300 BIOLOGY AND HUMAN LIFE 



cow and the soft-eyed deer browse on the herbage and are direct 

 destroyers of Hving things. Many of the predatory animals, 

 those that catch their prey, are in turn exposed to predatory 

 enemies : a frog catches a worm and is devoured by a snake ; 

 a bird eats an insect and is then killed and eaten by a cat; 

 and so on. 



A third means of obtaining food is to take it directly from 

 the body of a living organism without necessarily killing the 

 organism. In this way there are fungi living on the bodies of 

 almost every species of green plant. There are many species of 

 protozoa, worms, and other classes of animals living on the 

 bodies of almost every species of vertebrate. Plants and ani- 

 mals that get their food in this way are called parasites. 



We sometimes find an association of two species that appears to be a 

 parasitic one (that is. of one organism living upon and at the expense of 

 another), but in which the relation is really different. For example, on 

 the roots of plants in the bean family (clover, peas, vetches, lupines, 

 etc.) are found little swellings, or tubercles, containing millions of bac- 

 teria. These bacteria hve by using some of the food material in the 

 cells of the roots. So far, then, they appear to be parasites. But a closer 

 examination shows that these bacteria absorb and combine nitrogen 

 from the atmosphere, and so supply the host with material for protein- 

 making. There is thus an exchange of materials or services, and such a 

 relationship is one of mutualism (see Fig. 144).^ Certain species of ants 

 may be seen to ''milk" plant lice by stroking them with the antennae and 

 feeding upon the secreted juices ; but, on the other hand, the ants are 

 said to protect the plant lice from other enemies, and thus to give some- 

 thing in return for the food which they obtain. There are many other 

 cases of mutualism between different species. 



232. Parasite and host. The plant or animal upon which or 

 in which another organism finds its food is called the host. 



&•■ 



1 After a crop of clover has been grown on a piece of land, the soil will 

 contain more nitrogen than it did at the beginning of the season. Accordingly 

 the plants of the bean family not only contain a larger proportion of nitroge- 

 nous compounds (proteins) than most other plants, but are of sF)ecial value 

 in the rotation of crops. After a series of grain or root crops, for example, 

 have been removed from a piece of land, a season or two of a leguminous 

 crop will restore the removed nitrogen to the soil. 



