56 IOWA ACADEMY OF SCIENCE 



III. Swamp 



Typha, Juncus, Penthorum formation 

 Consocies Typha, Juncus 

 Consocies Penthorum 

 Consocies Ludwigia, Sparganium 

 Consocies Leersia 



IV. Pond 



Ranunculus, Sagittaria formation 

 Consocies Ranunculus 

 Consocies Sagittaria 



Every stable formation may be expressed as a summary of its ha- 

 bitat conditions, which are equal to the number of its associations. 



The chief causes of difference in associations are their habitat fea- 

 tures which are not common factors. 



The principal factors which are not common or which vary in 

 marked degree are (1) type of soil and (2) water content of habitat. 



Structure of soil has a direct bearing on water content. Water con- 

 tent depends on the type of soil, drainage, and rainfall. 



The main significance of temperature variation in this study is in 

 its application to the production of societies with reference to seasonal 

 aspects. 



