14 

 source of many of the foods, for example tlie nitrates and salts, 

 and is also tlae source of organic hurais. Soil is tlae best raediura 

 to keep plants stable and is also the best protection for plant 

 life and best for vegetative reproduction. The danger of too 

 little food supply is not so great as that of too little water. 



The relation between two life forms acting together is called 

 Symbiosis, and is sometimes defined as a relation which is mxitually 

 beneficial. Now the term includes all relations between plants 

 and animals. Insect and bird pollination are classed as symbiosis. 

 The relation of the pitcher-plant to animals is another example ; 

 dead animals producing food for plants ; destructive work of animals 

 influence of plants on plants ; relations of parasitisn as shown 

 in the dodder. The reflation of mutualism is beneficial symbifesis, 

 as oak and ivy ; Algae and fungus and lichens ; nitrogen tubercles 

 on clovers ; epiphytes^and lianas. A broader exainple is where a 

 shade plant is absolutely dependent upon the shade of another plant. 

 A still broader symbiosis is a plant society in which the individual 

 membeBS react upon one another. Another kind is the growth of one 

 kind of trees in soil where trees of other genera grew formerly ; 

 as- oaks succeeded by beech forests or vice versa. 



