w; 



CHAPTER 3 



SYMBIOSIS 



Now for as much as the Crocodile sojourneth in the water, 

 he hath his mouth all full of leeches within. Whensoever he 

 goeth up out of the water on the land, and thereafter yawneth 

 (which he is wont commonly to do when the west wind 

 bloweth) then entereth the Sandpiper into his mouth and 

 swalloweth down the leeches, and the Crocodile is pleased 

 at the help which he receiveth and hurteth not the Sandpiper 

 at all. 



Herodotus 



"HEN TWO different species of animal or plant live in close associa- 

 tion, from which they derive mutual benefit, the relationship is 

 known as symbiosis. A few biologists hold that the term implies 

 dependence and that its use should be restricted to those rare cases in 

 which neither partner can survive without the other. An assassin-bug, 

 [Rhodnius prolixus) and its symbiotic fungus [Actinomyces rhodnii) provide 

 a good example of this relationship. The fungus lives in the intestine of 

 the bug and is passed from generation to generation on the eggshells 

 and in its faeces which are then eaten by the nymphs when they hatch. 

 Without the fungus the insects' development only proceeds normally 

 until the fourth or fifth moult, and the majority fail to become adult. 

 The few which complete their metamorphosis are sterile and do not 

 reproduce. It is thought that the fungus provides a source of vitamin 

 B for the bug, essential to its proper development. At any rate the two 

 organisms are entirely dependent on one another for survival. The 

 majority of biologists, however, beheve that the word "symbiosis" 

 should not be interpreted too rigidly and can, therefore, be used to 

 describe any regular, though not necessarily obligatory, association in 

 which benefits are enjoyed by both partners. Thus, the world-wide 

 relationship between man and the cow is clearly symbiotic, although 



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