PRACTICAL STUDY OF PLANT FORMATIONS. 63! 



physiographic areas. It should be borne in mind always that all asso- 

 ciations of plants do not represent distinct formations. Some formations 

 may be incipient and others decadent and thus difficult to determine; also 

 more or less extensive areas may have a mixture of several formations 

 over a transition area. Again, mere patches representing a gregarious 

 condition of certain species within another formation should not be mis- 

 taken for a formation. 



The district selected for study should, if possible, include forest, low 

 marshy areas, and ponds or lakes, within which area will probably occur 

 a number of different soil conditions, as rocky, sandy, or clayey areas, 

 etc., ravines, bluffs, meadows, etc. It might be well to follow the plan of 

 mapping the area, or of mapping the distinct physiographic areas studied 

 when they are far separated, and indicating the location and extent of 

 different formations, with notes on kind of soil (gravel, sandy, rocky, 

 loamy, etc.), and in general the relation of the formations to topography, 

 i.e., to different degrees of the gradient of slope where there is radial 

 or lateral topographic symmetry, as on hillsides or borders of streams, or 

 ponds, etc., different depths of water, etc. Topographic maps already in 

 existence can be used or made the basis of the special chart. The 

 topographic maps now being issued by the United States Geological 

 Survey * are most excellent for use and reference in connection with this 

 study. 



The district studied would, of course, be more or less limited. In con- 

 nection with the work studies of literature, or lectures should connect the 

 work with larger districts and with the great climatic formations, or vege- 

 taiton provinces, or regions (See Section II of this chapter). 



Suppose the district to be studied lies within the woodland climate. 

 For practical purposes of an elementary study this could be divided into 

 mountain districts, and coastal plain, and continental valley districts, since 

 in a mountain region one would be chiefly concerned with problems some- 

 what different from those of the coastal plains or continental valleys, and 

 the two sets of districts could only be studied together in actual practice 

 when the work was extended over a long period. For the purpose of 

 subdividing a large area and getting at the chief physiographic areas which 

 are the units of the principal formations the more level districts of the 

 woodland climatic region might be divided as follows: 



* From the U. S. Geological Survey at Washington, D. C., can be ob- 

 tained a map showing on a small "scale the districts covered for each State 

 and indicating which ones are completed, as well as the price. By secur. 

 ing this map one can easily determine the topographic maps needed to 

 cover the area selected for study. 



