CLIMAX UNITS. 



133 



by taller later ones. Thus there may be distinguished prevernal, vernal, 

 aestival, and autumnal aspects, and corresponding societies. In boreal and 

 alpine regions the number of aspects is often but two, vernal and aestival, and 

 the societies correspond. The large majority of societies fall more or less 

 clearly within one aspect, but there are exceptions, as previously suggested. 

 Hence it is necessary to establish a major distinction into aspect societies and 

 permanent societies. Many of the latter are not true societies at all, but are 

 more or less imperfect expressions of undershrub and scrub consocies which 

 represent a potential climax. Such are the Gviierrezia, Yucca, and Artemisia 

 cana communities of the Great Plains. 



Layer societies. — ^As alreadyindicated, these usually have a seasonal relation 

 also, as they tend to develop successively rather than simultaneously. The 

 societies of thicket and woodland differ from those of grassland in being more 

 coherent and in falling into well-marked layers. The latter are found in 

 prairie, but they are usually incomplete and obscure. When the development 

 of the layers is clearly seasonal, the societies concerned may well be regarded 

 as aspect societies. As a rule, however, the layers are all developed before 

 midsimmier, and the forest presents a distinctively storied appearance. Nat- 

 urally, the layers are often fragmentary or poorly defined, and in closed or 

 mature forests they may be lacking. It seems best, then, to distinguish but 

 two kinds of layer societies at present, namely, societies of the shrub layer or 

 layers, and societies of the herbaceous layers. In cases where tall herb layers 

 overtop one or more of the shrub layers this distinction has Httle value, but as a 

 rule, the essential difference in the life-forms of the two layers or sets of 

 layers marks a convenient if not an important distinction (c/. Hult, 1881). 



Cryptogamic societies. — These in turn bear some relation to layer and sea- 

 sonal societies. The lowermost layer of a thicket or forest often consists of 

 mosses, liverworts, lichens, and other fungi. In mature forests of spruce this 

 is often the sole layer. Nearly all the parasites and many of the saprophytes 

 can not develop until stems and leaves appear, and hence exhibit both a sea- 

 sonal and a layer relation. While there can be no question of the distinctness 

 of cryptogamic societies, their treatment is a difficult matter. Many of them 

 are actual colonies in minute seres, such as the pure or mixed communities of 

 Marchantia, Funaria, or Bryum in burned spots. Distinctions into ground 

 societies, parasitic societies (i. e., those mostly on leaves and herbaceous stems, 

 which necessarily disappear each season), and bark societies (which persist 

 from one year to another) are convenient, but of minor importance. A dis- 

 tinction based upon life-form, i. e., moss, liverwort, lichen, and fungus, is 

 probably of greater value. Perhaps a more exact analysis would result from 

 the use of both life-form and location, but such a basis produces results too 

 detailed for our present needs. The soil in particular presents a virgin field 

 for the recognition and limitation of parasitic and saprophytic societies and 

 socies, especially of bacteria, but our knowledge is too slight to furnish the 

 necessary criteria. 



Terminology. — Societies have been designated by adding the locative suffix 

 -He to the name of the dominant genus, e. g., Iridile, Opulasterile, Androsadle 

 (Clements, 1905:299). Layers were named in similar fashion by adding 

 -anum to the generic name or group, Opulaster-Ribesanum. Since the society 

 is usually a group with a definite impress and a basic relation to habitat con- 



