524 RELATION TO ENVIRONMENT. 



often extensive patches * or areas of the cat-tail flag, which ap- 

 pears at a little distance to occupy the area to the exclusion of 

 all other vegetation. In similar locations are patches of arrow- 

 leaf, of rushes, reed grasses, etc. In low damp ground near 

 bodies of water or streams, the alders and willows often form 

 each a distinct patch or zone. In forests there are often dis- 

 tinct groups of white pine or of pitch pine, sometimes of maple 

 or beech. Smaller plants are often between the larger ones 

 composing these groups, or in their shade, but the larger plants 

 so closely associated are dominant and characterize the area. 



The various groups above described would then be called cat- 

 tail-flag formation (or Typha formation), arrow-leaf formation 

 (Sagittaria formation), Cassandra formation, white-pine for- 

 mation, etc. (The same formations are sometimes called also 

 typhetum, sagiltarietum, cassandretum, pinetum, etc.) In the 

 cases just cited above, a single species is dominant, and it is 

 a pure formation. In many cases the individuals of several 

 species may share the same spot or area of ground equally or 

 nearly so. In the former case the individuals of the one species 

 are able, in their competition with others, to occupy the ground 

 to the exclusion of their competitors, either because they possess 

 characters which enable them to gain the upper hand, or be- 

 cause competing species may not have been present at the 

 start. When the individuals of several species occupy equally 

 the same ground, the formation is mixed. The conditions are 

 favorable for the growth of several dominant species, and all, 

 as it were, feed at the same table, with no greater competition 

 between the individuals of several species than there would 

 be between the individuals of one species were only one species 

 present. This mixture of several species of the same grade 

 of dominance is sometimes called commensalism, and the term 

 is also extended to mixtures where individuals of smaller species 

 are mixed in between those of larger ones or in their shade, 

 and do not come into actual competition with them. Examples 



* All small patches should not be regarded as a formation. 



