LICHENS 113 



by repeated exposures to burning sunshine. A pro- 

 tracted period of drought does a Lichen no more dis- 

 service than long submergence in water. And Lichens 

 revel in light and air to a degree that would be the 

 death of the mycelium of a Fungus. 



The Alga-Fungus partnership of a Lichen is in all 

 respects remarkable. The Alga is involved in the 

 mycelium of the Fungus; it is protected by it, and it 

 enables the delicate organism to exist in situations 

 where it certainly could not live by itself. The Alga 

 owes more than protection to its Fungus-partner, for the 

 latter supplies it with water and salts in solution. In 

 return for these services, the Alga manufactures organic 

 food by means of its chlorophyll, using carbon dioxide 

 from the air, and grants the Fungus a supply sufficient 

 for its needs. Thus, the Fungus-partner, which can 

 live only on ready-prepared organic matter, is benefited 

 by its relations with the Alga-partner. One can hardly 

 say that the Fungus lives parasitically on the Alga, 

 although it feeds at its expense. It is a case of service- 

 for-service, and may be classed as symbiosis — i.e., 

 mutual living. But in the Lichen partnership the Fungus 

 seems to be predominant. Generally speaking, it con- 

 trols the Alga and takes the lead ; it also determines the 

 form of the Lichen. 



Some very interesting experiments have been carried 

 out in recent years in relation to Lichens. The partners 

 have been separated, and it has been demonstrated 

 that the Alga-partner, freed from the embrace of its 

 Fungal symbiont, can live and thrive independently, if 

 provided with suitable environment. The spores of the 

 Fungus-partner have also been reared in specially pre- 



15 



