MacDougal : Symbiosis and Saprophytism 527 



tion of the reproductive and other branches. The starch in the 

 apex of the coralloid mycorhiza is used in the construction of em- 

 bryonic tissue and a portion of it remains in the medio-cortex and 

 becomes available to the fungus as a highly advantageous food. 

 Janse and others have upheld the theory that endotropic my- 

 corhizas are similar in physiological value to leguminous tuber- 

 cles (ii), but the only actual proof adduced in favor of this view 

 is the evidence obtained by Nobbe and Hiltner from experiments 

 with Podocarpiis ( 1 2). That endotropic fungi may cooperate in 

 the fixation of free nitrogen in the roots of plants in which they 

 occupy only a portion of the absorbing system is readily ad- 

 missible and may be considered as proven. Such an explanation 

 is wholly inadequate to account for the arrangement of the my- 

 corhizal components and transpiratory structures in Corallorhiza^ 

 however, on purely anatomical grounds. The underground mem- 

 bers of this genus are furnished with a complete sub-epidermal 

 sheath of mycelium, which fills every cell of the outer cortex in 

 two or three layers, except a minute area at the tip of the coral- 

 loid branch, and usually the 10 to 15 layers of the medio-cortex. 

 It is obviously impossible for the CorallorJiiza to absorb substances 

 from the soil except through and by the agency of the fungus. 

 The fungus may be capable of accomplishing the fixation of free 

 nitrogen, but that it is not its sole, or its major function in the 

 symbiosis, since all of the food-material of the association must 

 pass through its hyphae ; a statement equally true of such forms 

 of ectotropic mycorhiza as those of Ptcrospora, Monotropa, etc. 

 The higher plant affords a lodgment for the fungus, from which it 

 sends out absorbent and reproductive branches. Food-material 

 taken in by the fungus is yielded to the higher plant and consti- 

 tutes its sole supply. To this extent the higher plant is parasitic 

 upon the fungus. But the higher plant accomplishes transforma- 

 tions of chemical energy in the food thus obtained of which the 

 fungus is incapable and yields the elaborated product in an ad- 

 vantageous form in the apex of the mycorhiza, where it serves as 

 a food for the advancing mycelium. The higher plant is, there- 

 fore, not a fungus-trap pure and simple, as the association is of 

 ^reat mutual advantage. 



The principal conclusions which may be drawn from the facts 



