Vegetation of Northern Cape Breton. 



403 



state of preservation, extended nearly to the botiom, where, at 

 the very base of the hummock and overlying the gravelly sub- 

 stratum, was a thin layer of Leucobryttm remains. It is evident 

 that for the most part the formation of these hummocks is a 

 result of the combined activity of the lichens, particularly the 

 cladonias, and the ericaceous shrubs. They have arisen some- 

 what as follows. Previous to their formation the ground was 

 covered by a thin mat of mosses and lichens in which grew vari- 



FiGURE 53. — Detail view of hummock in dwarf shrub heath association- 

 type; Cladonia alpestris, Chamaedaphne, Ledum, etc.; barrens in moun- 

 tains west of Ingonish. 



ous herbaceous plants and shrubs : essentially the same condition 

 which prevails in sedge-grass heath and which still persists in 

 the open depressions between the hummocks. Where edaphic 

 conditions are favorable the cladonias exhibit a marked tendency 

 to grow upward, but they are unable to do so to any extent with- 

 out some sort of support. The needed support is furnished by 

 the shrubs which, where they grow close enough together, 

 afford a sort of scaffolding upon or around which the lichens are 

 able to push upward. As the shrubs gradually become buried 



