Lecture VI — 71 — Environment 



ammoniacal state: this type has a moss and Hchen ground cover. In 

 raw-humus Melin visuaHzed a struggle between fungi and trees for 

 N, and the tree provided with mycorrhizae could compete with soil 

 organisms for that N. In mull soils, he found mycorrhizae (i.e., 

 obviously ectotrophic) poorly developed. Yet Engler (1903) had 

 supposed that mull but not raw-humus is available to mycorrhizal 

 fungi. The nature of the soil plays an important part in distribution 

 of these fungi (Peyronel, 1921) ; and also on fungal form since in 

 soils poor in organic materials rhizomorphs develop while in humus 

 a disperse mycelium is produced. Then, not only fungal but mycor- 

 rhizal form is influenced by humus (Bjorkman, 1940), for Type 

 C of Melin, formed by Boletus spp., was found on occasional pine 

 plants in "mor" and sand (humus mixed with sand in a volume ratio 

 of 1 :2). An interesting line of work is brought out by Magrou: "In 

 the cultivated field, manure is capable of elevating the osmotic pres- 

 sure of the soil solution and, in consequence, that of the sugar of the 

 plant, beyond the level at which tuberisation might commence." (Ann. 

 d. Sci. nat., Bot, XI, 4:97-102, 1943). 



Two ideas about humus have governed students of mycorrhizae, 

 vis., (1) that humus by its decomposition provides salts to the plants 

 growing in it, and (2) that humus buffers the soil by absorbing 

 deleterious substances. In other words, the influence of humus is 

 considered to be either chemical or physical. 



The first idea is the older. Frank, in his experimental work 

 recorded in 1888 and 1889, indicated that fungi living in humus obtain 

 their nutrient from it and change over the humus into directly assimil- 

 able N compounds or ammonia. For beech, Muller (1886) concluded 

 that the tree lives on the remains of its own vegetative activity, a 

 peat being built up under the tree in which mycorrhizae live. Not 

 only beech but spruce and fir live thus in the Bavarian Alps (Eber- 

 MAYER, 1888), the tree using ammonia salts directly and also mineral 

 salts derived from the humus layer. Of these salts, coniferous litter 

 was found to contain more N, deciduous litter more K and P. (Ferry, 

 1887). By removal of leaf-litter, the fungi are deprived of their 

 normal food-supply and are transformed from mycorrhizal into 

 parasitic fungi (Delacroix, 1897). 



The physical influence of humus on mycorrhizal development has 

 been adverted to by several earlier workers, as e.g. Hoeveler (1892) 

 who noted a rich branching of the mycorrhizal root system in humus, 

 while Shimizu (1930) thought that humus determines mycorrhizal 

 form in pine. But more recently the idea has been current that in 

 humus there are inhibitory or deleterious substances which interfere 

 with growth of plants, and that possession of mycorrhizae enables 



