CHAPTER XXIV. 

 SYMBIOSIS. 



455. Living contact. — Not only are different species as- 

 sociated through the influence of similar surroundings which 

 they find congenial, but certain plants adapt themselves to 

 such an intimate relation with others that they live in imme- 

 diate contact with them. This intimate association is known 

 as symbiosis. When the parties to symbiosis stand to each 

 other in the relation of partners, each furnishing certain 

 materials or conditions advantageous to the other, the asso- 

 ciation is called mutualislic sytnbiosis or mutualism. A\'hen the 

 relation of the parties is that of master and slave, one indi- 

 vidual deriving advantage from the labor of the other and in 

 return furnishing it suitable conditions for existence, the 

 association is a form of mutualism known as helolism. Finally, 

 when the relation of the parties is that of an unwilling host 

 and an unwelcome guest, one individual being fastened upon 

 by the other from whose presence it is unable to free itself, 

 the symbiosis is called parasitism. (See ^\ 51, 52, 53, 

 222. ) 



A. Mutualism. 



456. I. Between plants of the same species. — Mutual- 

 ism may occur between individuals of the same species. 

 Illustrations of this are to be seen in the massing of the 

 lower algfe into colonies, in some of which certain individuals 

 may be differentiated from others for the purpose of carrying 

 on a fimction of advantage to the colony. (See 12, 13, 



333 



