Ig1t] COWLES—VEGETATIVE CYCLES 181 
able tree may even dominate in each of the stages. The Douglas 
spruce differs from the amphibious plants in that it exhibits no such 
striking changes in leaf habit in the different conditions in which it 
lives; however, the change in the accompanying vegetation is much 
more profound than in the swamp, for at the outset the Douglas 
spruce may be accompanied by xerophytic pines and junipers, and 
at the close by the mesophytic hemlock and by a luxuriant carpet of 
mesophytic ferns and mosses. Thus it is clear that the life range 
of some plants is very broad and of others very narrow; obviously 
the latter are the best markers of habitat dynamics, for with a 
change of conditions they soon give way to other forms. Of 
especial interest to the physiologist is the situation in such plants 
as the Douglas spruce, whose leaves without change of form or 
structure seem equally fitted for light or shade, for dryness or 
humidity. 
7. Conclusion 
It is not to be supposed: that all the influences which are in- 
volved in plant succession have been outlined in the preceding 
Pages. Indeed, some minor contributory factors have been purpose- 
ly omitted, because of the brief time allotted upon such an occasion. 
However, it is to be hoped that the dominating factors, so far as 
known at present, are here mentioned. From a survey of the 
various agencies involved, it seems clear that the influences which 
bring about succession differ profoundly in their nature, and also 
In the rapidity of their: action. Although they grade into one 
another as do all phenomena of nature, we may recognize climatic 
agencies, which institute vegetative cycles whose duration is so 
long that the stages in the succession are revealed only by a study 
of the record of the rocks. Within one climatic cycle there may 
be many cycles of erosion, each with its vegetative cycle. The 
trend of such a cycle can be seen by a study of erosive processes 
as they are taking place today, but the duration of the cycle is so 
ong, that its stages can be understood only by a comparison of one 
district with another; by visiting the parts of a river from its source 
to its mouth, we can imagine what its history at a given point has 
been or is to be. Within a cycle of erosion there may be many 
Vegetative cycles, and among these are some whose duration is so 
