14:6 PLANT STUDIES 
amount of damage to plants. Many burrowing animals 
attack subterranean parts of plants, and interfere seriously 
with their occupation of an area. 
Various protective adaptations against such attacks have 
been pointed out, but this subject probably has been much 
exaggerated. The occurrence of hairs, prickles, thorns, 
and spiny growths upon many plants may discourage the 
attacks of animals, but it would be rash to assume that 
these protections have been developed because of the danger 
of such attacks. One of the families of plants most com- 
pletely protected in this way is the great cactus family, 
chiefly inhabiting the arid regions of southwestern United 
States and Mexico. In such a region succulent vegetation 
is at a premium, and it is doubtless true that the armor of 
thorns and bristles reduces the amount of destruction. 
In addition to armor, the acrid or bitter secretions of 
certain plants or certain parts of plants would have a 
tendency to ward off the attacks of animals. 
106. Plant rivalry, It is evident that there must be 
rivalry among plants in occupying an area, and that those 
plants which can most nearly utilize identical conditions 
will be the most intense rivals. For example, a great many 
young oaks may start up over an area, and it is evident 
that the individuals must come into sharp competition with 
one another, and that but few of them succeed in establish- 
ing themselves permanently. This is rivalry between in- 
dividuals of the same kind ; but some other kind of trees, 
as the beech, may come into competition with the oak, and 
another form of rivalry will appear. 
As a consequence of plant rivalry, the different plants 
which finally succeed in taking possession of an area are 
apt to be dissimilar, and a plant society is usually made up 
of plants which represent w T idely different regions of the 
plant kingdom. It is sometimes said that any well de- 
veloped plant society is an epitome of the plant kingdom. 
A familiar illustration of plant rivalry may be observed 
