330 INTRODUCTION TO BOTANY 



to corn. Their nourishment consists of the juices which they 

 suck from the tender roots. They excrete a sweetish sub- 

 stance called honeydew, which is used as food by ants and 

 sometimes by other kinds of insects. The aphids may begin to 

 live upon corn when the seedling is germinating, and continue 

 upon the growing plant until it is mature. The aphids are 

 sluggish insects and, although they reproduce rapidly when 

 food is abundant, they are not readily able to pass through 

 the soil or over its surface to the roots of 'new plants. 



There is a common black field ant which devours the honey- 

 dew, apparently with great relish. The burrows of these ants 

 may often be seen about the bases of corn plants. They dig 

 tunnels to the roots of the corn, then carry down some of the 

 aphids and place them upon the roots. There the aphids are 

 cared for by the ants, and the latter secure the honeydew as 

 food. Throughout the summer and autumn the ants con- 

 stantly care for the aphids and their young. Aphid eggs 

 are carried to the places that are most favorable for their 

 hatching, and when the young are hatched they are trans- 

 planted upon tender young roots. When disturbances of the 

 soil threaten destruction to the eggs, the ants seize them as 

 they would their own eggs and carry them away. At the be- 

 ginning of the winter aphid eggs are carried by the ants into 

 the deepest parts of the ant nests. At the return of the favor- 

 able season the eggs are brought forth again to places suitable 

 for hatching. In this case the aphids, which are parasitic upon 

 the corn roots, are themselves in slavery (helotism) to the ants, 

 and this interrelation obviously reaches a high degree of de- 

 velopment. When seed corn is treated with oil of lemon, the 

 aphids are said to be repelled, for a time at least, from the 

 young plants. Frequent cultivation disturbs the ant burrows, 

 and birds that prey upon ants, as flickers and woodpeckers, 

 also tend to reduce the damage done by the aphids. 



309. Resistance to disease. Every kind of economic plant 

 seems to have one or more plant or animal diseases. Each 

 presents special problems, many of which are yet unsolved. 



