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THE MICROSCOPICAL STRUCTURE OF LICHENS. 



By Egbert Paulson, F.L.S., F.R.M.S. 



[Read June 8th, 1920.) 



Before considering the microscopic details of the lichen thallus 

 it may prove of some advantage to preface these remarks with a 

 few. statements regarding lichens as they appear in their natural 

 habitats, for some of them are sufficiently conspicuous to claim 

 our attention by reason of size, form or colour. There are, for 

 instance, the " beard lichens" Usnea barbata and Usnea j^licata, 

 which frequently hang in long tangled festoons from the trunks 

 and branches of trees in pine forests in upland or mountainous 

 districts. A similar species, Usnea florida, stands erect from the 

 substratum, and closely resembles a miniature shrub ; they 

 all belong to the fruticose or shrub-like form of the lichen thallus. 

 The yellow "crottle" Xanthoria {Physcia) parietina, known by 

 sight to all of us, often covers, with bright orange patches, large 

 portions of the roofs and walls of farm buildings. It has a leaf- 

 like appearance, and serves as a good example of the foliose 

 thallus ; it is dorsi ventral in structure, the fruticose lichen being 

 radial. The third form, the crustose lichen, also dorsiventral, 

 develops as a crust, which in some species is of considerable 

 thickness, while in others the thallus is so attenuated that it 

 resembles a stain upon wood, stone or soil, whichever may be the 

 substratum (6). 



The three forms of thallus, crustose, foliose and fruticose, exhibit 

 stages of evolution ; the crustose, which in its simplest state con- 

 sists of a loose mass of tangled filaments enclosing green cells, 

 represents a low stage, while some of the highly developed 

 fruticose Lichens represent the highest point of development. There 

 is no straight line of development along which the lichens have 



