Kelley — 72 — Mycotrophy 



certain plants to grow in these soils. According to Freisleben 

 (1935), "The beneficial action of fungi on the growth of Ericaceae . . . 

 does not rest on a direct influence, as through the excretion of growth- 

 promoting substances, but on an inactivation, destruction or absorption 

 of the inhibiting materials. It is tO' be supposed that in natural soils 

 also the root fungi and the soil fungi which enter as components of a 

 peritrophic mycorrhiza, have a similar significance for the Ericaceae." 

 Again, Rayner's (1944) Wareham experiments indicate presence 

 of a toxic substance in this infertile soil and in a note, Neilson- 

 JoNES (1940) adduced some experimental evidence to prove the 

 hypothesis that emerged from Dr. Rayner's experiments in Wareham 

 forest : "The hypothesis was put forward that the local infertility on 

 the area is due to toxic residues formed during decomposition of 

 organic detritus by micro-organisms ; and that the effect of the 

 compost is to provide a substrate which, by altering the serial activities 

 of the different soil micro-organisms, modifies the chain of reactions 

 constituting soil decomposition ; with the result that the final residues, 

 instead of being toxic, are favourable to the growth of the trees, tO' the 

 mycelial growth of the fungi associated with them as mycorrhiza- 

 formers, and to the establishment and free-functioning of a normal 

 and balanced mycorrhizal relationship." 



Mycorrhizae as Soil Indicators: — Bernatsky (1900) con- 

 sidered mycorrhizae as indicators of poor although well-aerated soils 

 except in the case of Alnus. Certain generalizations can perhaps be 

 made : A coralloid mycorrhiza is associated with a raw-humus forest 

 soil ; racemose mycorrhizae of light colour mostly come from mineral 

 soil ; bushy-branched rhizothamnia are characteristic of sands ; while 

 pearl-necklace mycorrhizae are found in droughty soils. In mulls 

 the preceding sorts are not likely to be found, but endotrophic struc- 

 tures prevail. 



Mycorrhizae in the Soil Profile: — Mature natural soils present 

 a profile that is considered to exist in three "horizons", viz. the "A" 

 horizon, or zone of leaching and extraction of salts and colloids by 

 percolating waters ; the "B" horizon, or zone of concentration of 

 these materials ; and the "C" horizon, or zone of subsoil and rock 

 fragments where there is no visible concentration of leached materials. 



There is not enough data extant to formulate any comprehensive 

 picture of the occurrence of mycorrhizae in the soil profile but certain 

 cases are known. In northern coniferous forests where raw-humus 

 develops, mycorrhizae are developed chiefly near the surface in the 

 uppermost A horizon. Masui (1927) figured a soil profile for "woody 



