320 PLANT SOCIOLOGY 



Determination of the Construdiveness of Species. — For the determina- 

 tion of the constructiveness of species permanent quadrats (or areas) 

 are used. Changes in the quantitative relations, the sociabihty and 

 vitality of the species must be followed for years and decades before a 

 conclusive judgment can be reached. Number of seedlings and their 

 increase in size must be very accurately recorded. The factors of the 

 habitat and their reaction upon the interrelations of the species must 

 be investigated. 



Jenny-Lips marked off a quadrat of 15 sq. m. on a fresh gravel 

 slide in Switzerland, in 1922, and followed from year to year the 

 introduction and establishment of plants upon the area. Stipa 

 calamagrostis proved a species of great dynamogenetic value. Start- 

 ing with one small plant in 1922, it had in 1928 surpassed all other 

 species, with 59 individuals. This was in spite of the fact that in 1924 

 Dadylis glomerata and Kernera saxatilis dominated the heterogeneous 

 new community with 30 and 44 individuals respectively against one 

 individual of Stipa. Gypsophila repens, the characteristic species of 

 the order, did not put in an appearance until 1925 but had increased 

 to 54 individuals in 1928. The sociability of Gypsophila and Stipa 

 has steadily and significantly increased. The former covers about 

 1 sq. m. in one place, while Stipa has developed a stand 70 cm. in 

 diameter. In the intervening spaces there are various combinations 

 of characteristic species ever on the increase. 



2. Syngenetic Units. — Succession of vegetation consists, in last 

 analysis, of the exchange (appearance and disappearance) of species 

 or of changes in the quantitative relations of the species continuously 

 present. Not every exchange of species is to be regarded as a stage of 

 succession. Only when the existing equilibrium is obviously disturbed, 

 the uniformity of the prevailing vegetation visibly altered, are we justi- 

 fied in speaking of a substitution of one dynamogenetic unit by another. 



Stage. — A single clearly marked step in succession is called a stage — 

 the lowest, indivisible concrete unit of development. According to the 

 time of its appearance in the developmental series, it is designated as a 

 pioneer, transitional, or terminal stage. Pioneer stages, the initial 

 colonies of new land, are mostly sociologically simple, one layered, and 

 composed often of cryptogams: algae, fungi, lichens, and mosses. In 

 arctic and snowy climates and in deserts this pioneer stage is also the 

 climatically limited final stage. The Gyrophora cylindrica lichen asso- 

 ciation of the snowy siliceous peaks of the Bernese Alps (Frey, 1922) 

 is the beginning and the end of plant immigration. 



The number of transition stages depends upon the prevailing 

 cUmate and soil conditions, as well as upon the presence of dynamically 



