' 180 BOTANICAL GAZETTE [MARCH 
When a forest is destroyed by cutting, the succeeding vegetation 
commonly is more xerophytic than that which-was destroyed, 
because of increased light and decreased humus. The influence 
of fires is much more retrogressive, because the vegetation of the 
forest floor, as well as the trees, is destroyed, and also because the 
humus is more largely oxidized. Both in such areas as these which 
gradually return to the forest, and in other areas which are pre- 
vented from making such return, on account of their use for 
cultivation, or for habitation, or for grazing animals, there enter 
among the pioneers a large number of cosmopolitan weeds which 
follow in the train of man. Most of these weeds are of xerophytic 
tendencies, and hence are well fitted for these pioneer stages. In 
the revegetation of fallow land and in reforestation, these immi- 
grants soon disappear, giving way before the returning native forms 
which inhabited the region before man entered with his destructive 
axe and torch. 
#, PLANT PLASTICITY 
Before concluding this section on biotic agencies, there should 
be noted some instances where dynamics in the habitat meets 
with a reaction other than that of succession. Very frequently 
in the draining of a pond by humus accumulation, the same plants 
may be found in different stages, but characterized by a change of 
aspect. For example, the mermaid weed (Proserpinaca), the water 
hemlock (Sium), and the water smartweed (Polygonum amphibium) 
are fitted for existence in a shallow pond and also in a swamp where 
the soil level is above the water table. In the former instance the 
plants possess so-called water leaves, which vary greatly in form 
and structure from the air leaves, which are seen in the following 
swamp stage. Such amphibious plants thus have the power 
through their great plasticity of existing in two distinct plant 
formations; many of their companions, however, in the two situa- 
tions are quite unlike, indicating that the habitat range of the 
latter is narrower, on account of their smaller plasticity. : 
In the western forests, the Douglas spruce may be a xerophytc 
pioneer, and yet may remain through all the stages of forest devel- 
opment, including the culminating mesophytic forest; this remark- 
