Nostocacede 



43 



In Plectonema the filaments are falsely branched, and in Symploca they form compact, 

 erect, pointed tufts arising from a flat stratum. In some species of 

 Schizothrix the sheaths are strongly lamellose and brilliantly 

 coloured, but it is in the African genus Polychlamydum that the 

 sheath exhibits its maximum differentiation. 



The most extraordinary member of this family is Gomontiella sub- 

 tubulosa (fig. 20) found by Teodoresco in Roumania. The flattened, 

 rolled filaments are unique among the Hormogonese. 



Fam. Nostocacese. The filaments are simple, with delicate 

 gelatinous sheaths. The trichomes consist of a single series of 

 uniform and often torulose cells, amongst which are few or many 

 heterocysts. In a few forms the sheaths are distinct, but in others 

 they become confluent to form a mass of jelly enclosing a large 

 number of trichomes. This jelly attains its maximum development 

 in the genus Nostoc (fig. 31) in which the shape of the colony is 

 largely determined by the toughness of the gelatinous mass. 

 Thousands of trichomes occur in such a colony, and they are 

 generally contorted and densely crowded. In Anabsena the trichomes 

 are not associated to form definite colonies, as the gelatinous sheaths 

 are much more fleeting and delicate, although they may be aggregated 

 in masses. The trichomes may be rigid and straight or variously 

 twisted and contorted (as in so many of the plankton-species). 



Aphanizomenon occurs free-floating in the waters of pools and 

 lakes, and the trichomes are often agglutinated to form spindle- 

 shaped bundles. 



In Cylindrospermum there is only one terminal heterocyst (fig. 32 

 E (7) ; and in Nodularia the heterocysts are rather numerous and 

 occur at regular distances along the trie-home (fig. 32 H\ 



Fig. 30. Arthrospira 

 platensis (Nordst.) 

 Gom. Single fila- 

 ment, x 520. 



Fig. 31. A and B, Nostoc Linckia Bornet; 

 A, nat. size; B, small portion of thallus, 

 x 340. C, N. cceruleum Lyngbye, nat. size. 



Fig. 32. A D, Anabsena insequalis (Kiitz.) 

 Born. & Flah. E G, Cylindrospermum 

 stagnate (Kiitz.) Born. & Flah. H, Nodu- 

 laria sphserocarpa Born & Flah. (All x 

 480.) h, heterocyst; sp, resting-spore. 



