116 NUTRITIVE CONJUNCTIVE SYMBIOSIS 



in nearly every direction. In going toward the poles the last plants 

 one sees are pretty certain to be lichens. At the tops of the highest 

 mountains the plants are almost exclusively lichens. At the tops of 

 tall tropical trees, if any plants are found growing as epiphylls on 

 the leaves that are fully exposed to the hot tropical sun, they are 

 lichens. 



The exact physiological relationship between the two components 

 of a lichen is still very imperfectly known although it has been 

 studied and discussed for a very long time. Some writers have 

 thought that a lichen represents a sort of partnership between the 

 fungus and the algse each partner supplying to the other certain 

 necessities of life. Others have taken a somewhat opposite view and 

 have believed the algte to have been enslaved by the fungus. Still 

 others have considered the fungus as an ordinary parasite on the 

 algae, or have said that the fungus is diseased by the algse. It is 

 improbable that the physiological relationship between lichen- 

 fungus and alga is the same in all cases. It is reasonably certain, 

 however, that in the majority of lichens the fungus obtains organic 

 food from the algae, either as a parasite on the living gonidia or as a 

 saprophyte on dead ones. Similarly, it is certain that the algae 

 obtain water, at least, from the fungus, either directly or indirectly. 

 The relationship has apparently reached such a balance that it is 

 more or less normal for both fungus and alga, and both can endure 

 it, therefore, without suffering. This, however, must not be taken 

 to imply a sort of reciprocity agreement under which each party 

 supplies something to the other. Rather each party takes all it 

 can get from the other. In other words the lichen represents a case 

 of double, or reciprocal, parasitism, and must be classified as recipro- 

 cal nutritive conjunctive symbiosis. 



REFERENCES 



Surges, A.: On the Significance of Mycorrhiza, New PhytoL, 35, 117-131, 



1936. 

 CosENS, A.: A Contribution to the Morphology and Biology of Insect Galls, 



Trans. Canadian Inst., 9, 297-387, 1913. 

 Darbishire, O. v.: Some Aspects of Lichenology, Trans. British Mycol. Soc, 



10, 10-27, 1924. 

 Fagan, M. M.: The Uses of Insect Galls, Am. Nat., 52, 155-176, 1918. 

 Felt, E. P.: Gall-insects and Their Relation to Plants, Scientific Monthly, 



16. 509-525, 1918. 

 Fink, Bruce: The Rate of Growth and Ecesis in Lichens, Mycologia, 9, 138- 



158, 1917. 

 Fry, E. Jennie: The Mechanical Action of Corticolous Lichens, Ann. Bot., 



4(158), 397-417, 1926. 

 Hatch, A. B.: The Roll of Mycorrhiza; in Afforestation, Jour. For., 34, 22-29, 



1936. 



