CHAPTER VITI. 

 SYMBIOSIS. 



$ 14. MUTTALISM. 



Mutualism, or Symbiosis in the stricter sense, 1 has been 

 distinguished as a special case of parasitism. This condition 

 occurs when a parasite and its host mutually work for 

 the benefit of one another, each contributing to the other's 

 nourishment. The lichens furnish the most conspicuous example. 

 Here ftmgus-hyphae unite with algal cells, the algae furnishing 

 the fungi with assimilated organic nutriment, the fungi pro- 

 Nil ling water and dissolved salts for the algae. 



\Yliile it is by no means uncommon to find two organisms 

 taking a mutual advantage of each other, yet mutualism in its 

 strictest sense is a rare phenomenon. For it generally happens, 

 and is indeed to be expected, that one or both symbiotic 

 organisms modify in some degree their mode of life to suit the 

 altered conditions necessary for their mutual support. Thus 

 amongst the lichens, as a result of the union of fungus and 

 ali;a, a living organism originates, which in form, necessities, 

 ;ui<l mode of life is quite new, and differs completely from 

 either of its components. In the lichen-community, the fungus 

 aL me reproduces itself; yet the alga occurs as a free organism 

 in nature, while the fungus can only be reared in artificial 

 .-til tu re. This combination might perhaps be compared with 

 that iif oxygen and hydrogen to form water, also to a certain 

 extent with the union of the sexual cells to produce a new 



'Tin- term Symbiosis was applied by De Bary, (who introduced it), by Frank 

 and others, to denote those cases where a cohabitation or partnership was. 

 observed to take place between two different organisms. (Frank, Lehrbuch <l. 

 l;t nil,-, 1892). " Mutualism " was first used by Van Beneden. 



