Helotism 



37 



Some of the Myxophycese are regularly found associated with Fungi to 

 form the dual organisms known as Lichens. 

 The Alga3 which have thus lost their in- 

 dividuality become considerably modified 

 and generally lose almost all traces of their 

 original specific characters. It has been 

 customary to regard the thallus of the 

 lichen as a case of commensal symbiosis, 

 but in the great majority of Lichens the 

 Alga is a very subordinate constituent, 

 the fungus completely dominating it, even 

 to the extent of curtailing and absorbing 

 any undue increase in the algal cells. It 

 seems to be much more in agreement with 

 facts to look upon the Algse as existing 

 under conditions of beneficent slavery ! 

 The lichen-thallus thus provides an illus- 

 tration of helotism, which is a form of 

 symbiosis intermediate in character between 

 commensalism and parasitism. The fungal 

 hyphae are partially parasitic on the algal 

 cells, but at the same time they supply them 

 with various excretory products, mineral 

 salts, and water. It is true that inclusion 



within the lichen-thallus permits of the existence of the algal cells in situa- 

 tions in which they could not maintain themselves, but their multiplication 

 is kept within restricted limits by the undoubted parasitism of the fungus. 



The following list shows some of the genera of Lichens which contain certain of the 

 Blue-green Algse : 



Glceocapsa occurs in the thallus of ANEMA Nyl., COLLEMOPSIDIUM Nyl., CRYPTOTHELE 

 (Th. Fr.) Forss., FORSSELLIA Zahlbr., GONOHYMENIA Stnr., JENMANIA Wacht., LEPTOGIOPSIS 

 Nyl., PAULTA Fee, PECCANIA (Massal.) Forss., PHLCEOPECCANIA Stnr., PHYLLISCIDIUM 

 Forss., PSOROTRICHIA (Massal.) Forss., PYRENOPSIS (Nyl.) Forss., and SYNALISSA E. Fr. 



Chroococcus in PHYLLISCIUM Nyl. and PYRENOPSIDIUM (Nyl.) Forss. 



Nostoc in ARCTOMIA Th. Fr., COLLEMA (Hill.) Zahlbr., DICHODIUM Nyl., HASSEA 

 Zahlbr., HOMOTHECIUM Mont., HYDROTHYREA Buss., KOERBERIA Mass., LECIOPHYSMA Th. 

 Fr., LEMMOPSIS (Wainio) Zahlbr., LEPIDOCOLLEMA Wainio, LEPROCOLLEMA Wainio, LEP- 

 TOGIUM (Ach.)S. Gray, LOBARIA (Schreb.) Hue [Section LOBARINA (Nyl.) Hue], NEPHROMA 

 Ach. [in part], PANNARIA Del., PARMELIELLA Mull. Arg., PELTIGERA Willd., PHYSMA 

 (Mass.) Zahlbr., PYRENIDIUM Nyl., PYRENOCOLLEMA Reinke, SCHIZOMA Nyl., SOLORINA 

 Ach. [in part], and STICTA Schreb. [Section STICTINA (Nyl.) Hue], 



Scytonetna in COCCOCARPIA Pers., DICTYONEMA Ag., ERIODERMA Fee, HEPPIA Nag., 

 LEPTODENDRISCUM Wainio, MASSALONGIA Koerb., PETRACTIS E. Fr., PLACOTHELIUM (Ach.) 

 Harm., POROCYPHUS Koerb., STEREOCAULON Schreb. and THERMUTIS E. Fr. 



Fig. 23. Richelia intracellularis Jobs. 

 Schmidt. A, one extremity of cell 

 of Rhizosolenia styliformis Btw. con- 

 taining four filaments of the Richelia; 

 B and C, two isolated filaments 

 showing the basal position of the 

 heterocyst. x about 450 (after 

 Jobs. Schmidt). 



