SERAL UNITS. 139 



A colony does not have a fixed rank, but it may develop later into any com- 

 munity of higher rank in the developmental series. As already indicated, 

 it is primarily a mixed invasion group, which is inevitably worked into the 

 history of the sere as development proceeds. The term colony is itself an 

 index of this pioneering quality. Colonies may be designated by the suffix 

 -ale, as in Hordeale, Ambrosia-Ivale, etc. 



Family. — The use of the term family for an ecological group was proposed 

 by Clements (1904 : 297, 299; 1905 : 297; 1907 : 228, 237). The fundamental 

 identity of such families of plants with those of animals and man is thought to 

 make such use of the word unavoidable in spite of the established usage for a 

 systematic unit. While the possibility of confusion from the double use of 

 the term is slight, it may prove desirable to avoid this objection altogether 

 by using the term famile for the ecological unit. As is evident, it is from the 

 same root as family, and has essentially the same meaning (plate 42 b). 



A family is a group of individuals belonging to one species. It often springs 

 from a single parent plant, but this is not necessarily the case, any more than 

 in a human family. It may consist of a few individuals or may extend over a 

 large area. The group of cells within a Gloeocapsa sheath is a family, and not 

 a colony in the proper sense. The coating of Pleurococcus on a tree-trunk is a 

 family, as is also a tuft of Funaria at its base, or the group of Helianthus which 

 fills a large field to the exclusion of all other flowering plants. Families, 

 however, are usually small, since they are more readily invaded when large, 

 and consequently pass into colonies. They are especially typical of bare 

 areas and initial stages. They rarely appear in dense vegetation, except where 

 local denudation occurs. As the individuals of a family become more numer- 

 ous, adjacent families merge into a colony; or migrules from one family may 

 invade another at some distance and convert it into a colony. Since the family 

 always consists of a siugle species, it may be designated in the usual way by 

 adding the patronymic sufiix to the generic name, as in Sedas, Aletas, Erio- 

 gonas, etc. Where greater definiteness is desired, the specific name in the 

 genitive form may be added, e. g., Rubas strigosi. 



Summary of units. — ^The following table is intended to show the relation of 

 climax and serai or developmental units to the formation, the relation of the 

 units of each series to each other, and the correspondence of units in the two 

 series. 



Formation. 

 Climax Units: Serai Units: 



Association Associes. 



Consociation Consocies. 



Society Socies. 



Clan Colony. 



Family. 



Mixed communities. — Clements (1905 : 304; 1907 : 235) has considered 

 briefly the mixiug of communities as a consequence of juxtaposition or of 

 succession. The former applies to the characteristic mingling of dominants 

 where their corresponding communities touch. It may occur between two or 

 more formations, associations, consociations, or societies, or between associes, 

 consocies, or socies. In every case the mixing takes place at the borders of the 

 communities concerned, producing an ecotone or tension. This is often very 

 extensive, and frequently its relations are very puzzling. Difficult as the task 



