CHAPTER II. 



SYMBIOSIS. 



INTRODUCTION. 



Since Schwendener's epoch-making researches, which have fully 

 established the dual nature of lichens, other investigators have made 

 valuable discoveries in regard to the nutritive interdependence of 

 different organisms. The student will readily comprehend the im- 

 portant bearing this subject has upon the study of lichens. A gen- 

 eral consideration of the subject of symbiosis will enable us to have 

 a more definite opinion in regard to the life-history of the individual 

 lichen. 



The study of the symbiotic relationship (consortism of Reinke, 

 symbiosis of De Bary) of different organisms is of comparatively 

 recent origin. As generally admitted, the phenomenon of sym- 

 biosis was first explained by De Bary in the year 1879 l . By symbi- 

 osis this author understood a form of nutritive commensalism or con- 

 sortism of different organisms which proved mutually beneficial. It 

 was, therefore, placed in opposition to parasitism, which is a form of 

 cpmmensalism in which one of the organisms is benefited at the ex- 

 pense of the other. Within recent years the term symbiosis has been* 

 used to designate all forms of commensalism, whether parasitic, in- 

 different or mutually beneficial. It is in this broader sense that the 

 term is here used. 



We shall now briefly consider the gradation from antagonistic 

 symbiosis (parasitism) to the most highly developed mutualistic sym- 

 biosis (individualism) as we find it in lichens. 



I. ANTAGONISTIC SYMBIOSIS. (PARASITISM.) 



It is assumed that the student is sufficiently familiar with the an- 

 tagonistic symbiosis of fungi with higher plants as well as with ani- 



1 The phenomenon of symbiosis had, however, been previously explained by 

 Reinke. lie used the term " consortism " to distinguish this relationship from true 

 parasitism. His views are explained in the following publications : 



Reinke, J. Gottinger Nachrichten, p. 100. 1872. Reinke, J. Morphologische 

 Abhandlungen. Leipzig. 1873. Reinke and Grisebach, A. S. Oersted's System 

 der Pilze, Lichenen und Algen. 1873. 



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