Fig. i6i Microcystis aeruginosa. A, colony. B, portion of a colony 

 ( X 750)- (Aj after Geitler; B, after Tilden.) 



Chroococcaceae : Microcystis (micro, small; cystis, bladder). Fig. 

 161 

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

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

 dividual cells are confluent with the colonial envelope. Reproduc- 

 tion of individual cells takes place by means of fission in three 

 planes, whilst reproduction of the colony is through successive 

 disintegrations, each portion growing into a new colony. The shape 

 of the colony is primarily determined by the environmental condi- 

 tions, and it can be changed by altering the environment artificially. 

 M. aeruginosa is a very common waterbloom alga, whilst M. toxica, 

 a new species from South Africa, has appeared in sufficient quan- 

 tity to cause the death of thousands of cattle. 



* Chroococcaceae : Chroococcus {Chroo, colour; cocct^s, berry). 

 Fig. 162 

 The cells are single or else united into spherical or flattened 

 colonies each containing a small number of cells, the individual 

 sheaths being either homogeneous or lamellated. Plants grown in 

 water produce a concentric envelope but when grown on damp soil 

 the sheath is often asymmetrical. The outer integument is not very 

 gelatinous and indeed is quite thin in some species. A study of the 



283 



