PLANT SUCCESSIONS Of' MT. ROBSON 217 



rapidly, thus giving rise to large accumulations of talus at their 

 bases, and continually exposing fresh surfaces above. Rock 

 areas stable enough to allow the development of a rock surface 

 succession were not studied, though without doubt they are 

 abundant enough. 



Studies in other regions have shown that two types of rock 

 surface succession may be distinguished. (1) An area of slope 

 slight enough to prevent removal of material by gravity may be- 

 come covered by a layer of fragments, the products of its own 

 weathering. Such will present conditions similar to talus, and 

 vegetation upon it will develop along similar lines. (2.) If on 

 account of smoothness and relative freedom from joints the 

 rock resists weathering, vegetational development will be much 

 slower, and will begin with lichens upon the smooth areas and 

 crevice plants in the crevices, the lines of progress originating 

 with these two sets of pioneers finally merging in a mat of vege- 

 tation resting upon the practically unweathered rock and anchored 

 in the crevices. The latter is the common type of development 

 upon the rock shores of Isle Royale, Lake Superior. ^ Doubtless 

 both types could be found in the Mt. Robson region, but I 

 have no observational data to offer. The process as suggested 

 above must be regarded as hypothetical. 



2. The Talus Succession 



The observations upon which this account is based were made 

 mainly in the vicinity of Lake Kinney; also near Emperor Falls. 

 The normal course of plant succession is from lower plants to 

 higher, both in size and in stage of evolutionary development. 

 Thus a succession may be initiated by lichens or algae, and these 

 will normally be followed by plants higher in the evolutionary 

 scale and larger in size. In either case the final stage will be 

 forest, chmate permitting. In the talus succession, in the local- 

 ity studied, the order is reversed, for the pioneers are trees, and 

 the smaller and lower plants come at a late stage. The natural 

 result of this anomaly is that the stages of the succession are 



2 Bot. Gaz. 55: 115-140; 1913. 



