ECOLOGICAL 175 



( xtemal or chiefly external root-fungi serve to absorb water and 

 salts from the soil, and thus function as extraneous root-hairs. It 

 is noteworthy that many of the plants that have external root-fungi 

 are poorly equipped with root-hairs. As to the investing fungus, it 

 cannot be benefited unless it also sends hyphae into the root; but 

 when it does so there may be a profitable absorption of carbon- 

 compounds which the green plant has made. 



When the mycorhiza is strictly internal and intracellular, the 

 puzzle of interpreting the symbiosis is even greater. How is the 

 green plant benefited when the fungus has little or no connection 

 with the soil? And although the host often digests the fungoid 

 hyphae, this may be comparable to phagocytosis in animals, when 

 active amoeboid cells digest intruding micro-organisms. In the case 

 of the mycorhiza, the digestion may be the host's method of keeping 

 the fungus within bounds. One would not, indeed, press the point 

 that what organic material the flowering plant may absorb from 

 the endotrophic fungus has been previously absorbed by the fungus 

 from the cells of the flowering plant. For it may well be that the 

 organic matter is changed by the fungus into more useful form, just 

 as happens in the symbiotic organisms of some insects, e.g. cock- 

 roaches and wood-boring beetle larvae, which can alter the cellulose 

 of the wood into more useful form. But what these symbions digest 

 is usually pure cellulose, as is well illustrated by the symbiotic 

 Infusorians (Trichonympha, etc.) in termites; and this is against 

 our suggestion. The fact remains that in some cases of mycorhiza 

 there is an indubitable increase in the protein-content of the roots 

 after the fungoid infection has occurred. 



The advantage to the internal fungus is least obvious in cases 

 where the flowering plant is destitute of chlorophyll, as in the 

 peculiar Bird's Nest Orchis {Neottia nidus-avis), Indian Pipes 

 (Monotropa), and the rootless Coral-root (Corallorhiza), where the 

 mycorhiza is in the rhizome. The first of these plants, if not the 

 others as well, seems to have relapsed into complete saprophytism 

 or absorption of decaying organic matter in the soil ; and it is difficult 

 to say why the fungus should not do this sufficiently for itself, as 

 indeed so many fungi do. 



Perhaps we get some light from experiments, still few in number, 

 which indicate that certain internal fungi are able to fix atmo- 

 spheric nitrogen. This has been proved for the fungus of the orchid 

 Podocarpus and of some heaths. More facts must be established, but 

 in the meantime it seems safe to suspect that one of the ways in 

 which mycorhizal fungi may help their flowering partner is by 

 fixing free nitrogen. 



The remarkable symbiosis in the common heather has been well 

 worked out by Miss Rayner. This characteristic plant of the moor- 

 land, which flourishes in the unready soil where little else will grow, 



