No. 17] FLOWERING PLANTS AND FERNS 27 
succession is from extreme hyrophytic¢ conditions to mesophytie. 
Succession may thus progress along two lines, from hydro- 
phyte societies on the one hand, and xerophyte societies on the 
other, in either case tending toward mesophyte societies. Un- 
disturbed successions tend always toward mesophytie condi- 
tions, and generally terminate in the most mesophytie plant 
associations permitted by climate. These associations are 
called climax associations, and become static—no further 
change being possible, except through changes in the factors 
that influence vegetation. 
