Kelley — 84 — Mycotrophy 



rain, which followed dry weather, has been sufficient to . . . conclude 

 that new rootlets followed by a complete change to mycorrhiza have de- 

 veloped within ten days." 



As to phenology of internal anatomy, several observations may be 

 cited: In Hippophae, Arcularius (1928) said the fungus grows 

 best in summer and there are new vesicles present in winter. In Vitis 

 (Rives, 1923), vesicles appear at the end of the season, in August 

 and September. In Orchis, fungal digestion occurs chiefly from 

 autumn into winter (A. Fuchs, 1924). In Fraxinus, Kelley (1943) 

 found phagocytosis occurring in April and May, in Pennsylvania. 



Mycorrhlzae in Relation to Habitat: — Apart from influence 

 of soils on mycorrhizal form, more recent studies have been directed 

 to influence of habitat as a whole on mycorrhizae. It is obvious that 

 environmental influences of the habitat react first on the fungus, as in- 

 dicated by Curtis (1937) : "There is an apparent correlation between 

 ecological habitat and fungus type, rather than between orchid species 

 and fungus." In conifers the possession of mycorrhizae seems de- 

 pendent on edaphic conditions (Dominik, 1937), and the more my- 

 corrhizae are developed the better the growth. Naturally, conditions 

 that favour the fungus result in a greater development of mycorrhizae. 



In his review of soil fungi and root infection, Surges (1939) 

 considered the soil flora with its microhabitats ; and the relative abun- 

 dance of fungi, which is one-thirtieth that of bacteria but greater in 

 numbers than that of any other group. The biological groups of fungi 

 present in the soil the author considers as (a) root parasites, (&) 

 casual parasites and mycorrhizal fungi, (c) facultative parasites and 

 primary saprophytes, and {d) true soil fungi. The last group com- 

 prises those of a "humus type", the second group being most difficult 

 to study and some seem to be obligate parasites. 



The term "microhabitat" graphically expresses the situation of a 

 mycorrhiza, for it is in a little cosmos of its own. Here it is subject to 

 the inorganic and biological influences of the immediate neighbourhood, 

 the "rhizosphere" as it has been called. Hiltner is said to have origi- 

 nated this term for the space about a root which is subject to root ex- 

 cretion, in which he thought there is an aggregated microflora. But 

 KiJRBis (1937) pointed out that fungi live in and on tree roots and 

 separate the root from the purely rhizospheric fungi. Consequently 

 Jahn (1934) invented the term "peritrophic mycorrhiza" and defined 

 the peritrophic fungus as one that lives in an outer zone, mantling 

 the root, between soil-portion and root-epidermis. Ordinarily con- 

 sidered saprophytes, they bear a definite relation tO' the root. He 

 said that in many cases endo-, ecto- and peritrophic fungi are present 



