The major divisions of prairie and forest formations are regularly due to alternation. There is an inherent tendency to the segregation of facies, arising out of physical or historical reasons, or from a combination of both. Not all formations show this, but it is characteristic of the great majority of them. The primary areas which thus arise have been called associations: they are naturally subordinate to the formation. To avoid the confusion which inevitably results from using the word association in two different senses, it is proposed to term this primary division of the formation, a consociation, or better, a consocies. This term is applied only to an area characterized by a facies, or less frequently, by two or more facies uniformly commingled. The consocies of grassland are determined by grasses, those of forests by trees, etc. From the different position of the facies in these two types of vegetation such areas are readily seen at all times in the forest, but they are often concealed in grassland by the tall-growing principal species of the various aspects. When definite consocies are present, they are often found to mingle where they touch, producing miniature transition areas, and, very rarely, they sometimes leave gaps in which no facies appears.
Fig. 77. Late (serotinal) aspect of the alpine meadow, characterized by Campanula petiolata, Rydbergia in fruit.
The seasonal changes of a formation, which are called aspects, are indicated by changes in composition or structure, which ordinarily correspond to the three seasons, spring, summer, and autumn. The latter affect the facies relatively little, especially those of woody vegetation, but they influence the principal species profoundly, causing a grouping typical of each aspect. For these areas controlled by principal species, but changing from aspect to aspect, the term society is proposed. They are prominent features of the majority of herbaceous formations, where they are often more striking than the facies. In forests, they occur in the shrubby and herbaceous layers, and are consequently much less conspicuous than the facies. A close inspection of the societies formed by principal species shows that they are far from uniform. Since they usually fail to exhibit distinct parts, it becomes necessary to approach the question of their structure from a new standpoint. Such is afforded by aggregation, which yields the simplest group in vegetation, i. e., that of parent and offspring. This is so exactly a family in the ordinary sense that there seems to be ample warrant for violating a canon of terminology by using the word for this group, in spite of its very different application in taxonomy. It has already been shown that aggregation further produces a grouping of families, which may properly be called a community. As they are used here, family and community become equally applicable to the association of plants, animals, or man. Both families and communities occur regularly in each society of the formation, and they represent its two structures. In some cases, all the families are grouped in communities, two or more of which then form the society. Very frequently, however, families occur singly, without reference to a community, and the two then constitute independent parts of the same area. This is typically the case wherever gregarious species are present, since these are merely family groups produced by aggregation.
Fig. 78. Calthetum (Caltha leptosepala), a consocies of the alpine bog formation.
Fig. 79. Iridile (Iris missouriensis), a society of the aspen formation.
Objection may be made that this analysis of formational structure has been carried too far, and that some of the structures recognized are mere interpretations, and not actual facts. Such a criticism will not come from one who has got beyond the superficial study of formations, for he will at once recognize that certain probable features of structure have not been considered. On the other hand, the ecologist or the botanist who has not made a careful investigation from the standpoints of development and structure will naturally refrain from expressing an opinion, until he has obtained an acquaintance at first hand with the facts. Over-refinement is the usual penalty of intensive work. The unbiased investigator, however, will not be misled by the suddenness with which new concepts appear. It seems plausible that the structure of a formation, if not as definite, is at least nearly as complex as that of an individual plant. Few botanists will insist that the refinement of tissues and tissue systems has been carried further than the differentiation of the plant warrants. Yet, if these had been defined within a period of a few years rather than slowly recognized during more than a century, they would have been called seriously in question. As a matter of fact, the consocies, under the term association, and the society, under various names, have been recognized by ecologists for several years. They are definite phenomena of alternation which can be found anywhere. The family and the community, though the latter is less distinct in outline, are equally valid structures, the proof of which anyone can obtain by thorough methods of study.
348. Nomenclature of the divisions. The suffix -etum is used to designate a consocies of a formation, e. g., Picetum, Caricetum, etc. When two or more species characterize the area, the most important, or more rarely, the two are used. The termination used to designate a society is -ile, as Asterile, Sedile, Rosile. The suffix which denotes the community is -are, and for the family, it is -on, viz., Giliare, Bromare, Bidenton, Helianthon, etc. Layers are indicated by the affix -anum, as Opulasteranum, Verbesina-Rudbeckianum, etc. It is evident that these suffixes, like the terms to which they refer, must be used always for the proper divisions if they are to have any value at all. There has been a marked tendency, for example, to use -etum in connection with the names of groups of very different rank. It is hardly necessary to point out that such a practice does not promote clearness. The following tabular statement will illustrate the application of both terms and suffixes: