Lecture XI — 153 — Theories 



Nitrogen Theory: — It is Frank's nitrogen theory that has been 

 received with most attention, a theory that is briefly stated by Lind- 

 QUiST (1939) : The nitrogen theory was first clearly formulated by 

 Frank (1894), [in a short discussion of the various possibilities of 

 significance of mycorrhizae, in which he supposes the fungus makes 

 available the N-compounds contained in forest humus and duff. More 

 probably, it was said, the fungus aids intake of humic compounds of 

 calcium], who considered that the beneficial influence of the mycor- 

 rhizal fungus consists mainly, for the higher symbiont in the pro- 

 vision of organic N-compounds for the latter. As basis of this con- 

 ception, Frank, etc., pointed out that the tissues of a mycorrhizal 

 tree are nitrate free. As it is known that the fungus can readily take 

 up NH4 and organic N-compounds, he considered it self-evident that 

 such compounds were taken up from nitrate-free or nitrate-poor 

 forest soils and into the mycorrhiza-bearing tree. The N-nutrition 

 hypotheses were extended later by numerous successors, as e.g. von 

 TuBEUF, MoLLER, MuLLER, Weiss, and especially Melin, who in the 

 years 1917-1927 conducted numerous investigations into the mycor- 

 rhizae of forest trees and their role as N-absorbing organs. . . . With- 

 out wholly renouncing the mineral nutrition hypothesis, he assigned 

 to the mycorrhiza no great role in the intake of mineral nutrients, 

 showing in this connection that the trees have ability to obtain rich 

 nutrition from the mineral soil by their deeply penetrating roots. 



A similar reciprocity was claimed by Rayner (1927) for Calluna, 

 and she said: "it seems probable that Melin's conclusions respecting 

 the beneficial effects resulting from the presence of the mycorrhiza 

 in acid humus may be extended to the more specialized case of Calluna. 

 Under such conditions, the mycorrhiza of this species, and doubtless 

 of other cricoids, probably functions in a similar way to that of trees, 

 conferring on the host plant the power of drawing upon the organic 

 food reserves locked up in humus. . . . Like certain conifers and other 

 trees, Calluna and its allies are not strictly autotrophic in respect to 

 their nitrogen metabolism, and they are singularly well equipped for 

 successful competition in the struggle to obtain the requisite nitro- 

 genous food materials, whether in sandy soils, poor in organic con- 

 stituents, or in acid humus soils deficient in nitrates. They have 

 solved the problem of growth upon the poorest and most unpromising 

 soils, but they have solved it at the price of their independence." 



Chemical Studies of the Nitrogen Theory: — We come next to 

 a chemical study concerning presence of NH^ salts in plants by 

 Weevers (1916), who found NH^ salts present in all species in- 

 vestigated except mycotrophic and insectivorous sorts. He said that 



