6 DEVELOPMENT AND STRUCTURE OF VEGETATION 



viduals. The latter exhibits within itself peculiar changes, 

 likewise rhythmic and progressive. These changes consti- 

 tute or produce the fundamental phenomena of vegetation. 



Prom the nature of this primary dependence upon envir- 

 onment, vegetation is essentially dynamic, but, on account 

 of its marked reaction upon the habitat, it exhibits a con- 

 stant tendency to become static. Its characteristic phe- 

 nomena, then, are manifestations of change, in which each 

 successive stage is less dynamic and correspondingly more 

 static. The physical and biological factors which constitute 

 a habitat are so diverse and their interaction so complicated 

 that complete stasis is never attained, though forest and 

 grassland sometimes closely approximate this condition. 

 Even in such formations, however, the facies alone are 

 static, the subordinate species being in constant readjust- 

 ment to slowly intensifying conditions. Furthermore, in all 

 formations, whether they have attained a degree of stasis 

 by progressive changes or not, there is still a seasonal 

 rhythm of appearance, reproduction and decadence, which 

 remains always and essentially dynamic. Finally, the phe- 

 nomena of vegetation illustrate this primary distinction be- 

 tween the dynamic and the static. Invasion and succession 

 are completely dynamic in their operation, i. e., develop- 

 mental, while association, zonation and alternation are re- 

 latively static, i. e., structural. The further consideration 

 of the dynamics of vegetation will, in consequence, be found 

 under invasion, and, especially, under succession. 



The fundamental phenomena of vegetation are associ- 

 ation, invasion, succession, zonation and alternation. Of 

 these, association represents the stage to which vegetation 

 has been brought by the cumulative changes of the past. 

 It corresponds essentially to the historical fact in vege- 

 tation. Invasion and succession are dynamic forces ro phe- 

 nomena which are at work modifying the present facts of 

 association. They are initiated by physical factors, and are 

 wrought out by the interaction of physical and biological 

 factors. Hence, they are representative of the physical fact 



