Lecture XI — 161 — Theories 



Working with aqueous extracts of fall-litter, Melin found that 

 "all types of litter contained substances soluble in water which 

 favourably influenced mycelial growth of thiamin-heterotrophic fungi 

 (mycorrhizal Hymenomycetes and Gasteromycetes, as well as litter- 

 decomposing Hymenomycetes) in a nutrient medium containing 

 sugar, salts and thiamin. The litter extracts likewise exercised a 

 favourable influence on the development of thiamin-autotrophic soil 

 fungi." These results apply only to tree-litter examined and not to 

 litter from the grass, Glyceria. The litter extracts appear to contain 

 growth substance like Robbins' "Z" factor (Symbol. Bot. Upsaliensis 

 8(3) :1-116, 1946). 



Limitation of Mycotrophic Hypotheses: — It must be realized 

 in any study of mycotrophic hypotheses that the protagonists of each 

 hypothesis have been specialists in some one field of mycorrhizal 

 study, and their hypothesis naturally reflects experiences with their 

 own material. It may be that each investigator is correct, as un- 

 doubtedly he is so far as his understanding of his own data is con- 

 cerned, but no one person has yet had a broad enough grasp of all 

 phases of mycotrophy to develop an explanation which will fit all 

 cases. 



Again, it must ever be remembered that a mycorrhiza is not a 

 static thing: Endrigkeit (1937) has neatly expressed the case: 

 "Since the parasitic acquisition of nutrients by the fungus is of a 

 very restricted order in both mycorrhizal groups, the higher plant is 

 evidently the chief gainer by the association until the activity of the 

 roots begins to decrease with age, the balance inclines in favour 

 of the fungus. Comparative membrane and permeability observations 

 on plants in a colonized and uncolonized condition revealed a pro- 

 gressive loss of independent assimilatory capacity of the roots with 

 increasing fungal activity." 



The Intaking Mechanism: — The point at which current theories 

 of mycotrophy break down is with respect to the mechanism for 

 intake of liquid materials from the habitat. It is after all of little 

 consequence whether the plant is to receive N or P or any other ion 

 if there is no means of transport for these substances. Of course 

 the plant does receive them, but not much attention is paid to the 

 method or the apparatus by which the substances enter the mycor- 

 rhiza or mycotrophic organ. The presumption is that they enter 

 through communication-hyphae, but so far as we are aware there is 

 not a single research devoted to the structure and functioning of these 

 hyphae. They seem to have been forgotten by the protagonists of 



