THE INTERDEPENDENCE OF LIVI\(. TIIIN(;S 



W I III III I'll rci 



An encnistiiif^ licht-n. Why docs lliis pl;inl suc- 

 ceed in such an unfavorable cm ironiucul i' 



mutual partnership in 



an interesting way. A 



lichen is composed of 



two kinds of plants, a 



green alga and a fungus, 



one of which at least 



may live alone. The 



two plants form a part- 

 nership for life, the alga 



making the food and 



nourishing the fungus, 



while the latter gives the 



alga raw food materials, 



protects it, and keeps 



it from dying when the 



humidity of the air is low. 



Other examples are bac- 

 teria and the mycelial 



filaments of fungi {my- 



corhiza) which live sym- 



biotically on the roots of certain plants, taking food from the plants, 



but giving them nitrogen in a usable form in return. 



A common example of symbiosis between plants 

 and animals is the green Hydra {Chlorohydra viri- 

 dissima), which holds in its body wall a unicellular 

 alga known as Zoochlorella. These plants contain 

 chlorophyll, using the sun to make food. In this 

 partnership, the algae get carbon dioxide and ni- 

 trogenous wastes from the animal, to which, in 

 turn, they give food and the oxygen set free in the 

 process of starch-making. There are numerous 

 examples of this kind of symbiosis in tlio animal 

 world, as is seen in many of the protozoa, sjionges, 

 A root' tip of the coelenterates, flatworms, molluscs, and sea urchins. 



European beech xhe symbiotic relationship of animals to each 



{Fagus sylvalica), ^^^ -^ ^j^^^^.^^ y ^j^^ ^j, protozoans iixing in 



showing ectotrophic '^ „ . i •. * 



mycorhiza, the fun- the digestive tracts of termites or white ants. 



gal hyphae forming These Httlc animals act as digestive cells for the 

 :nS:the?tLtLt termites, making it possible for them to use 

 -After Frank. wood fibers on which they live. In return th(> 



H. W. H. — 5 



