Kelley — 180 — Mycotrophy 



(3) The hyphae break down. By dissociation of complexes in the 

 cell-sap, free H-ions are left in solution. These ions, acting on the 

 fungal arbuscle or the undifferentiated hypha, cause it to break down 

 and extrude its plasm into the host-cell. Demeter (1923) had shown 

 the breaking down of hyphae in vitro at an optimum acidity of 

 0.025N HCl. This concept is in agreement with Magrou (1921), 

 who said that the fungus is limited to certain parts of the plant 

 through toxic constituents of the cell-sap. It is also indicated by 

 Hatch's (1937) statement that susceptibility to infection by mycor- 

 rhizal fungi is apparently controlled indirectly by the internal concen- 

 tration of nutrient elements in short-roots. These "toxic substances" 

 are apparently ions normally present and not special humoral bodies. 

 RouTiEN & Dawson (1944) suggest an increased H-ion output in the 

 mycorrhiza, arising from carbonic acid, but leave unsettled the ques- 

 tion of its origin. 



(4) The fungal material is digested. The presence of proteolytic 

 enzymes enables the host's digestion-cell to utilize the extrav- 

 asated plasm of the fungus. Hitherto the hypha was utilizing the 

 host's substance; but now the host gets back not only what it had 

 previously lost but all that the fungus brought in from the soil. In this 

 sense there is a total intake of mineral salts, organic substances and 

 water by the mycorrhiza, but all combined as protoplasm of the fungus. 



(5) The digestion-area is strictly localized. Since the ionizable sub- 

 stances which pass from the stele to the cortex are subject to definite 

 physical laws, the rate of diffusion is specific for a given sort of plant, 

 being conditioned by the nature of the substances through which 

 diffusion must take place. For this reason, phagocytosis must neces- 

 sarily be limited to a certain region of the cortex. "I think," said 

 Emberger (1924), "that localization of infection is conditioned by 

 differences of osmotic pressure." 



The apical meristem and other growing points are richly supplied 

 with ionizable substance by the flow of liquid materials into such 

 regions. Through these rich supplies, actively growing tissues can 

 readily repel the endophyte by breaking it down at a distance from 

 the meristem to which the ionizable substances extend. Chloroplasts 

 in green tissues and perhaps leucoplasts in tubers probably exert 

 a similar influence. 



(6) The mechanism of phogocytosis is apparently ionic. A plant 

 is not static, and the more active its growth the more ionizable material 

 it will have at its disposal, and the more certainly will the fungus 

 be destroyed in its tissues. Hence it may be understood what Reed 



