Lecture I — 9 — Rise of Mycotrophy 



seedling and if a mycorrhiza results, to consider the fungus mycor- 

 rhizal. The pioneer in synthesizing mycorrhizae was Josef Fuchs, 

 and one of the most prominent investigators of such syntheses is 

 Melin. Still more recently, Modess has reported many such 

 syntheses ; and Fries has formed synthetic mycorrhizae with mono- 

 spore mycelia. 



The Nitrogen Theory of Mycotrophism: — As to the nature 

 of the mycorrhizal symbiosis, numerous hypotheses have been ad- 

 vanced. It has already been noted that R. Hartig and others con- 

 sidered the symbiosis to be a case of harmless parasitism, and that 

 Kamienski maintained that there is a beneficial symbiosis only in 

 the case of Monotropa. Frank had concluded that the parasitism 

 was just the reverse, — that the higher plant was a parasite on the 

 fungus ! He had found that "the tissues of a mycorrhizal tree are 

 nitrate free". As it was known that the fungus could readily make 

 use of ammonia and organic nitrogen compounds, he considered it 

 self-evident that in this way such compounds were taken up from 

 nitrate-free or nitrate-poor soils and liberated in the mycorrhiza. 

 The nitrogen nutrition hypothesis which Frank originated was 

 elaborated by subsequent investigators, von Tubeuf, Moller, Mul- 

 LER, Weiss, to mention a few ; and especially by Melin, who has 

 conducted many researches into the nature of mycorrhizae of forest 

 trees. Melin appears to consider that, in Swedish forests, the prob- 

 lem of the mycorrhiza is above all a nitrogen problem. 



The Stahlian Hypothesis: — On the other hand, Stahl in 1900 

 emphasized the mycorrhizal intake of all minerals used by the higher 

 plant. Trees being brought into competition with fungi and bacteria 

 for nutrient materials contained in the soil are seriously limited in 

 their food supplies ; and in the mycorrhizal fungi find provisioners 

 that bring water and dissolved salts into the roots. It is not merely 

 nitrogen that is brought into the root, according to this hypo- 

 thesis, but all the minerals the soil solution contains. As a corollary 

 the supposed relation of transpiration to mycotrophy was pointed 

 out. Plants possessing a large transpiration stream bring a 

 considerable quantity of mineral salts into the higher plant and 

 deposit them in its tissues. Plants with a smaller transpiration stream 

 have a smaller salt intake and are presumably compensated with 

 the salts provided by the mycorrhizae. But the Stahlian hypothesis 

 ran into difificulties which are detailed later; and it met with little 

 favour until supposedly revived by Hatch in 1937. In reality Hatch 

 originated a new hypothesis which states among other things that 



