712 



ECOLOGY AND EVOLUTION 



bare rock surfaces. Therefore, the alga 

 doubtless has a greater population as the 

 result of its symbiosis. However, as in the 

 legumes, there is no proof that selection 

 pressure through the association with the 

 fungi has modified the germinal constitu- 

 tion of the alga. With our present know- 

 ledge, Hchens cannot be used as a convinc- 

 ing example of the evolution of mutuaUsm. 



The mycorrhizal association, in which an 

 enveloping sheath of fungus myceUum 

 surrounds short thick rootlets of flowering 

 plants, may be beneficial to both partners, 

 but the interdependence of the two plants 

 has not been adequately proved. Eighty 

 per cent of flowering plants in both tem- 

 fungi form associations with a single 

 perate and tropical habitats have mycor- 

 rhizal associations. A number of different 

 species of flowering plant, and the same 

 species of fungus may be found in the 

 mycorrhiza of different species of flowering 

 plants (Rayner and Neilson- Jones, 1944). 

 Mycorrliiza may be examples of mutuaUsm, 

 but further experiments are necessary to 

 prove the point conclusively. 



Some instances of interdependence be- 

 ^tween orchid roots and fungi are known. 

 The Indian Pipe {Monotropa uniflora), a 

 saprophytic plant related to the heath 

 family, is also dependent upon a fungus 

 for nutrition, and the fungus seems to have 

 developed its mutualism from a more 

 ancient parasitism (Freeman, 1937). 



Intracellular and intestinal yeastHke and 

 bacterium-like symbiotes Hving within the 

 bodies of insects have been described in 

 great numbers, and these micro-organisms 

 may possibly be examples of the evolution 

 of extreme mutuaUsm. The details of the 

 functional relationships of these organisms 

 with their hosts are still vague, and it may 

 be that many of the species are actually 

 parasitic, their hosts having moved far in 

 the direction of toleration. It has been 

 found, however, that by injecting penicillin 

 into roaches, the bacteroids (cells in the 

 fat body filled with bacteria) are destroyed, 

 and with this destruction the roach dies 

 (Brues and Dunn, 1945). The drug does 

 not seem to produce direct toxic effects on 

 the roach. These experiments make it ap- 

 pear that there is probably a mutualistic 

 relation between roach and micro-organism 

 (Altenburg, 1946). Other cases are re- 

 viewed by Portier (1918), Buchner, 



(1921), Uichanco (1924), Steinhaus 

 (1946), and Hughes-Schrader (1948). 



Cleveland, Hall, Sanders, and Colfio' 

 (1934, p. 317) briefly discuss the wood- 

 feeding roach, Panestliia javanica, which 

 does not harbor intestinal flagellates, but 

 has an enlarged foregut, from which a bac- 

 terial organism has been isolated that di- 

 gests cellulose rapidly in vitro. Cleveland 

 suggests that this species of roach and 

 other wood-eating insects without symbiotic 

 protozoans may have developed a mutuahs- 

 tic relation with bacteria. 



Hungate (1944) isolated an anaerobic 

 bacterium {Clostridium cellobioparus) from 

 the rumen of cattle. This organism was still 

 capable of digesting cellulose after four 

 years of pure culture in an inorganic me- 

 dium with the addition of biotin and a 

 carbohydrate. The chief digestion product 

 of cellulose proved to be cellobiose, and 

 not cellulose dextrins or glucose. The or- 

 ganism can ferment glucose, but glucose 

 is not a normal product of cellulose hy- 

 drolysis by this bacterium. If the host ab- 

 sorbs much material from the rimien, it 

 would have to be the products of fermen- 

 tation rather than sugars. 



Certain green-colored flagellates {Zoo- 

 chlorella) and certain yellow or brown-col- 

 ored flagellates (Zooxanthella) are often 

 closely associated with different animals 

 (protozoans, sponges, coelenterates, turbel- 

 larians, bryozoans, rotifers, mollusks, anne- 

 lids, ascidians, and the eggs of amphib- 

 ians). These symbiotes are justifiably 

 classified as algae or as protozoa. Zoo- 

 chlorellae are most common in fresh-water 

 hosts, and zooxanthellae in marine hosts. 

 It is possible that the flagellate gains some 

 benefit from the carbon dioxide and nitro- 

 genous excreta of its host, and from its 

 position and protection. The host may gain 

 oxygen and carbohydrates from the sym- 

 biote, and have metabohc wastes removed. 

 Many of these symbiotes, especially the 

 zooxanthellae, are probably never free-living 

 in nature, but are always associated with 

 animals upon which they depend. The 

 animal, in these instances, is not depend- 

 ent upon the flagellates. 



The turbellarian, Convoliita roscoffensis, 

 does not harbor green flagellates at the 

 time of hatching (Cleveland, 1926; Yonge, 

 1944). At first the turbellarian is holozooic, 

 but it soon acquires flagellates (probably 



