246 



ANALYSIS OF THE ENVIRONMENT 



zation of animal bodies, in special forms of 

 Protozoa, Porifera, Coelenterata, Platyhel- 

 minthes, Aschelminthes, and Mollusca by 

 green algae— the zoochlorellae— or by the 

 yellow or brown zooxanthellae (flagellates), 

 is well known. Familiar animal hosts arc 

 Amoeba viridis, Chlorohydra viridissima, 

 and the flatworm Convoluta roscoffensis of 

 the European sea coast. The vast extent and 

 biological importance of this type of mutu- 

 alism are evident when it is recalled that 

 zoochlorellae and zooxanthellae are present 

 in the individual polyps of most reef corals. 

 A high perfection of such metabolic equilib- 

 rium between host animal and guest plant 

 is indicated by the long life of Chlorohydra 

 viridissima, with its zoochlorellae when it 

 is sealed ofiF in water in a glass tube (Buch- 

 ner, 1930). 



The studies of Yonge and A. G. Nichols 

 (1931) on the relation of zooxanthellae 

 and coral polyp indicate that the host polyp 

 is not dependent for carbohydrates or pro- 

 teins on its plant associate; but they leave 

 unquestioned the mutual benefit of oxygen 

 supplied to the polyps and carbon dioxide 

 to the zooxanthellae, plus benefit of removal 

 of nitrogenous wastes, in this partnership. 

 Their studies suggest that nutritional 

 aspects of the plant-animal mutuafisms re- 

 cently enumerated require experimental 

 re-examination. 



The breadth of the physiological basis 

 for such metabohc mutualism is shown by 

 the ingenious experiments of the Buchs- 

 baums (1934) who showed that when a 

 culture of the green alga Chlorella is com- 

 bined with embryonic chick tissue cells, 

 both are evidently favored, as compared 

 with either algal or tissue culture alone. 



These metabohc relations of plant and 

 animal may be as intimate as the mutualistic 

 relations of plants with plants. Algae and 

 fungi associate to form the varied group of 

 lichens, which, from their successional posi- 

 tion on bare rock and from their abundance 

 under the severe chmatic conditions of the 

 tundra, may be supposed to carry this type 

 of mutualism backward toward the earliest 

 geologic times in which life was present. 

 The equally intimate association of fungi 

 with the roots of higher plants in the my- 

 corrhiza (apparently present in the majority 

 of plant species) are clearly symbiotic and 

 apparently mutualistic. The relation of fun- 

 gus to higher plant may be either extraor- 

 ganismic or intraorganismic, without much 



significant physiological difiference (Weaver 

 and Clements, 1929). The close relation of 

 nitrogen-fixing bacteria with leguminous 

 plants is discussed in Chapter 35 (p. 711). 



An extremely intimate type of plant-ani- 

 mal association into mutually beneficial 

 partnership, in which the animal is the dom- 

 inant partner, is exhibited by the series of 

 fungus-growing beetles, by the fungus-gar- 

 den ants, and the fungus-growing termites. 

 The relation of man with food plants that 

 no longer are found in the wild state, Hke 

 wheat or Indian corn, may be thought oi 

 as exhibiting essentially the same type of 

 relation. The fungus-growing forms repre- 

 sent sharply definable taxonomic groups, 

 which attests to the fixity of the relation. 

 The corresponding development of fungi 

 specific (as species) to the particular group 

 of beetles engaged in growing them appears 

 to be demonstrated. The agricultural status 

 of the fungi grown by ants and termites re- 

 sembles that of plants cultivated by man 

 that are not genetically distinct from natu- 

 ral populations, since the ant and termite 

 fungi are beHeved to occur independently 

 (seep. 714). 



Especially notable is a graded series of 

 increasingly complex means of transmittin.j 

 the fungus to new colonies among the var- 

 ious families of fungus-growing beetles, all 

 of which are wood-boring forms the larvae 

 of which are associated with the adults in 

 burrows in Hving or at least in sound wood. 

 Thus in certain platypodid beetles, the 

 spores of the fungus and fungus fragments 

 are carried by the adult female beetle in au 

 elaborate external apparatus on her head 

 from the burrow in which she has passed 

 her larval life to the new excavation in 

 which she will establish a new colony. In 

 the Scolytidae the fungus is carried in the 

 midgut and is regurgitated in the new bur- 

 row. The females of the beetles of the fam- 

 ily Lymexylonidae have an apparatus con- 

 nected with the ovipositor that effectively 

 smears the egg with fungus spores as it is 

 laid. An even more elaborate apparatus for 

 injecting mycelium and spores into a new 

 excavation is that of the wood-boring wasp 

 Sirex (and of closely related forms), in 

 which the whole fungus-insect relation is 

 still under investigation. 



The more advanced fungus-growing ants 

 are sometimes referred to as "parasol ants" 

 because of a fancied resemblance of the 

 green leaf fragments being carried into the 



