president's address. 777 



into account in attempting to explain the relations that exist in 

 different plant associations. 



A. Climatic factors. 



(a) Temperature. 



(b) Moisture. 



B. Ecological factors. 



(a) Edaphic (the soil and its properties). 



(b) Atmospheric (heat, light, and influence of wind). 



(c) Hydrodynamic (action of tides and waves on strand 



vegetation). 



(d) Biotic factors (struggle for existence). 



C. Historic factors. (Factors which involve the element of 

 time, e.g., certain geological and physiographic forces). 



In tracing the genetic development of a forest, Whitford 

 selected certain islands, etc., in the vicinity of Lakes Michigan 

 and Superior, and examined four sets of physiographic forma- 

 tions : — 



1. The Sand Societies. 



2. The Clay Societies. 



3. The Rock Societies. 



4. The Swamp Societies. 



Taking (1), he deals in succession with the "lower beach," 

 "middle beach," "fossil beach," "heath,'' showing how a coniferous 

 forest established itself in course of time on the heath, which is 

 eventually succeeded, and even superseded, by the maple, beech 

 and hemlock forest. The dune societies are incidentally touched 

 upon, 



(2) The Clay Societies are more difficult to trace than those 

 found on the sand; but " in due time a^ maple-beech-hemlock con- 

 dition is the result." 



(3) The Rock Societies are traced, and in the locality selected 

 it is shown that on the rocks (mostly granites and quartzites) the 

 mesophytic forest (of maple, etc.) is not reached until first pre- 

 ceded by a coniferous forest. 



51 



