CHAPTER VII 



PHYLOGENY 



I. GENERAL STATEMENT 

 A. ORIGIN OF LICHENS 



THOUGH lichens are very old members of the vegetable kingdom, as 

 symbiotic plants they yet date necessarily from a time subsequent to the 

 evolution of their component symbionts. Phylogeny of lichens begins with 

 symbiosis. 



The algae, which belong to those families of Chlorophyceae and Myxo- 

 phyceae that live on dry land, had become aerial before their association 

 with fungi to form lichens. They must have been as fully developed then 

 as now, since it is possible to refer them to the genus or sometimes even to 

 the species of free-living forms. The fungus hyphae have combined with a 

 considerable number of different algae, so that, even as regards the algal 

 symbiont, lichens are truly polyphyletic in origin. 



The fungus is, however, the dominant partner, and the principal line of 

 development must be traced through it, as it provides the reproductive organs 

 of the plant. Representatives of two great groups of fungi are associated 

 with lichens: Basidiomycetes, found in only a few genera, and Ascomycetes 

 which form with the various algae the great bulk of lichen families. In 

 respect of their fungal constituents lichens are also polyphyletic, and more 

 especially in the Ascolichens which can be traced back to several starting 

 points. But though lichens have no common origin, the manner of life is 

 common to them all and has influenced them all in certain directions: they 

 are fitted for a much longer existence than that of the fungi from which they 

 started ; and both the thallus and the fruiting bodies at least in the sub- 

 class Ascolichens can persist through great climatic changes, and can pass 

 unharmed through prolonged periods of latent or suspended vitality. 



Another striking note of similarity that runs through the members of this 

 sub-class, with perhaps the exception of the gelatinous lichens, is the formation 

 of lichen-acids which are excreted by the fungus. These substances are 

 peculiar to lichens and go far to mark their autonomy. The production of 

 the acids and the many changes evolved in the vegetative thallus suggest the 

 great antiquity of lichens. 



