10 



CYANOPHYCEAE 



Characium (figs. 5, 26), Chroococcus and Pleurococcus (figs. 3, 44), 

 Lynghya and Hormidium. 



As a group, the plants are extremely widely distributed over the 

 face of the earth under all sorts of conditions, frequently occurring 

 in places where no other vegetation can exist, e.g. hot thermal 

 springs. Their presence in great abundance in the plankton often 

 colours the water and is responsible for the phenomenon known as 

 water bloom, whilst they may also form a large constituent of the 

 soil algae (cf. Chapter x). 



The class is divided into two orders : 



CoccoGONALES, which reproduce by means of single cells. 

 HoRMOGONALES, which reproduce by groups of cells or hormo- 

 gones. 



COCCOGONALES 



Curoococcaceae: Microcystis {micro, small; cystis, bladder). Fig. 2. 



The thallus, which is free-floating, varies much in shape and 

 contains a mass of single spherical cells, but the sheaths of the 



Fig. 2. Microcystis aeruginosa. A, colony. B, portion of a colony ( x 750). 

 (A, after Geitler; B, after Tilden.) 



individual cells are confluent with the colonial envelope. Repro- 

 duction of the single cells takes place by means of fission in three 

 planes, whilst reproduction of the colony is through successive 



