MYXOPHYCOPHYTA 279 



which has been shown to be a 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. Anabaena. This is usually composed 

 of a pectic material, although in the Scytonemataceae it may be 

 made of cellulose. There is considerable variation in the composi- 

 tion of the different cell sheaths, and the amount of material laid 

 down frequently depends upon the external environment. Colour 

 of the sheaths is modified by acidity of the medium and also by 

 fungal parasitization. In any case the secretion of pectins by these 

 plants is regarded as a primitive characteristic. In the unicellular 

 and filamentous forms this material is produced at the periphery 

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

 in the cytoplasm. Protoplasmic connexions between mature cells 

 are known in one order, the Stigonematales (see p. 291). 



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

 reproductive organs ; there are no sexual organs and no motile re- 

 productive bodies have ever been observed. It has been suggested 

 that the lack of sexuahty can be correlated with the complete 

 absence of sterols, 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 

 some other types the cell contents give rise to a number of non- 

 motile bodies that are termed endospores (gonidia in the past) (Fig. 

 164). All stages from simple binary fission to endospores can be 

 found : 



(a) binary fission, e.g. Chroococcus turgidus; 



(b) quadrants and octants formed, e.g. C. varians; 



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

 is retention of individual sheaths, e.g. Gloeocapsa spp. (These 

 are sometimes called nannocytes or gonidia). 



(d) the same without individual sheaths, e.g. Chroococcus macro- 

 coccus ; 



(e) abstricted endospores, e.g. Entophysalis. 



Many of the filamentous forms produce specialized cells known 

 as heterocysts. These are enlarged cells which possess thickened 

 walls, particularly at their poles, and they usually occur singly, 

 though occasionally they may be formed in rows. They develop 

 from an ordinary vegetative cell, but during development they are 

 said to remain in protoplasmic communication with neighbouring 



