Kelley — 70 — Mycotrophy 



phic plants included but decided that there was no "physiological 

 dryness" of the habitat, nor was there any lessening of transpiration 

 in these plants. They assimilate Al so greatly that they could be termed 

 "Al-plants". All sorts of mycotrophic conditions were described by 

 Takamatsu (1930) for the solfatara soils at Hakkoda, and the 

 mycorrhizal structure was similar to that of mycorrhizae from forest 

 soils. 



Humus: — Whatever name is placed on the complex of substances 

 usually denominated "humus", it is manifest that these substances 

 are determinate in the distribution of mycorrhizae. Two principal 

 forms of humus, — raw-humus and mull — are associated with "ecto- 

 trophic" and "endotrophic" mycorrhizae respectively; and these in 

 turn are related to certain edaphic conditions, especially water. Hence 

 there is an interplay amongst organic detritus, microorganisms and 

 moisture that determines existence of mycorrhizae. This organic 

 detritus is undecomposed ; and when it is broken down with forma- 

 tion of mineral salts, it ceases to be humus. It is futile, therefore, to 

 speak of absorption from humus of water and mineral salts. It is 

 equally futile to say that mycorrhizae make use of humus : what mycor- 

 rhizae use is the protoplasm of invading fungi. These invading fungi 

 utilize humus through the soil portion of their mycelium, either directly 

 or through the mediation of microorganisms ; and the humus has 

 ceased to be humus when these organisms have made use of it . . . 

 Hence mycorrhizae never take in or "absorb" mineral salts ; and how 

 they acquire water is, to the best of our knowledge, still a matter of 

 conjecture. 



Humus is naturally formed by the partial breaking down of 

 organic matter, chiefly vegetable, by the action of microorganisms. 

 Earlier workers, such as Nikitinsky (1902), had shown that humic 

 substances could not be used directly by higher plants but that they 

 were broken down by bacteria and fungi into simpler products that 

 could be used. In his well-known studies of humus-formation, 

 Falck (1923) used the term "Mykokrinie" to describe the chemical 

 changes involved in the decomposition of forest duff; and by this 

 process fallen branches, dead leaves, etc., are transformed into humus 

 and finally into "mineral salts" that the higher plant can utilize. In 

 Sweden, where Melin (1925) has studied so intensively, the forest 

 soils may be divided into a mull type wherein there is a more or less 

 rapid destruction of plant residues resulting among other things in 

 nitrate formation ; and this type has a rich herbaceous ground-cover ; 

 or a raw-humus type resulting from a less rapid destruction with 

 scarcely any nitrification while the organic matter remains in an 



