FUNGI 321 



homoiomerous. The heteromerous thallus mainly consists of the fungus 

 body of the lichen differentiated into a cortical layer and a medullary 

 layer, the algae occurring either as a definite layer where the cortical 

 and the medullary hyphae join, or they are scattered throughout the 

 medulla, or form a dense mass in it. Such thalli exhibit considerable 

 variety in forms of growth, and are &&..& foliaceous,fruticose, crustaceous^ 



FIG. 284. Roccella tinctoria DC. A filamentous lichen. Small plant (natural size). 



&c., in descriptive works. The homoiomerous thallus consists of algal 

 cells and hyphae more or less equally distributed and alike in bulk. 

 Collema, referred to below, is a gelatinous lichen, exemplifying this 

 structure. Though the fungus does not actually prevail in bulk, it 

 modifies the form of the thallus. 



Until comparatively recent times, lichens were considered ' to be 



Y 



