consisting of from 

 one to three cells, and the whole appearance is that of a Rivularia, the 

 filaments being immersed in a homogeneous jelly, and spreading from 

 a central spot. No distinct sheath has been observed, nor any resting- 

 spores or hormogones. In Petalonema Berk, the mucilaginous sheath 

 forms a kind of broad coloured wing to the filament. Mastigocoleus 

 (Lagerheim, Notarisia, 1886, p. 65) is a marine genus growing attached 

 to the shells of molluscs, with both terminal and lateral heterocysts ; 

 the filament sometimes ends in a hair, as in the Rivulariaceae. Drilo- 

 siphon Julianus Ktz., frequent on the damp walls of greenhouses, is 

 characterised by an outer calcareous sheath, and is a remarkably pleo- 

 morphic organism. According to Zukal (Oestcrr. Bot. Zeitschr., 1883, 

 p. 73) it forms two kinds of hormogones, and displays a kind of alterna- 

 tion of generations. The ordinary hormogones produce only more and 

 more slender filaments, which gradually assume a moniliform character, 

 and are then known as Nostoc parietinum (Rabh.). Eventually the 

 cells of these nostoc-filaments separate, and assume the character of 

 an Aphanocapsa, or, in other cases, become the organism known as 

 Gloeocapsa fenestralis (Ktz.) ; or very slender filaments are produced, 

 constituting the Leptothrix parasitica and muralis (Ktz.), which forms 

 are distinctly connected genetically with Drilosiphon. These leptothrix- 

 filaments may again break up into vibrio- and bacillus-forms. The 

 second kind of hormogone has a fusiform shape, and consists usually of 

 from four to eight pseudocysts. It may remain dotmant for a time, 

 and, on germinating, reproduces the ordinary thick filaments. 



Wille (Ber. Deutsch. Bot. Gesell., 1883, p. 243) and Scott (Journ. 

 Linn. Soc, xxiv., 1887, p. 188) have determined the presence of a cell- 

 nucleus in Tolypothrix. Wille states also that in Stigonema compactum 

 (Kirch.) the necklace- like pseudocysts are in direct communication 

 with one another through perforations in their cell-walls. When this 

 species passes into the Gloeocapsa-condition, the perforations disappear, 

 in consequence of the gelification of the common sheath, and the 

 separate cells then carry on their existence as distinct individuals. 

 Under the name Tolypothrix amphibia, Zopf (Ber. Deutsch. Bot. 

 Gesell., 1883, p. 319) describes an organism having both an aerial and 

 an aquatic form, the latter being a true Tolypothrix with its filaments 



