366 Relations between Species 



Rhizobium, which form nodules on the roots of leguminous plants and 

 live symbiotically with their hosts (Fig. 10.2). The bacteria are 

 somewhat specific as to host, and obtain carbohydrate and other sub- 

 stances from its juices. In return the rhizobia fix gaseous nitrogen 

 and pass it on to their plant host. If available fixed nitrogen is absent 

 from the soil, the plant is completely dependent upon its symbiotic 

 nitrogen-fixing bacteria. 



Fig. 10.2. Vertical section through the root of a soybean plant, showing the root 

 nodules within which Hve symbiotic bacteria { Rhizobium japonicum) . 



A somewhat similar mutualism is seen in the fungi that form mycor- 

 rhizal structures either inside the roots of certain plants or on their 

 outside surfaces (Fig. 10.3). Ectotrophic mycorrhizae are found on 

 various kinds of trees such as pines, oaks, hickories, and beech; endo- 

 trophic mycorrhizae occur in the red maple and are particularly com- 

 mon in roots and other tissues of many orchids and heaths. Fungi of 

 this type are nourished by organic material that they absorb from 



