1912] SHERFF—SKOKIE MARSH 433 
different demands upon the soil, yet edaphic competition is un- 
doubtedly sharp. The almost complete absence, in these small 
areas, of stoloniferous or loosely spreading species makes it seem 
certain that there exists some mechanical competition in which 
species of compact and frequently caespitose habit or species 
capable of reproducing extensively from seed are successful. The 
extent, however, to which their success is achieved because of their 
growth-form or because of their superior adaptation to the particu- 
_lar complex of soil and moisture conditions in these small areas 
is of course incapable of accurate estimation without further study. 
The idea of mechanical competition (i.e., 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 compactness to the locomotion of 
their subterranean organs) is opposed by CLEMENTS (3, pp. 285- 
289); but WARMING (16, p. 324), in accounting for the usual absence 
of vegetative locomotion among perennial herbs of the meadow 
formation, seems inclined to accept this idea in part. 
Summary and conclusions 
1. Atmometer readings, taken for seven weeks at four different 
levels among Phragmites plants and at five different 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 (20). 
2. Data accumulated at Skokie Marsh appear to support the 
conclusion of Massart (12) that it is a matter of importance to 
perennial plants that their hibernating organs occupy a definite 
level in the soil. 
3. Certain observed cases of variation in this level (Teucrium 
occidentale, Polygonum Muhlenbergii, 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. 
4. Two or more species may live together in harmony because 
(1) their subterranean stems may lie at different depths; (2) their 
roots may thus be produced at different depths; (3) even where 
roots are produced at the same depth, they may make unlike 
