Kelley — 160 — Mycotrophy 



evaluated. Young (1941, p. 91) makes it clear that it is not merely 

 C that the plant receives from the soil through its endophyte but 

 "Inorganic salts and perhaps nitrogen compounds are probably also 

 supplied to the plant." 



Growth-Promoting Substances: — In more recent years atten- 

 tion has been turned to growth-promoting substances, and Lindquist 

 (1939) presented a Growth-substance Hypothesis, based on his 

 studies which indicated an excretion of a substance from mycorrhizal 

 mycelia that stimulated growth of spruce in pure culture. "The 

 mycelia . . . influenced so markedly the nutrient liquid into which they 

 were drawn during these experiments that a decided increase in 

 growth of needles, as well as of stem and root, could be marked." 

 Lindquist considered it established that "the nutrient materials 

 derived from fungi are of essential, indeed of vital, significance." 

 BuRGEFF (1934) also found the higher plant stimulated by the fungus, 

 in this case specialized orchids of the Vanda group, which always 

 develop slowly in absence of their natural symbionts. In promoting 

 growth, the dead fungus was found to be just as effective as living 

 hyphae, because of presence of a "growth- factor" resembling "Bios 

 11" although its chemical nature was otherwise unknown. In signi- 

 ficant experiments, Noggle & Wynd (1943) tried the effect of 

 various growth-promoting substances, and found "good germination 

 and excellent development of the seedlings when nicotinic acid 

 (P - P factor) was supplied in the nutrient medium." 



But RoMELL (1939) rejected Lindquist's hypothesis that "the 

 chief function of mycorrhiza formation would be the exchange of 

 growth-promoting substances between the symbionts", and "returned 

 to Frank's original ideas". Yet it is established that growth-promot- 

 ing substances do influence mycorrhizal fungi. Melin (1939) found 

 increased growth with aneurin in synthetic (glucose-containing) 

 medium; yeast filtrate gives even better results while biotin and 

 inosit failed to improve growth except in combination with aneurin. 

 Aneurin (vitamin B^) also gave better growth when added to cul- 

 tures of endophyte of Arum (Magrou, 1939). Later, Melin (1942) 

 found that aneurin is replaceable by its pyrimidin and thiazol compo- 

 nents, and equimolar quantities in synthetic cultures of certain mycor- 

 rhizal fungi. The development of M. r. atrovirens, which is aneurin- 

 autotrophic, is retarded by addition to the medium of aneurin or its 

 components, especially pyrimidin alone or mixed with thiazol. Ex- 

 perimenting with orchids, Meyer (1944) concluded that thiamin 

 (vitamin B^) substantially aided growth of seedlings. 



