THE PLANT F0RMATI(3N 221 



no regular order, but seem to fit together in a haphazard fashion, 

 which is termed alternation. 



To avoid repetition, the development of the formation is treated 

 together with that of vegetation in the three chapters that follow. 

 Since the formation is the working unit in the study of vegetation, 

 its structure and classification are taken up in detail here, while 

 the general discussion of zonation and alternation is reserved for 

 the final chapter. 



235. Structure of the formation. No formation is uniform 

 throughout its entire extent. Practically all show more or less 

 striking differences at every step, so that minute and universal 

 variation may be regarded as a law of formational structure. 

 This characteristic variation is the result of the action of physical 

 factors and of formational activities, i.e., migration, competition, 

 etc., upon the number and grouping of individuals and species. 

 It finds expression in three ways. Species come to be distinguished 

 from each other with respect to number and to their importance 

 in the formation. They adjust themselves to seasonal changes 

 in such manner that they appear only at a certain time, or are 

 more characteristic at one season than at another. Finally the 

 variations within the habitat, together with aggregation, migration, 

 and competition, arrange individuals and species in more or less 

 typical groups or areas. 



236. Facias. The importance of the part which each species 

 plays in tlie formation is largely determined by the number of its 

 individuals. Other factors also have considerable influence in 

 determining this matter. It has already been pointed out that 

 the size, i.e., height and width, of the indi\aduals is an important 

 factor. In addition other qualities of the plant, especially its 

 duration, are of much importance. In determining the control 

 which a species exerts in the formation, all of these points are 

 taken into account, though in many cases number alone affords 

 a satisfactory basis. 



The dominant or controlling species of a formation are termed 

 fades. These are, as a' rule, the most abundant, or they make up 

 in size or duration what they lack in number. They are the most 

 widespread species of the formation, though they are not neces- 

 sarily found everywhere in it. In forests, the facies are the domi- 

 nant species of trees; in thickets, of shrubs; and in grassland, 

 they are the controlling grasses or sedges. When grasses are of 



