42 



Myxophycese 



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 Oscillatoriace* and Camptotrichacere they are 

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

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

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

 and more rarely by resting-spores. 



A. Psilonemateee. Trie-homes 1 cylindrical, sometimes narrowed at the 

 extremities, but never gradually attenuated towards the extremities. 



A 



D 



Fig. 28. A, Lynybya major Menegh. B and C, 

 L. terugineo-ccernlea (Kiitz.) Gom. D, Phor- 

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

 tenue (Menegh.) Gom. All x 460. 



Fig. 29. A, Oscillatoria limosa Ag. 

 B, O. irrigua Kiitz. C, 0. tennis 

 Ag. D, O. spleiidida Grev. E, 

 0. acuminata Gom. All x 4GO. 



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

 Psilonernftteas 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 

 conidal 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 Spirutina, 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 Microcolcus and Schizothrix (fig. 3), several 

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

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

 Lyngbyese). 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 (7) 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. 



