THE LICHENS 255 



lichens have sac fruits closely resembling those of the cup 

 fungi. There is one small group of tropical lichens whose 

 fungal portions are basidia fungi, or Basidiomycctes, and not 

 Ascomycetes. 



The algal portions of a lichen may be scattered, but in some 

 types they are arranged in definite layers. The kinds of algae 

 differ in various lichens. Some of them are unicellular green 

 forms, evidently of the genus Pleurococcus (Plate V, C). Most 

 of the species belong to the blue-green algae, one-celled forms 

 being commonest, though some complicated filamentous types, 

 such as Nostoc, are found in certain lichens. One curious lichen, 

 which grows 011 the leaves of the coffee plant, contains a species 

 of Coleochccte (Sec. 222). 



The development of the present clear understanding of the 

 composite, or fungal and algal, nature of lichens makes one of 

 the most interesting chapters in the history of botanical science. 

 First came the recognition of the colorless portion of the lichen 

 as fungal and the colored elements as algal in character. Then 

 these portions were separated and cultivated independently of 

 one another, which proved that they remained respectively algae 

 and fungi ; for example, the lichen spore never developed into 

 algal cells, but only into fungal filaments. Finally, lichens were 

 created by bringing germinating spores in contact with wild 

 algae of a suitable kind, and these lichens have in some cases 

 lived for many months, finally developing typical lichen sac 

 fruits (ascocarps), thus completing the life history. 



The lichens are perhaps chiefly interesting for the relations 

 which the algae and fungi bear to one another. When two 

 organisms live in intimate physiological association, so that 

 both receive some benefit from the partnership, the condition 

 is called symbiosis (meaning a living together). The mycorrhiza 

 relationship (Sec. 278) is an excellent illustration of symbiosis. 

 It is not easy to analyze critically the relationships between 

 the algae and fungi in a lichen association, but some points 

 seem clear. 



