428 THE ALGAE 



apparently cannot live unless infected with the alga, whilst under 

 certain conditions it also digests the green cells. 



Examples of a looser form of symbiosis (almost a commensalism) 

 are provided by Anabaena cycadearum which hves in the root 

 tubercles of species of Cycas, and Anabaena azollae found in hol- 

 lows in the leaves of the water fern Azolla filiculoides. Species of 

 Nostoc found in the thaUi of the Hverworts Blasia and Anthoceros 

 are probably no more than space parasites obtaining shelter. 



Epiphytism is extremely common among the algae, whilst there 

 are also a number of epizoic forms. One may also find endophytic 

 species, such as Schmitziella endophloea in Cladophora pellucida and 

 S. cladophorae in C.feredayae, and endozoic species, such as Rhodo- 

 chorton endozoicum in the sheaths of hydroids. The origin of the 

 symbiotic habit among the algae is probably to be explained as 

 cases of epiphytism in which the relationship between host and epi- 

 phyte became more intimate : similarly the relatively few cases of 

 parasitism probably arose either directly from an epiphytic habit 

 or else passed through the symbiotic phase. Examples of algal para- 

 sites are Notheia anomala^ in the Phaeophyceae, Choreocolax poly- 

 siphoneae, Harveyella mirabilis, the entire genus Jancewskia and 

 Colacolepis in the Rhodophyceae, and Phyllosiphon arisari in the 

 Chlorophyceae. 



^ Hemi-parasite. 



REFERENCES 



Blinks, L. R. (1951). Manual of Phycology, 263-285. Chron. Bot., Wal- 



thanij Mass. 

 Fogg, G. E. (1953). The Metabolism of Algae. Methuen. 

 Keeble, F. and Gamble, F.W. (1907)- Quart. J. Micro, Sci., 5i> 167. 

 Raper, K. (1952). Bot. Rev., 18, 447. 

 Smith, G. M. (195 1). Manual of Phy oology, 231-240. Chron. Bot., Wal- 



tham, Mass. 

 Yonge, C. M. and NichoUs, A. G. (1932). Reports of the Gt. Barrier Reef , 



Exp. J, 135. Brit. Mus. Publ. 



