Kelley — 76 — Mycotrophy 



development. After a rain there is a rapid root growth but as available 

 soil moisture lessens in amount growth slows and may cease, to be 

 renewed with the next rainfall. Mycorrhizae therefore are not neces- 

 sarily structures of a steady growth that finally comes to a definite 

 end, but growth can be renewed. After a rain there is rapid mycor- 

 rhizal growth but as available moisture lessens, growth is retarded, 

 to recommence at the next rainfall. Every student of mycorrhizae has 

 seen old brown or even black mycorrhizae that have split their mantle 

 and protruded a new white mycorrhizal apex. With some plants the 

 periods of growth and quiescence are marked by constrictions or 

 rings, and the mycorrhiza assumes in consequence a beaded appear- 

 ance. Even in winter a warm rain starts new mycorrhizal growth 

 and one finds abundant white mycorrhizal root-tips. 



Soil Solution: — Much is known of the composition and physics of 

 the soil solution but its relationship to actual plants growing in native 

 habitats is problematical. For, if plants are nourished through a mycor- 

 rhizal apparatus located in the uppermost A horizon, of what particu- 

 lar interest to them is a soil solution in the mineral B horizon? 

 Scientists, with that habit so ingrained in the human race, have gone 

 into the utmost minutia of research regarding the soil solution, but 

 they have never gone to the trouble to find out whether roots of 

 naturally growing plants actually come into contact with this solution. 

 Even for the mycorrhizae that do occur in the B horizon it is not 

 known what significance the soil solution has for them because the 

 mycorrhiza is buffered, so to speak, by fungal structures that more 

 or less isolate the mycorrhiza from the soil. Hence the whole question 

 of soil solution and mycorrhiza is largely conjectural, and because 

 of its character the various theories of mycotrophy are conditioned. 



Some of the more recent studies have thrown incidental light on 

 the soil solution-mycorrhizal relationship. For some years Melin 

 has emphasized the importance of N salts to mycorrhizal plants as 

 indicated by laboratory tests. He has found that the simpler N com- 

 pounds, such as asparagin, can be utilized but that more complex com- 

 pounds, as peptone and nucleic acid, are used with difficulty. Harley 

 (1937) found with reference to beech that if the fungus supplies N 

 to the tree, it does not overcome low N content of the soil, and vigour 

 of the beech tree is more attributable to soil variations than to varia- 

 tions in infection of roots; yet infected roots had a greater N content 

 than uninfected. In the case of "fairy rings", Guinier (1937) found 

 that ammonia content of the soil was markedly higher within the ring 

 and grass found here was dark green. Also, in coniferous forests the 

 ammonia content of the soil was greater immediately beneath the 



