ORGANIC MATTER AND NITROGEN 209 



with some certainty in artificial cultures, and which very possibly 

 occurs to some extent in farm manure from legume hay, may fur- 

 nish bacteria with feeble action for a time, but ultimately, no doubt, 

 with full power. Of course, this process of forcing bacteria to live 

 upon a legume to which they are not naturally adapted has little 

 or no practical value, because it is unnecessary, if there is a species 

 of bacteria which naturally live upon the same legume. On the 

 other hand, if, by any such process of breeding, or evolution, a 

 species of nitrogen-fixing bacteria could be developed which could 

 live on a nonleguminous plant, as corn, for example, it would be 

 of incalculable value. As yet, the efforts of bacteriologists, working 

 on this problem, have given only negative results, so far as known 

 to the author. 



Attention is called to the fact that there are numerous instances 

 where two different kinds of plants live together in intimate part- 

 nership relation. If only one of the two plants receives benefit 

 from this relationship or association, then the plant receiving the 

 benefit is called a parasite. Thus the mistletoe is a parasite upon 

 the elm or gum or other tree on which it lives. The mistletoe 

 draws its nourishment from the tree. The tree is injured rather 

 than benefited by the mistletoe. Dodder is also a parasitic plant, 

 living upon other plants, except during the early part of its growth. 

 Ticks and lice are common examples of animal parasites, living 

 upon other animals. 



In some cases a relationship exists which is not parasitic, but 

 symbiotic. The term symbiosis, which is commonly used by 

 biologists to define this relationship, means living together in 

 mutual helpfulness. The association of bees and flowers may 

 serve to illustrate this mutual helpfulness, although this is not an 

 example of intimate symbiosis. Thus, the bees obtain their food 

 from the flowers and, in turn, the flowers, many of them, are in- 

 capable of producing seed or fruit unless the pollen is carried from 

 the male flower to the female flower by bees or other agencies. It 

 is well known that plant lice and ants are mutually helpful. 



Likewise, the association of nitrogen-fixing bacteria and legume 

 plants is a relationship of mutual helpfulness, and this is one of 

 the best illustrations of what is meant by symbiosis. The legume 

 furnishes a home for the bacteria and also furnishes in its juice or 



