T4 



CYANOPHYCEAE 



frequently absent. The apical cell is sometimes provided with 

 a cap or calyptra. There are a number of common species, 

 O. limosa being frequently found on very damp soils, wet stones 

 and omer moist places. 



OsciLLATORiACEAE : Lytigbya (after H. C. Lyngbye). Fig. 8. 



This genus differs from Oscillatoria in the presence of a sheath of 

 variable thickness and colour, the character of which is largely 



.12//. 



Pig. 8. Lynghya aestuarii. A, apex. B, C, portions of threads. (After Chapman.) 



dependent upon the environment. The plants are either attached or 

 free-floating. When the hormogones and trichomes escape from the 

 sheaths it is frequently very difficult to determine whether they 

 belong to Oscillatoria^ Lynghya^ or some other similar genus. 



ScYTONEMATACEAE : Scytonema (scyto, leather ; nema, thread). Fig. 9. 



The threads differ from those of the preceding genus in the 

 presence of heterocysts. The filaments (trichome and sheath) have a 

 base and apex, and the false branches arise either between two 

 heterocysts or else adjoining a heterocyst. The intercalary growth 

 results in strong pressure being applied to the sheath, which finally 

 ruptures so that the trichome forms a loop outside (fig. 9 A-C). 

 Further growth causes this loop to break, thus producing twin 

 branches, one or both of which may subsequently proceed to 

 additional growth, the branch sheaths extending back into the 



