956 BATTLES OF PLANTS IN THE WORLD 
singular but beautiful and important members of 
a forest society. Some of them, however, become 
enemies of trees, entering at wounds and rotting out 
the heart. Others attack the leaves, and by injuring 
or destroying these food-getting organs weaken the 
life of the tree. Others attack branches and deform 
or blight them. 
Mosses and lichens, in the temperate and arctic 
forests, greatly influence the character of the tree 
trunk which they cover and colour. Those hanging 
on branches give a grotesque appearance but seldom 
do injury. In sub-tropical and tropical forests there 
is a tendency to a change of position of the smaller 
members of the society from the forest floor to the 
tree tops, where hanging moss and tree-dwelling orchids 
and ferns abound. 
Desert societies. The oddest looking of plant societies 
are desert societies,—the great trunks of different 
kinds of cactus, with no leaves on them, or the 
sprawling opuntias, many of the cacti covered with 
spines. These large fleshy trunks do not lose water so 
rapidly as thin leaves do; so these plants are well 
suited to grow in the dry climate of the desert, where 
the soil is often parched and little water can be found 
by the plants. This character of the vegetation is the 
result of ages of warfare with uncongenial conditions. 
All plants not suited to grow here either have been 

