INTRODUCTION 7 



modified plasmatic membrane. In addition there is an outer cell 

 sheath which may surround the whole cell, e.g. Chroococcus, or 

 form a cylindrical sheath, e.g. Oscillatoria, or an interrupted sheath, 

 e.g. Anahaena. This is usually composed of a pectic material, 

 although in the Scytonemataceae it may be made of cellulose. There 

 is considerable variation in the composition of the different cell 

 sheaths, and the amount of material laid down frequently depends 

 upon the external environment. In any case the secretion of pectins 

 by these plants is regarded as a primitive characteristic. In the 

 unicellular forms this material is produced at the periphery of the 

 cell, whilst in a few, e.g. Chroococcus tiirgidus, it accumulates in the 

 cytoplasm. Protoplasmic connexions between mature cells have 

 been recorded for one genus, Stigone?na. 



The group is characterized by a general absence of well-marked 

 reproductive organs; there are no sexual organs and no motile 

 reproductive bodies have ever been observed. It has recently 

 been suggested that the lack of sexuality can be correlated with the 

 absence of sterols in the group, an hypothesis that might well repay 

 further study. The coccoid forms (spherical cells) multiply by cell 

 division which takes place by means of a progressive constriction, 

 whilst in other types the cell contents divide up to give a number of 

 non-motile bodies that are termed gonidia (fig. 5). Crow (1922) 

 has pointed out that all stages from simple binary fission to gonidia 

 can be found : 



[a) Binary fission, e.g. Chroococcus turgidus. 



[b) Quadrants and octants formed, e.g. C. varius. 



(c) Numerous small daughter cells are produced in which there 

 is a retention of individual sheaths, e.g. Gloeocapsa sp. and variants 

 of Chroococcus macrococcus. 



(d) The same without individual sheaths, e.g. C. macrococcus, 

 Gloeocapsa crepidinum. 



{e) x\bstricted gonidia, e.g. Chamaesiphon. 



Many of the filamentous forms produce specialized cells known 

 as heterocysts. These are enlarged cells which possess thickened 

 walls, and they usually occur singly though occasionally they may 

 be formed in rows. They develop from an ordinary vegetative cell, 

 but during development they remain in protoplasmic communica- 

 tion with neighbouring cells and if they contain contents, as they 



