FUNGI AND FUNGOUS DISEASES OF PLANTS 249 



236. The structure of a lichen. A lichen is not a single 

 plant, but a combination of fungi and algse living together 

 in such a close relationship that it looks like a single plant. 

 There may be many individual fungi and many individual 

 algae in this relation, but the combination is spoken of as the 

 lichen plant. The fungal part of the lichen is usually, though 

 not always, a member of the sac-bearing class of fungi, and 



FIG. 194. The "bearded moss" lichen (Usnea barbata) growing upon the 

 branches of a spruce tree 



consequently lichens are often classified with sac fungi. This 

 is obviously a somewhat questionable classification, but for 

 lack of a better one we shall use it The alga) that enter into 

 the formation of lichens are usually unicellular forms resem- 

 bling Pleurococcus. When the lichen is dissected, the green 

 cells of the alga) and the white threads of the fungi may be 

 seen (fig. 195). The chlorophyll of the alg3 enables the 

 lichen to manufacture carbohydrate foods. Individual alga 

 cells are often closely wound about by tlireads of the fungus ; 



