INTRODUCTION 9 



may affect the shape of colonies of Microcystis and Chroococcus 

 turgidus and determines the size of Rivularia haematites (cf. p. 337). 

 Certain lines of morphological development have been followed by 

 the group and the various lines may be depicted schematically as 

 follows (cf. also fig. i): 



Plate-like 

 colonies 



Single floating unicell 



" / 



Regular spherical or^ 

 cubical colonies 



'Single attached 



unicell 



Irregular 

 spherical 

 colonies 



/ 



PLANKTONIC / 

 / 



/ V 



/ Attached single Aggregated 



filament ^ filaments 



Filament with 

 false branches 



True branched 

 filament 



BENTHIC 



10/Z 



A B C D E F G 



Fig. I. Types of trichoma in the Cyanophyceae. A, Hapalosiphon arboreus. 

 B, Calothrix parietaria. C, Schizothrix purpurascens. D, Oscillatoria margari- 

 tifera. E, O. proboscidea. F, O. irrigua. G, Arthrospira jenneri. (After Crow.) 



As rnay perhaps be expected from a primitive group there is 

 evidence of homoplastic or parallel development when compared 

 with plants from other primitive groups, especially the Chloro- 

 phyceae. Homoplasy can be seen in Gloeothece and Gloeocystis, 

 Merismopedia elegans and Prasiola (figs. 4, 40), Chamaesiphon and 



