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 alge living together 
in such a close relationship that it looks like a single plant. 
There may be many individual fungi and many individual 
alge 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 
Fic. 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 alge that enter into 
the formation of lichens are usually unicellular forms resem- 
bling Pleurococeus. When the lichen is dissected, the green 
cells of the algze and the white threads of the fungi may be 
seen (fig. 195). The chlorophyll of the alge enables the 
lichen to manufacture carbohydrate foods. Individual alga 
cells are often closely wound about by threads of the fungus ; 
