216 PLANT PHYSIOLOGY AND ECOLOGY 



a meadow may pass so gradually on the one side into a swamp 

 and on the other into a prairie that it is impossible to say exactly 

 where the one stops and the other begins. Nevertheless, meadow, 

 prairie, and bog are three different formations, as is readily seen 

 when areas tj'pical of them are compared. In consequence, the 

 real test of a formation is its character or composition rather than 

 the sharpness of its limits. Adjacent formations of the same 

 general nature usually shade gradually into each other, e.g., 

 meadow and prairie, forest and thicket, etc. Those that are very 

 different in character, e.g., meadow, pond, and forest, stop abruptly 

 at the line of contact. 



231. Recognition of formations. The formation is the unit 

 of vegetation. The plant covering of the earth is a vast complex, 

 largely made up of different formations. In recognizing these 

 units or formations, one or two precautions are necessary. The 

 unit itself shows parts which may be mistaken for formations. 

 This danger is considerable when a formation has been so broken 

 up by natural or artificial forces that one or more of its parts have 

 been separated from the original. The chance of confusion is 

 greater when the original formation has disappeared from a region, 

 leaving only one or two incomplete parts to represent it. In 

 such cases the most careful study is necessary to determine the 

 proper standing of these isolated areas. The need of caution is 

 well illustrated in forested countries that have been largely cleared, 

 and in grassland regions which have been almost wholly plowed up. 

 Fragments of the original forest or prairie are left here and there, 

 all more or less widely separated. Some of these are so different in 

 composition that they appear by comparison with each other to be 

 distinct formations. When these are all brought together, and 

 especially when they are compared with larger areas in other 

 places, they are found to be merely the more or less different 

 portions of an original formation. In studying small areas of a 

 few square miles or less, it must constantly be kept in mind that 

 these are probably not different formations, but merely parts of 

 an originally extensive formation which have now become 

 separated. This is especially true of rugged regions, such as 

 mountains, in which the pieces of one formation are very small 

 and widely scattered. 



Similar care is necessary in regard to areas which show different 

 stages of development. One stage or formation changes slowly 



