CYANOPHYCEjE 441 



enclosed in sheaths and breaking up into hormogones, from which is 

 derived the aerial form with the nature of a Chroococcus, and dividing 

 in three directions. 



Rabenhorst and Cooke regard Stigonema, and the latter authority 

 also Hapalosiphon, as genera of lichens ; and Bornet and Flahault 

 state that several organisms described as species of Sirosiphon and 

 Stigonema are really lichens in a more or less advanced stage of deve- 

 lopment. Hansgirg (Oesterr. Bot. Zeitschr., 1884, and Bot. Centralbl., 

 xxii. & xxiii., 1885) considers the genera placed under ScytonemeK to be 

 the highest forms of development of various organisms hitherto mostly 

 placed under Oscillariacese. In the same way, from Tolypothrix and 

 Scytonema may arise, by further development, the corresponding forms 

 of Hapalosiphon, Mastigocladus, Sirosiphon, Stigonema, Fischera, and 

 other genera usually placed under Sirosiphonese. 



With the exception of Mastigocoleus, the Scytonemacese are found 

 only in fresh water, in bog-pools, or very commonly on wet rocks or 

 trunks of trees, or among moss. They may form mats of considerable 

 thickness, but the individual filaments, including the sheath, seldom 

 exceed o'04-o-05 mm. in thickness. Several Scytonemacese are known 

 to enter into the composition of lichens (see fig. 279 d.) 



Literature. 



Bornet — Notes Algol., fasc. I, 1876, pp. iv.-v. ; fasc. 2, 1880, pp. 135-156. 

 Bornet and Flahault— Ann. Sc. Nat., v., 1887, p. 51. 



Order 4. — Oscillariace^e (including ChamjEsiphonace^). 



The Oscillariaceae or Oscillatorieae, in which the Lyngbyese are also 

 included, consist of delicate blue-green threads, occurring singly or in 

 large floating masses in fresh running, or more abundantly in stagnant, 

 less often in salt, water. The filaments are cylindrical and unbranched, 

 straight, or (Oscillaria princeps, Vauch.) with the terminal portion laent 

 at an obtuse angle with the rest of the filament ; in Spirulina' (Lk.) the 

 whole filament is coiled in a corkscrew-like manner. The filament is 

 divided by very delicate transverse septa into disc-shaped pseudocysts ; 

 there is no differentiation between the two extremities. The cell-wall has 

 the property of transforming its outer layers into copious mucilage, which 

 forms a gelatinous sheath investing either single filaments, as in Lyngbya 

 (Ag.)and Symploca (Ktz.), or a number of filaments, as in Inactis (Ktz.) 

 and Microcoleus (Desm.). In most species of Oscillaria (Bosc.) and 

 Spirulina a distinct sheath is either wanting or it is extremely thin and 

 delicate, but the filaments are often imbedded in structureless jelly. 



