504 ECOLOGICAL CLASSIFICATION OF PLANTS 
Glories and Bindweeds cut off the light and break plants down, 
and Grape vines often injure the trees over which they spread. 
In a number of ways plants are adjusted to animals. The 
presence of thorns, stinging hairs, and bitter juices may pro- 
tect plants against destruction by animals. Insect pollina- 
tion is a notable example of the dependence of plants upon ani- 
mals. The flowers of some plants are so adjusted that they 
require insects and often certain specics of insects to pollinate 
them. Thus bees are required to cross-pollinate Red Clover, 
and Sweet Clover and Alfalfa, although they do not require 
cross-pollination, require insects which can trip their flowers, 
so that the pollen can get on the stigma. It is also recognized 
that bees are essential to good pollination in orchards. Orchids 
and Yuccas are two of the most notable examples of plants 
which have flowers so constructed that only certain types of 
insects can pollinate them. In such cases it is obvious that 
propagation by seeds depends upon the presence of the insects 
which are required to pollinate the flowers. For securing the 
dissemination of their seeds, plants are adjusted to animals in 
a number of ways, but chiefly by developing hooked or spiny 
fruits or seeds which cling to the coats of animals. 
The above factors with minor ones largely determine the 
modifications and distribution of plants. These factors work 
together and not singly, and the combinations of factors are 
numerous. According to their adjustment to the ecological 
factors, plants fall into groups or classes known as societies. 
Thus all plants adjusted to a water habitat belong to a 
hydrophytic society and are called Hydrophytes, while those 
adjusted to a drought habitat belong to a xerophytic society 
and are known as Xerophytes. 
Ecological Societies 
Since the ecological factors and their combinations vary 
widely, there are many different habitats and hence many eco- 
logical societies. With reference to the water factor plants are 
grouped into Hydrophytic, Mesophytic, and Xerophytic soci- 
etics. 
Hydrophytic Societies.— These are the societies of water 
plants called Hydrophytes and include plants which live sub- 
