IXTERDEPEXDEXCE OF ORGANISMS 7 



potato a wild, straggling, solitary weed in the hills and 

 propagates it and covers whole acres with it. Thus he 

 disturbs the balance, for the enemies of the corn and 

 potato, with food supply enormously increased, multiply 

 and spread as never before; and to maintain his artificial 

 conditions man must continually put forth his hand to 

 assist his chosen species and to stay their enemies. A few 

 species (such as the bison and the auk) he may by persistent 

 slaughter exterminate; a few (such as cockroaches and 

 some weeds) multiply in spite of his efforts against them; 

 but most are merely held in check, and when his efforts 

 against them cease, they speedily reoccupy their former 

 place. 



All economic procedure that deals with plants and 

 animals is based upon the knowledge of their relations to 

 each other. The arts that feed and clothe the human 

 race make progress as such knowledge is advanced. 



It is the purpose of the studies of this chapter to give a 

 closer acquaintance with some common phenomena il- 

 lustrating close interdependence and intricate vital relations 

 between organisms. Three subjects have been selected as 

 especially available and serviceable to this end: 



1. The relations between flowers and insects. 



2. Galls. 



3. The relations between ants and aphids. 



THE RELATIONS BETWEEN FLOWERS AND INSECTS. 



Inter-relations of mutual advantage are excellently 

 shown by flowers and insects, and may be studied anywhere 

 during the flowering season. The more abundant the 

 flowers, and the more sunshiny the weather, the better will 

 be the opportunities. Two products of the flowers are 

 eagerly sought by insects for food, nectar and pollen: 

 Nectar is the sugary sap of the plant secreted by nectaries 



