372 PKACTICAL BOTANY 



are dependents. Usually the expression dependent organisms 

 refers to an intimate life association between two or more 

 organisms. All such life associations are included under the 

 term symbiosis. 



A good illustration of the different kinds of symbiosis is 

 sometimes seen in the common Indian-corn plant and other 

 plants and animals that live upon and within it. Upon the 

 growing cornstalk, ear, or tassel, the dependent corn smut 

 often appears (Fig. 197). Most of this fungus grows within 

 the tissues of the living corn plant, from which it secures its 

 food. This particular kind of symbiosis is known as parasit- 

 ism, and the dependent plant is known as a parasite. Later 

 the corn plant may die and fall upon the moist ground, when 

 molds and bacteria may grow upon it. These take their nour- 

 ishment from the dead corn plant, and as they do so they 

 assist in bringing about its decay. Such a relation is known 

 as saprophytism, and the plants or animals which live upon 

 dead plants or animals are known as saprophytes. 



Still another phase of dependency is often shown by small 

 insects known as aphids, or plant lice, one kind of which 

 thrives upon the roots of corn. Their nourishment consists of 

 the juices which they suck from the tender roots. They also 

 excrete a sweetish substance 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 ger- 

 minating, and continue upon the growing plant until it is 

 mature. Evidently these aphid insects are parasites. It is to 

 be further noted that they 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. Often about the bases of 

 corn plants the burrows of these ants may be seen. 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 



