591 



The almost complete absence, in these small areas, of stolonifer- 

 ous or loosely spreading species makes it seem certain that there exists 

 some mechanical competition in which species of compact and fre- 

 quently cespitose habit or species capable of reproducing extensively 

 from seed are successful. The extent, however, tO' which their suc- 

 cess is achieved because of their growth-form or because of their 

 superior adaptation to the particular complex of soil and moisture 

 conditions in these small areas, is of course incapable of accurate 

 estimation without further study. The idea of mechanical competi- 

 tion (/. ^., a struggle either among the various species because of the 

 mutual bodily resistance of any or all of their growing parts, or of 

 individual species because of the resistance offered by the soil's com- 

 pactness to the locomotion of their subterranean organs) is opposed 

 by Clements ('05, pp. 285-289) ; but Warming ('09, p. 324), in ac- 

 counting for the usual absence of vegetative locomotion among per- 

 ennial herbs of the meadow formation, seems inclined to accept this 

 idea in part. 



Summary and Conclusions 



1. Atmometer readings at a uniform height of 25 cm., taken for 

 a period of 147 days at four different stations, show that the evapora- 

 tion rate is lowest in the center of the reed swamp and gradually in- 

 creases as conditions approximating those of forest are reached. 



2. The evaporation rate found to obtain in the swamp white oak - 

 white ash forest, conforms with the commonly known fact that with 

 successive increases in the mesophytism (attended with decreasing 

 hydrophytism) of a forest, trees such as Quercus hicolor, Fra.rinus 

 nigra, and F. aiiicricana are antecedent to trees like Fagits grandi- 

 folia and Acer saccharum. 



3. Atmometer readings, taken for seven weeks at four different 

 levels among Phragmitcs plants and at five dift'erent levels among 

 Typha plants, show that among marsh species of compact social 

 growth evaporation is proportionate to the height above the soil. 

 These results thus coincide with those of Yapp ('09). 



4. Data accumulated at Skokie Marsh support the conclusion of 

 Massart ('03) that it is a matter of importance to perennial plants 

 that their hibernating organs occupy a definite level in the soil. 



5. Certain observed cases of variation in this level (Teucrium 

 occidcntale, Polygoniun Miihlcnhergii, etc.), corresponding to changes 

 in the water-level, indicate that with certain species, at least, the 

 depth of the water-table is much the most potent controlling factor 

 (cf. Yapp, '08), 



