154 



PLANT MORPHOLOGY 



algae live on the surface of the mycelium, closely covering it. With only 

 a few rare exceptions, lichen-forming fungi are ascomycetes belonging 

 either to the Pezizales or to the Pyrenomycetales. In three genera of 

 lichens the fungus is a basidiomycete, the best-known species being Cora 

 pavonia, which is widely distributed throughout Central and South 

 America. The lichen-forming algae are members either of the Cyanophy- 

 ceae or Chlorophyceae, most of the latter belonging to the Chlorococcales. 



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Fig. 125. Group of common lithens, natural size. A, a crustose form {Placodium) grow- 

 ing on rock; B, a foliose form {Parmelia) growing on bark; C, a fruticose lichen (Cladonia) 

 which grows erect on the ground; D, a branching form {Usiiea) that hangs from the limbs 

 of trees. 



Lichens were once regarded as single plants. In 1868, their dual nature 

 was demonstrated. In 1889, lichens were first synthesized by sowing 

 spores from the fungous element of a lichen among appropriate free-living 

 algae. The developing mycelium was seen to enclose the algae and 

 develop into a lichen. Although the algal symbionts are forms that may 

 exist independently, the fungi are known only as constituents of lichens. 

 Vegetative reproduction takes place mainly by soredia, globular or 

 scale-like bodies composed of a few hyphae closely investing one or more 

 algal cells. They arise as buds on the upper surface of the thallus, become 

 detached, and are scattered by the wind. The algal components multiply 



