VEGETATION TYPES. 
181 
natural causes has had a profound influence on vegetation in this 
part of the world, and it is much more frequent in some types than 
in others, depending on the location with respect to bodies of water, 
etc., and also on the character of the vegetation itself, especially the 
amount of grass and of humus. Where fire is rarest woody vines 
are most abundant, and vice versa. 
Some of the relationships between the different vegetation types 
have already been pointed out in the descriptions. They can also 
be illustrated graphically and more concisely by diagrams or tables. 
Some plant sociologists have constructed diagrams in which all the 
different vegetation types or plant associations of a given area are 
connected by arrows showing the supposed direction of succession, 
i. e., the tendency of one type of vegetation to encroach on another. 
Although it is reasonable to conclude that of the vegetation types 
here described the sandy hammocks are tending to encroach on the 
scrub wherever the two are in contact, and would encroach on the 
high pine land if the frequency of fire was diminished, and the high 
hammocks may be slowly invading the red oak territory, we have 
little or no evidence of any other tendencies to succession in this 
area. Most of the types of vegetation are pretty closely correlated 
with soil and topography, and can hardly be expected to supersede 
one another without changes in soil or topography, and those take 
place so slowly that we cannot detect them. The vegetation is there¬ 
fore practically in a state of equilibrium (except for the interfer¬ 
ence of m'an, which is not taken into account here). 
The geographical (or more strictly speaking, chorographical) 
relations between the various types are shown by the vegetation 
map. Naturally there can be no suceessional relation between types 
which are nowhere contiguous, such as flatwoods and red oak woods. 
The fioristic relationships can be best exhibited perhaps by statis¬ 
tics showing how many species any two types have in common, not 
in absolute figures but in “coefficients of community/’ The coef¬ 
ficient of community between any two floras, or lists of plants, is 
the ratio of the species common to the two to the total number in the 
two taken together,* and is usually expressed as a percentage. The 
following table shows the percentage coefficients of community be¬ 
tween all the vegetation types herein described, except the last, 
*See Harshberger, Trans. Wagner Free Inst. Sci. 7:185-186. 1914. The 
total number of species in two areas is of course not the sum of the two taken 
singly, but that sum minus the number common to both. 
