SAPROPHYTISM AND SYMBIOSIS 797 



tween leguminous plants and their tubercle bacteria. Careful study in 

 certain forms has shown that starch and other carbohydrates disappear 

 from the root cells coincidently with an obvious increase in the fungal 

 cytoplasm. In some forms there is evidence that food passes from 

 the root to hyphae in the soil, one observer regarding the root tannins as 

 the source of food. On the other hand, it has been observed that the 

 entrance of fungi into a root may be followed by increased activity in 

 the root cells and by nuclear enlargement, and also by a considerable 

 increase in proteins; in Podocarpus proteolytic enzyms appear upon 

 fungal entrance. It is now thought that a number of fungi are able to 

 fix nitrogen; among these are such common forms as Aspergillus and 

 Penicillium. It has been shown that the endotrophic fungus of Podo- 

 carpus is able to fix nitrogen, and several species of Phoma that have been 

 taken from the roots of ericads have been found able to fix nitrogen more 

 economically than Azotobacter. Thus it is becoming increasingly likely 

 that root fungi are important sources of nitrogenous compounds for 

 mycophytes. However, experiments on the orchid fungi as yet give 

 negative results, and it may be that nitrogen fixation is a function of cer- 

 tain root fungi rather than of root fungi in general. Also of interest 

 in this connection is the demonstration of nitrogen fixation by the para- 

 sitic fungi that frequently inhabit the aerial organs of Lolium, result- 

 ing in increased nitrogen content and greater vigor in the latter. 



From the foregoing it seems likely that the fungus appropriates carbon 

 compounds from the green plant, while the latter probably appropriates 

 nitrogenous substances from the fungus. It has been suggested also 

 that ectotrophic fungi and even endotrophic fungi may take the place 

 of root hairs as organs of absorption of water and salts; in any event 

 root hairs are scarce in mycophytes. Since fungal hyphae are better 

 absorptive structures than are root hairs, mycophytes may be better off 

 nutritively than autophytes, especially if they are able to utilize the 

 substances absorbed from the soil by their fungi. 



In cases of mycosymbiosis where neither symbiont is green (as in Monotropa and 

 Corallorhiza), the gain to the fungus is less evident and to the larger symbiont more 

 evident than usual. Monotropa appears usually to be entirely dependent upon its 

 fungus, since its roots are completely invested; sometimes, however, Monotropa has 

 elongated roots without fungal sheaths, a fact which seems to indicate that its 

 nourishment then is derived saprophytically. Corallorhiza must also depend upon 

 its fungus, unless it is able to absorb food saprophytically from the humus, a matter 

 that is as yet uncertain. 



Recently there have come into prominence two further theories concerning 



