Lecture XI — 159 — Theories 



Hydrocarbon Hypothesis: — That nutrient exchange in mycor- 

 rhizae is carbonaceous rather than nitrogenous was advocated by 

 McLennan (1926). "A critical discussion of [some of] the earlier 

 work on mycorrhizae, more particularly that dealing with the physical 

 relations between the two forms, discloses the fact that the most 

 generally accepted ideas of this relation are those connecting it with 

 nitrogen fixation without any real evidence that such is the case. . . . 

 The demonstration of many infecting strands [in Lolium mycor- 

 rhizae], together with the appearance of fat, firstly in the conducting 

 and travelling hyphae of the root, with its subsequent removal to the 

 sporangiole, and then to the host-cell, accompanied by collapse and 

 shrivelling of the fungal mechanism, have led to the conclusion that 

 a metabolic exchange takes place from the fungus to the higher plant, 

 with the result that the latter obtains a supply of fat or oil." Knud- 

 son's results were thought to favour this hypothesis and "The idea 

 that the exchange is carbonaceous rather than nitrogenous is also 

 compatible with Bernard's suggestion" of the relation between 

 tuberisation in plants and the presence of endotrophic mycorrhizae. 



As a postscript we may add: "Although these homogenous 

 globules do not stain black with osmic acid after bichromate, they 

 have, nevertheless, been proved to be fat globules." 



Carbohydrate Hypothesis: — Another carbonaceous hypothesis 

 was proposed by Young (1940) as a "mycorrhizal theory regarding 

 the cause of fused needle disease" in pine. "According to this hypothe- 

 sis, normal mycorrhizas supply the tree with an essential part of their 

 carbohydrate supply, and it is to the inefficient functioning of the 

 mycorrhizas in this respect that the fused needle condition is due. The 

 supplying of additional phosphorus to soils low in this element results 

 in a more abundant phosphatide excretion from the pine roots, 

 thus stimulating normal mycorrhiza formation and bringing about a 

 satisfactory balance of conditions for correct mycotrophic activity. 

 The amount of vegetable detritus present is important in this respect, 

 as it is from this source that the carbohydrate supplied to the higher 

 plant by the mycorrhizal fungus is obtained. The addition of phos- 

 phates to the soils in question stimulates the growth of natural 

 vegetation, and thus aids the development of the necessary supply 

 of vegetable detritus." 



This explanation was not accepted by Neilson-Jones (1941), 

 who ascribes development of needle fusion to a sudden shortage of 

 water in the plant as the leaves start to expand, due to a failure to 

 produce mycorrhizae at the critical juncture. But the hypothesis of 

 carbon intake through mycorrhizae remains to be experimentally 



