664 General and Applied Biology 



In certain higher plants, especially trees, a close relationship may exist 

 between the mycelium of certain fungi and the roots. This association 

 does not seem to harm the tree, and the mycelium may form an encircling 

 mantle around the finer roots, causins; them to enlars:e and become much 

 branched. This close association of fungus and roots is known spe- 

 cifically as mycorrhiza (mi ko -ri' za) (Gr. mykos, fungus; rhizos, root), 

 in which both plants derive mutual benefit ; the fungi may prepare nitrog- 

 enous foods for the roots and receive food from the roots in return. 



Certain types of nitrogen-fixing bacteria live in the small swellings 

 (nodules) on the roots of leguminous plants such as clovers, alfalfa, 

 beans, soybeans, etc. These specific bacteria take free nitrogen from the 

 atmosphere and convert it into certain nitrogen compounds, which are 

 changed by other bacteria eventually into usable nitrates. The nitrogen- 

 fixing bacteria secure food and protection from the plant, while the lat- 

 ter profits from the foods formed by the bacteria. The nitrogen-fixing 

 bacteria (Fig. 325) which are present in the root nodules of plants are 

 refen^ed to as symbiotic nitrogen-fixing bacteria to diflferentiate them 

 from other bacteria which can accomplish a similar phenomenon, but the 

 latter bacteria live free in the soils. The latter phenomenon is known as 

 7ionsymbiotic nitrogen-fixation. 



III. COMMENSALISM 



An association of members of two or more species of organisms in 

 which one (commensal) is benefited but not injured while the other 

 (host) is neither benefited nor injured, but both using the same supply 

 of food, is known as com.mensalism (kom -en' sal izm) (L. com, together; 

 mensa, table, or food). In this type of association there is not quite the 

 close relationship of organisms as found in symbiosis. A special type of 

 tropical fish known as the shark sucker (Remora) attaches itself by means 

 of a sucker to the body of sharks, turtles, whales, etc. Part of the food 

 captured by the animal host is used by the Remora. In this case the host 

 does not appear to receive any benefit from the association. Certain small 

 birds, such as one of the grackles, may build nests near the nests of larger 

 birds, such as a fish-eating osprey, thus securing protection. The so- 

 called "rudder fish" secures shelter and protection from the stinging 

 tentacles of large jellyfish but apparently gives nothing in return, but 

 both may eat of the same food. Protozoa, yeasts, and fungi may live in 

 the digestive tract of man and other animals, doing neither harm nor good 

 but using some of the common food and receiving protection from the 

 host. 



