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. 
