THE STRUCTURAL CHARACTERISTICS OF THE COMMUNITY 47 



indirectly determined in these cases by the time elapsed before 

 germination. 



Periodicity.— The works of Diels (1918), Scharfetter (1922), and 

 Alechin (1926) illuminate anew the general significance of rhythmic 

 phenomena in the social life of plants. This includes much more than 

 purely phenological phenomena. 



Sociological periodicity depends upon the beginning, the duration, 

 and the seasonal course of the struggle for existence. Superficially 

 striking phenomena like flowering and fruiting (phenologically impor- 

 tant characters) are less important than growth of shoots, duration of 

 foliage, leaf fall, and root renewal (formation of absorptive roots). 

 The important root relations, however, which greatly influence the 

 seasonal development of higher plants have been little studied. 



For an understanding of the simultaneous and successive life 

 activities of plants, chance observations will not suffice; continuous 

 and systematic records are necessary. In every study the stage of 

 development of the species should be given. 



For this special signs may be used, but in general the following 

 abbreviations suffice: 



fol. = in foliage, 



s.fol. = leafless. 



b. = buds. 



fl. = flowering. 



fr. = fruiting, 



sdl. = seedlings, 



ass. = assimilating (photosynthetic). 



Aspect. — While the communities of the unfavorable climates of high 

 mountains and the north show mostly only two clearly marked seasonal 

 aspects — a hibernal and an aestival — in the temperate and subtropic 

 regions there are several. Physiognomically they are shown by suc- 

 cessive waves of flowering which give to each stage or aspect a wholly 

 different appearance (Figs. 26, 27, 28, 29). The changes in the array 

 and aggressiveness of the various species in the course of the year 

 are sociologically more important. The greatest aggressiveness is not 

 usually at the flowering time but at the height of shoot development. 



Diagrams of Periodicity. — The best insight into the social rhythm 

 of vegetation is given by graphic representations which cover the 

 photosynthetic activities or the space demands of the species from time 

 to time. 



Salisbury (1925) distinguished four seasonal periods in the Quercus 

 robur forest according to photosynthetic activities: a prevernal type, 

 photosynthetic from February to June (Ficaria, Anemone nemorosa 



