RHODOPHYCEAE 



217 



haps be added, since it is always found on one particular host, 

 Ascophyllum. The order is principally marine, but there are a few 

 fresh-water representatives, e.g. Batrachospermum, Lemanea and 

 Hildefibrandtia, which are usually confined to fast-flowing streams 

 where there is an abundance of aeration. 



Proto-florideae 



Bangiaceae: Porphyridium cruentum (porphyridium, diminutive of 

 purple dye; cruentum, blood red). Fig. 142. 



This alga has had an extremely varied history, having been 

 placed at various times in both the Palmellaceae and Schizogonia- 

 ceae of the Chlorophyceae, near to Aphanocapsa in the Cyano- 

 phyceae, and among the Bangiaceae in the Rhodophyceae where 



ABC D E 







•'F 



Fig. 142. Porphyridium cruentmn. A-E, stages in nuclear and cell division 

 ( X 1280). F, cells connected by stalks after division ( x 1280). (After Zirkle and 

 Lew^is.) 



it finds a home at present. The single cells are united into a one- 

 layered, gelatinous colony of a blood red colour which is found on 

 the soil. Cell divisions take place in all directions, and when a 

 cell divides the sheath elongates to form a kind of stalk which 

 eventually ruptures. So far no form of sexual reproduction has 

 been observed. In each cell there is one large chromatophore with 

 cyanophycin granules around the periphery and also a central 

 nuclear-like body, composed largely of anabaenin, which undergoes 

 a primitive form of mitosis at cell division. Whether this alga 

 represents a primitive form or else is a much-reduced type cannot 

 at present be determined. 



