16 



PLANT SOCIOLOGY 



atrata and A. moschata, Rhododendron hirsutum, and R, ferrugineum. 

 In fact the pronouncedly basophilous and calcicolous A. atrata and 

 R. hirsutum never survive long on acid soils, whereas A. moschata 

 and R. ferrugineum being acidophiles do not appear on calcareous basic 



soils. The life requirements of these 

 pairs of species are so different that the 

 question of competition cannot arise. 

 Unfortunately this long-recognized error 

 (c/. Braun-Blanquet 1913, p. 141) still 

 crops up as an example in the latest text- 

 books like Lundegardh's (1925, p. 299) 

 and also in the book of Clements, Weaver, 

 and Hanson (1929). Indeed, it is con- 

 stantly extended by new and equally 

 untenable examples (Riibel, 1912, p. 417; 

 Vierhapper 1922, p. 41). 



Competition between Plant Communities. 

 In the competition between plant commu- 

 nities the aggressive domineering species 

 of great dynamic and genetic significance 

 play the role of advance guard and main 

 army; the more or less constant com- 

 panions furnish the reserves. 



Most conspicuous is the conquering 

 ^ „ ^ , , , march of communities of few species or 



i^iG. 8. — Bark surface of a 



beech trunk 122 cm. in diameter, the advance of swarms of foreign immi- 

 unrolied to show competition grants with great powers of expansion. 



among epiphytes. {After Ochsner.) °^ ° '^ , r r • x xu 



Lee. = Lecanora subfusca; Phi. Ihe Spread 01 Etodea and 01 J ussieua oXthQ 

 = Phiyctis argena;Le. = Lepraria Q^pQj^QQ of ^he European aquatic flora 



glauca; Ea. = Radula complanata; . 



Fru. = Fruiiania diiatata; Me. = Came With the suddcnncss 01 an explosion, 

 Metzgeria furcata; o. L. = Ortho- ^s also did the Spread of the river-bank 



trichum lyellii; U. = Ulota crispa; o 7 • i a 1 



Dr.= Drepanium fiiiforme; Is. = socictics of American Sohdago, Aster, and 

 isotheciu7nmyurum; Ho. = Homa- Stenactis species in Central Europe; Diplo- 



lia trichomanoides; + = sparse. 



taxis erucoides and Xanthtum macrocarpum 

 in the vineyards of southern France; and Spartina townsendi in the tide 

 basins of the canals, where it dispossessed the native S. stricta. 

 Similarly in North America the rivers of Florida were rapidly invaded 

 and navigation obstructed by the water hyacinth, Eichornia crassipes, 

 from Brazil, while the European moneywort, Lysimachia nummularia, 

 is found abundantly on many flood plains in the United States. The 

 seed and sprout production of these foreigners is so overwhelming, and 

 the distribution follows so suddenly, that a change of the local climatic 



