146 PLANT RELATIONS. 



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 wictaly different regions of the 

 plant kingdom. Tt 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 



