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 species of insects to pollina'te 

 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- 

 eties. 



Hydrophytic Societies. — These are the societies of water 

 plants called Hydrophytes and include plants which live sub- 



