140 STRUCTUBE AND UNITS OF VEGETATION. 



may be, however, the real nature and significance of such an ecotone can be 

 determined only from a study of the adjacent communities. 



The mixing of two stages in development is much more complex and puz- 

 zling. This is due to the fact that mixing may take place throughout the area 

 and in varying degree. There are consequently in such places no distinct areas 

 of the two stages with which comparison can be made. Hence it is necessary 

 to turn to other examples of the same development and to make a comparative 

 study extending over a wide region and over several years. While there is 

 inevitably some mixture in all stages of the sere, it is only when the dominant 

 species of two, or rarely more, stages are present on somewhat of an equality 

 that a real mixture may be said to result. It is now proposed to restrict the 

 term mictium (Clements, I. c.) to this developmental mixture, and to use 

 ecotone for an actual transition area on the ground between two communities, 

 regardless of whether the latter are climax or serai. Thus, a Poffidus-Pinus- 

 mictium is a more or less uniform mixture of two successive consocies, while a 

 Popidus-Pinus-ecotone is a band of mixed aspen and pine between two pure 

 communities of each (plate 43, a, b). 



Nomenclature of units. — The whole task of ecological nomenclature is to 

 secure a maximimi of characteristic with the minimum effort. A long step 

 toward this result is taken by having a definite concept and name for every 

 distinct unit. The method of suflSxes, first used by Schouw (1823 : 65) in 

 designating groups by adding -etum to the generic name, has furnished the 

 model for the designation of all groups in which life-form and dominance are 

 the chief characteristics. Such are the consociation, consocies, society, etc. 

 Where the habitat is of primary importance, as in the formation, association, 

 and associes, it is necessary to employ a separate word, poion, helion, hylion, 

 eremion, etc., to indicate it. This must then be qualified by the use of the 

 generic name for actual floristic definiteness (Clements, 1902 : 16). Difficul- 

 ties arise, however, when two or more genera are concerned, or when it is 

 necessary to indicate the species in order to secure the requisite definiteness. 

 In both cases a balance must be struck between usability and definiteness, and 

 the latter must often be sacrificed. In the case of the Great Plains grassland, 

 definiteness would demand that it be termed the BovMoua-BulMlis-Aristida- 

 paion. Such a name is impracticable, as taxonomy long ago proved in the case 

 of polynomials. The use of two generic names is the most that convenience 

 permits, and one is better still. In the case cited, since BowLeloua is the domi- 

 nant of the widest range and greatest importance, the grassland might well 

 be called the Bovielouor-poion. Once the names of units become generally 

 recognized, such a designation is no more indefinite or incomplete than 

 Solanacese, for example. 



In this connection. Moss (1910:41) states that: 



"The naming of a pure association [consociation] by its dominant species 

 is comparable with the plan of naming a systematic group after an easily 

 recognizable character; and in neither case does such a name exhaust the char- 

 acters of the group it denotes." 



This statement does not seem wholly consistent with the further statement 

 that: 



"This name, EriophorymfirSdrpus-Oxodion, would not, however, be really 

 definitive, as no indication would be given of the species of Eriophorum or 



