798 ECOLOGY 



mycosymbiosis. One theory is merely the revival of the old hypothesis that the 

 fungus alone is parasitic; favoring this view is the fact that in the bryophytes fungal 

 symbiosis seems to cause diminished rather than increased luxuriance, and that the 

 endotrophic mycorhizas of various plants (as the olive) is best developed on the 

 weaker rather than on the stronger trees; also it has been observed that the seed- 

 lings of oaks and chestnuts have been killed in large numbers when their ectotrophic 

 fungus is more vigorous than usual, and especially when it becomes endotrophic. 

 The latter instances, however, may be explained in line with the prevailing theory 

 as merely a disturbance of the usual balance between the two symbionts. The 

 other theory is that the fungus is a harmless endosaprophyte, living upon such root 

 excreta as tannin or upon such foods as sugar. Supporters of the theory of 

 parasitism and of that of endosaprophytism generally agree that the fungus alone 

 is benefited by the symbiosis, the digestion of the fungus by the root cells being 

 regarded merely as the destruction of a foreign organism, that is, as phagocytosis. 

 It is conceivable that all the theories here mentioned are valid, for the possibility 

 must be recognized that in the many cases of mycosymbiosis there is opportunity 

 for fundamental differences in the nutritive relations of the symbionts. 



Probably to be compared with the mycorhizas are the fungus-containing root 

 tubercles of Alnus, Shepherdia, and Elaeagnus ; as in the Leguminosae, cultures 

 of these plants without tubercles result in depauperate individuals, and there is 

 supplementary evidence that the fungi are able to fix nitrogen. As in the 

 orchids, the fungi may occur in hyphal clumps and may exhibit various stages of 

 disintegration, thus indicating digestion or phagocytosis by the root cells. A recent 

 investigator regards the organisms of the alder tubercles as bacteria rather than as 

 filamentous fungi, and he claims to have demonstrated the existence of infection 

 threads, rodlike forms, and bacteroids, as in the tubercles of the Leguminosae; 

 Myrica and Ceanothus are said to have similar root swellings containing similar 

 organisms. The roots of Cycas revoluta have coralloid galls which contain colonies 

 of bacteria, fungi, and blue-green algae. Recent studies appear to demonstrate 

 the existence here of reciprocal parasitism, the host cells showing structural altera- 

 tions, nuclear degeneration, and the loss of starch, while there also occur clumps of 

 fungal hyphae in various stages of degeneration; after the digestion or phagocyto- 

 sis of the fungi has taken place, the host cells contain various excretory products. 

 Unlike legume tubercles, cycad and alder tubercles are perennial. 



The significance of mycosymbiosis. The discovery of mycosymbiosis 

 alters previous conceptions concerning the prevalence of autophytism. 

 Apparently, ordinary green plants may be divided into autophytes and 

 mycophytes; the latter also include a few plants without chlorophyll. The 

 former are not only free from root fungi, but they also reach their opti- 

 mum development in sterilized soil, probably because their root hairs 

 are more imperfect absorptive organs than are fungal hyphae. Con- 

 trasted with these are the mycophytes, 1 which attain their optimum 



1 The relation here pictured also has been termed symbiotic saprophytism, since the 

 relation of the symbiotic complex to the soil is saprophytic. Most cases of so-called 



