CHAPTER A III. 

 SYMBIOSIS. 



^ 14. MUTUALISM. 



]\Iutiuilisin, or Symbiosis in the stricter sense,^ has been 

 distinguished as a special case of parasitism. This condition 

 occurs when a parasite and its host mutually work for 

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

 nourishment. The lichens furnish the most conspicuous example. 

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

 the fungi with assimilated organic nutriment, the fungi pro- 

 viding water and dissolved salts for the algae. 



While 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 

 alga, a living organism originates, which in form, necessities, 

 and mode of life is quite ntiv, and differs completely from 

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

 alone reproduces itself; yet the alga occurs as a free organism 

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

 culture. This combination might perhaps be compared with 

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

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



1 The term Symbiosis was applied by De Baiy, (who introduced it), by Frank 

 and otiieis, to denote tliose cases where a cohalntation or partnership was. 

 observed to take place between two different organisms. (Frank, Lehrhuch (L 

 Botanil; 1892). " Mutualism " was first used by Van Beneden. 



