CHAPTER IX 



NUTRITIVE CONJUNCTIVE SYINIBIOSIS 



Conjunctive symbiosis is said to be nutritive whenever there is 

 a direct food relation between two or more of the symbionts. It 

 thus inchides all interrelations of a parasitic nature. By parasitic 

 nature we mean any case in which one organism absorbs food or 

 food materials directly from another. It obviously includes, there- 

 fore, not only the ordinary parasitic relations that we commonly 

 call disease but also the relations existing in all gall-like structures 

 where the gall-forming organism is parasitic on a host and often 

 the host is in turn parasitic on the gall-forming plant or animal. 

 Only some of the more common or otherwise interesting examples 

 of nutritive conjunctive symbiosis can be discussed in this chapter. 



63. Antagonistic and Reciprocal Conjunctive Symbiosis.— In 

 antagonistic nutritive conjunctive symbiosis one or more, but not all, 

 of the svmbionts derive food or food materials from other symbionts 

 while those from which the food is taken receive no benefit but often 

 great injury from the relationship. Ordinary parasitism as exempli- 

 fied by disease, some mycorrhizas, and very many kinds of galls 

 belong to this type of symbiosis. 



Reciprocal nutritive conjunctive symbiosis differs from antago- 

 nistic symbiosis in that all of the symbionts receive benefit from the 

 symbiotic relation, usually in the form of food. Any organism that 

 receives food directly from another organism is parasitic. The 

 symbionts concerned in this type of symbiosis are, therefore, para- 

 sites and our commonest examples are those in which two unlike 

 kinds of organisms are living together in intimate contact and each 

 is parasitic on the other. There is thus a double or reciprocal 

 parasitism. Root and leaf tubercles caused by bacteria, lichens, and 

 some mycorrhizas are examples of this type of symbiosis. 



64. Parasites.— Parasites are plants or animals that derive food or 

 food materials from other living organisms. Parasites which lack 

 chlorophyll and so are dependent upon their hosts for all food are 

 called holoparasites while those that have chlorophyll and thus are 

 able to manufacture carbohydrates are called partial parasites. 

 The latter obtain food materials rather than food from their hosts. 

 It is not known in most cases whether parasites, either holoparasites 



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