42 



MyxopliycesB 



enclosed within a sheath, which varies from a hyaline, indistinct envelope to 

 a tough, coloured investment. In some instances several rows of cells occur 

 within the same sheath. In most of the families heterocysts occur in more 

 or less abundance, but in the Oscillatoriacese and Camptotrichaceae they are 

 absent. In some genera the filaments are branched, and in others a peculiar 

 type of false branching occurs. Most of the Algre in this group occur 

 attached to a substratum. The plants multiply by means of hormogones, 

 and more rarely by resting- spores. 



A. Psilonematese. Trichomes 1 cylindrical, sometimes narrowed at the 

 extremities, but never gradually attenuated towards the extremities. 



:..._j 



,.,..i 



p 



feiiij 



Fig. 28. A, Lyntjltya major Menegh. B and G, 

 L. serugineo-ccerulea (Kiitz.) Goru. D, Phor- 

 midium molle (Kiitz.) Gom. E and F, Ph. 

 tenue (Menegh.) Gom. All x 460. 



Fig. 29. A, Oscillatoria limosa Ag. 

 B, 0. irriyua Kiitz. C, 0. tennis 

 Ag. D, O. splendida Grev. , 

 0. acuminata Gom. All x 460. 



Fam. Oscillatoriaceae. A large family distinguished from the other families of the 

 Psilonematese by the absence of heterocysts. The trichomes consist of cylindrical or disc- 

 shaped cells, often somewhat modified at the extremities, the apical cell being frequently 

 conical and not uncommonly provided with a thickened calyptra. The cells may be so 

 compactly joined that the trichomes are exactly cylindrical, or there may be a slight 

 constriction between each pair of adjacent cells. In the genera Spirulina, Arthrospira 

 (fig. 30), and in some species of Lyngbya (fig. 19 F H} the filaments are twisted into 

 a regular spiral. In certain genera, such as Microcoleus and Schizothrix (fig. 3), several 

 trichomes are included within a single sheath (sub-fam. Vaginariese), whereas in most of 

 the other genera there is never more than one trichome within a sheath (sub-fam. 

 Lyngbyeee). The most abundant genus is Oscillatoria (fig. 29), in which the sheath is so 

 close and delicate as to be easily overlooked. In Lyngbya (fig. 28 A C) the trichomes 

 possess a strong sheath which is often lamellose. Phormidium is almost as abundant as 

 Oscillatoria and occupies a position half-way between that genus and Lyngbya. It is 

 characterized by the sheaths becoming agglutinated or else entirely diffluent. 



1 The term ' trichome ' is universally applied to the filament of a Blue-green Alga without its 

 sheath, the term 'filament' being retained for the trichome together with its sheath. 



