LICHENES 67 



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The Lichens are ascomycetous fungi enclosing in their my- 

 celium unicellular or filamentous algae on which they live para- 

 sitically. On account of a supposed symbiosis of the two groups 

 in the same individual lichen, there has been a hesitancy to place 

 the lichens in their true position in the system among the Pezi- 

 zales, Sphaeriales and other orders of the Ascomycetes. There is 

 every gradation from forms regularly enclosing algae to those in 

 which algae are only incidentally present, and the strongest 

 reasons for keeping lichens in a distinct class are custom and 

 convenience, neither of which should have weight in a natural 

 system. 



A few basidiomycetous fungi also enclose algae in a similar way, 

 so that there are Ascolichenes, Hymenolichenes and Gastrolickenes. 

 The greater part of our familiar species are allied to the Pezizales. 



