VOLUME LI NUMBER 3 
DME 
DOTANICAL Caer 
MARCH roi 
THE CAUSES OF VEGETATIVE CYCLES: 
CONTRIBUTIONS FROM THE HULL BOTANICAL LABORATORY 143 
HENRY C. COWLES 
I. The demonstration of vegetative cycles 
The work of the past decade has shown most clearly that there 
are cycles of vegetation, which are comparable precisely to cycles 
of erosion; in each there is a period of youth, which is characterized 
by vigor of development and by rapidity of change; in each there 
is a period of maturity, and finally one of old age; which is charac- 
terized by slowness of transformation and by approach to stability, 
or at least to equilibrium. At the close of the vegetative cycle 
there is no such universal feature as the base level of the physiog- 
rapher, since the final vegetative aspect varies with the climate, 
and hence is called a climatic formation. In the eastern United 
States, the final stage is a mesophytic deciduous forest; farther to 
the north and in the Pacific states, it is a coniferous forest; in the 
great belt from Texas to Saskatchewan, the final stage is a prairie; 
’ and in the arid southwest, it is a desert. In every case, the ulti- 
mate or climatic plant formation is the most mesophytic which 
the climate is able to support in the region taken as a whole. Ina 
Prairie climate there may be trees, but they occur for the most part 
near lakes or streams, or in protected depressions, and in the base- 
leveling of the region they give way to the prairie; quite the same 
may be said of trees in a desert climate. 
It has been ascertained that the original plant formations in any 
habitat give way in a somewhat definite fashion to those that come 
"Address delivered as retiring president of the Association of American Geog- 
Ttaphers, Pittsburgh, December 29, I9IO. 
161 
