14 



PLANT SOCIOLOGY 



been carried out by Clements and his associates (1929). Long before 

 this Bonnier had studied the spread of Hchens at the expense of mosses, 

 a theme which has recently been revived by McWhorter. Y. Bogdan- 

 owskaia (Leningrad, 1926) approached the problem from another 

 side. She followed minutely the increase by seedlings in different 

 associations and arrived at very interesting conclusions that prove the 

 significance of phytosocial structure for the rising generation of 

 plants. 



Under natural conditions the aggressiveness of species is shown in 

 every permanent quadrat used in the study of the development of 

 communities (Fig, 6). 



FiQ. 6. — Permanent quadrats showing the results of competition between Peltigera 

 spuria and Barbula convoiuta on an old charcoal kiln near Praspol, 1,680 m., Swiss 

 National Park. H = Helianthemum. grandiflorum; ] = Pinus silvestris seedling 

 (f dead). The same quadrat, 1 sq. m., in 1922 (A) and in 1927 (B.) 



In order to determine experimentally the aggressiveness of the 

 species of a given group it is necessary to observe their reaction to the 

 varying intensity of a single factor such as salinity, acidity, or grazing. 

 Everyone has noticed with what readiness Polygonum aviculare, 

 Plantago major, and Lolium perenne spread over little-trodden foot- 

 paths. When the paths are more frequented these species lose their 

 luxuriance and finally disappear, but they also disappear when the 

 paths are too little used, being crowded out by other species. Their 

 optimum growth, therefore, is maintained within very narrow limits. 



The mutual displacements of position on the ground are measured 

 on small areas ; on extended areas one must be content to estimate the 

 abundance, cover, and sociability of each species. On small permanent 



