296 



PHYSIOLOGY, SYMBIOSIS, ETC. 



impossible to grow the blue-green alga separately and it may thus 

 have lost its power of independent growth. Geosiphon, which is 

 variously regarded as a Siphonaceous alga or as a Phycomycete, 

 possesses small colonies of Nostoc enclosed in the colourless pear- 

 shaped vesicles that arise from an underground weft of rhizoidal 

 threads. Reproduction by the formation of new vesicles is said to 

 occur only in the presence of the Nostoc. The presence of chitinous 

 material in the vesicular wall suggests a fungal nature for Geosiphon^ 

 the vesicles perhaps being galls that are formed on the threads as a 

 result of the presence of the alga. 



J^ 



«^fcM5^ 



en 



mg 



Fig. 189. Symbiosis. Zooxanthellae in the tissues of a coelenterate, Pocillopora 

 bulbosa ( X 375). ec = ectoderm, ew = endodemiis, ^2: = dead zooxanthellae, 

 fg = fat globule, w^ = glands, 5m = structureless lamella, s' = zooxanthellae. 

 (After Yonge.) 



The principal genera taking part in lichen synthesis are Nostoc^ 

 ScytonemUy Cystococcus, Gloeocapsa and Trentepohlia. To what 

 extent the lichen body is a case of true symbiosis is a problem that is 

 still not wholly settled: under normal conditions it is probably a 

 real symbiotic relationship but under abnormal conditions the 

 fungus may become a parasite and devour the algal component. The 

 green bodies which are found associated with the cells of Coelenter- 

 ates and Radiolarians are usually placed in what may be called a 

 *'form" genus, Zooxanthella (cf. fig. 189). Most of the species 

 belong to the Cryptophyceae, but in certain of the Coelenterata the 

 motile phases of some of the algae which have been discovered 

 suggest an affinity to the Dinophyceae, whilst Chlorella (Chloro- 

 coccales) is also said to behave as a symbiont of this type. The 



