386 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1950 



These plants are widely distributed from the Arctic to the Tropics, 

 consisting of thousands of species and innumerable varieties and 

 forms. They have one feature in common that distinguishes them 

 from all other plants. Each of them consists of two different and 

 separate entities living together in such a balanced relationship that 

 they not only form a successful organism but are able to reproduce 

 the unit. One component is a fungus, usually an ascomycete but in 

 a few cases a basidiomycete, whose intertwining, compact hyphae give 

 form to the thallus. The other component consists of a species of 

 green or blue-green algae enmeshed between the hyphal strands of 

 the fungus. In this combination, each component is able to extend 

 its activities into habitats that would be inimical to it as an independ- 

 ent organism. Together they form a particular species of lichen 

 with specific morphologic, taxonomic, ecologic, and sometimes physio- 

 logic characteristics, the fungal part growing by extension of its 

 hyphae, the algal cells by division. 



This intimate relation of fungus and algae is a physiological union 

 usually regarded as one of symbiosis, i. e., of mutual benefit to each 

 component, the fungal element deriving food from the green alga, and 

 the alga benefiting by having its moisture and mineral nutrition 

 maintained through the water absorption and water retention charac- 

 teristics of the fungus. The presence of fungal haustoria, however, 

 and the penetration of hyphae into the algae have been cited as evi- 

 dence that this relationship is merely another case of parasitism. 

 Furthermore, the algae are commonly found freely growing in nature ; 

 lichenized fungi are not known to survive independently. 



As a taxonomic group lichens are open to fair and persistent criti- 

 cism. The International Eules of Botanical Nomenclature (art. 64) 

 definitely rejects any taxonomic group derived "from two or more 

 discordant elements." This should legally dissolve the biological 

 union traditionally accepted as the class Lichenes. The dommant 

 element of the union is the fungus, and through it the union is able to 

 perpetuate the unit ; the sexual reproductive elements are fungal, re- 

 sulting in the development of typical apothecia or perithecia in which 

 are developed spores. In the process of thinking about and describ- 

 ing the unit, the fungal characteristics are usually uppermost. The 

 inevitable result has been that many mycologists have segregated the 

 various groups among those fungi that appear to have a close relation- 

 ship. 



However, the thallus is a specialized type of structure, and the 

 fungus-alga relationship makes possible specialized functional rela- 

 tionships peculiar only to lichens. They may be conveniently treated 

 as a homogeneous group, for they have their own literature and spe- 

 cialists who concentrate their studies on them. 



