Kelley — 10 — Mycotrophy 



mycorrhizae greatly increase the absorbing surface of the rootlet. 

 The Hatchian hypothesis quietly drops the Stahlian conception of 

 transpiration streams. Still more recently, in 1943, Routien and 

 Dawson stated that "mycorrhizae increase the salt-absorbing capa- 

 city of the roots primarily by adding to the supply of exchangeable 

 hydrogen-ion derived in part at least from carbonic acid." 



Carbon Hypotheses: — Besides the nitrogen- and mineral-nutri- 

 tion hypotheses, there are carbon hypotheses which have been advanced 

 by two investigators. McLennan in 1926 stated that the more 

 generally accepted ideas connecting mycotrophism with nitrogen 

 nutrition were insufficiently founded ; and she concluded on the basis 

 of her researches on Lolium that "a. metabolic exchange takes place 

 from the fungus to the higher plant, with the result that 

 the later obtains a supply of fat or oil." McLennan believed that the 

 researches of Knudson and of Bernard lent support to this con- 

 ception. Another carbon hypothesis was advanced by Young in 

 1940, in these words : "It is thought that as well as providing a more 

 efficient absorptive system on the tree roots so that mineral salts and 

 nitrogen compounds are more readily available, the mycorrhizas also 

 furnish a means of augmenting the carbohydrate supply ... In the 

 author's conception the fungus manufactures its own carbohydrate 

 supply from the available soil organic matter, and a portion of this 

 is transferred to the higher plant by means of the intimate associ- 

 ation existing in the mycorrhizal structures." 



Growth Promoting Substances: — Complex substances of the 

 humus suggested still further hypotheses. Link, Wilcox and Link 

 in 1937 had suggested that "heteroauxin applied to a plant can 

 either substitute in part for its autoauxones or augment their ac- 

 tion, and the well-known fact that soil fungi and bacteria produce 

 heterauxin suggests that some of the beneficial effects of humus soil 

 may be due to the auxones of decaying plant debris or soil flora." 

 Still more specifically, Magrou in 1939 reported a more luxuriant 

 development in Arum as a result of supplying it with aneurin 

 (Vitamin B^). Melin in 1939 and 1940 found increased growth 

 in seven mycorrhizal fungi when these organisms were given aneurin 

 or yeast filtrate. In mycorrhizae of Monterey pine, MacDougal and 

 DuFRENOY reported in 1943 that the "independent growth of isolated 

 segments of mycorrhizal roots makes it obvious that through these 

 hyphal branches the root receives from the soil not only the C, N, 

 O, P necessary to build up the nucleus, the cytoplasm and its inclusions 

 (mitochondria and plastids), cell-walls, but also the mineral com- 



