MICHIGAN ACADEMY OF SCIENCE. 215 



Table 1. Moisture Holding Capacity. 



M. H. C. % 

 Station. Association. of Dry Weight. 

 A. Maple-Beech, Kalamazoo (for comparison) 62.1 



5. Jack Pine 42.7 



11. Jack Pine 41.3 



4. Jack Pine-Oak 41.7 



3. Oak 42.7 



6. Big Pines 41.8 



7. Big Pines 43.7 



9. Big Pines 43.1 



14. Big Pines 40.7 



15. Mixed Hardwood 42.5 



13. Mixed Hardwood 41.7 



Similar conclusions would seem to be justified from the following average 

 daily evaporation data : 



Table 2. Average Daily Evaporation. Aug. 4-Aug. 31, 1918. 



No. Stations. 



Association. No. Atometers. Evaporation. Range. 



1. Standard (Maximum) 2 — 2 19.4 cc 19.3 — 19.5 cc 



2. Big Pines 7 — 11 12.79 cc 8.3 — 18.6 cc 



3. Oak 2 — 2 19.0 cc 18.4 — 19.5 cc 



4. Jack Pine-Oak 2—2 17.5 cc 16.7— 18.4 cc 



5. Jack Pine 1 — 2 12.7 cc 12.2 — 13.2 cc 



The lower average daily evaporations in the big pines and in the jack- 

 pine association is due not to the effect of the canopy, but entirely to the 

 sheltering effect of the herbaceous and shrubby layers. This is evident in the 

 great average daily range of from 8.3 cc to 18.6 cc (124%) in the big pines, 

 corresponding as these rates do with the maximum and minimum of protec- 

 tion from the influence of air-currents. On the other hand, by the selection 

 of stations similarly protected from wind influence records equally low could 

 be readily obtained in the jack pine-oak and oak associations. On the basis of 

 these results it would appear entirely unwarranted to assign evaporation as 

 a casual factor in succession, thus econfirming the results of Gleason and Gates 

 ('12, '17) and raising a question as to the validity of Fuller's claim ('11, '14) 

 that "the differences in the I'ates of evaporation are sufficient to indicate that 

 atmospheric conditions are efiicient factors in causing succession," at least 

 in this region. 



In the introductory paragraphs, attention was called to the inadequacy 

 of any existing classification of the plant formations. Upon the basis of the 

 observational and experimental results just presented, which are mutually 

 supportive, the following ecological classification of the upland plant forma- 

 tions of this region is proposed. 



