308 MYCORRH1ZAE AND MYCOTROPHY 



(a) continued elongation, (b) increased diameter, (c) dichoto- 

 mous branching, (d) delay in suberization of cortex, and (e) ac- 

 quisition of additional surface area, the composite of that of the 

 hvphae. His three interpretations made in conclusion are: (1) 

 that mvcotrophic relationship is a symbiotic mechanism to in- 

 crease the absorption of soil nutrients; (2) that the extent of the 

 surface area of short roots is determined bv the availability of 

 minerals, mvcorrhizal roots being rarely formed in fertile soils 

 but produced in abundance in infertile soils; and (3) that trees are 

 dependent upon symbiotic association with mycorrhizal fungi for 

 all their mineral nutrients and therefore for their ability to exist in 

 all except the most fertile soils. Experiments by Mitchell, Finn, 

 and Rosendahl (1937) on mycorrhizae as related to mineral ab- 

 sorption by coniferous seedlings led them to arrive at conclusions 

 similar in all essentials to those of Hatch. Bjorkman (1942) found 

 that light, nitrogen, and phosphorus are each decisive factors gov- 

 erning the formation of mycorrhizae. 



Buries (1936) postulated that the higher plants benefit from 

 association with fungi by absorbing the nutrients made soluble as 

 a result of decomposition by the soil fungi. He does not believe 

 that there is any mutualistic relationship between tree roots and 

 fungi but that mycorrhizae represent a controlled parasitic at- 

 tack. Some support of this idea appears from Rayner's (1934, 

 1936) experiments, in which, after inoculation with mycorrhizal 

 fungi, she noted markedly improved growth of pine seedlings at 

 a period in advance of the actual formation of mycorrhizae. She 

 attributed this stimulation to the elaboration of growth-promoting 

 substances by the fungus and to the nutrients liberated to the 

 seedlings by the activity of the fundus. In view of the body of 

 evidence that is being accumulated on the elaboration of auxins 

 by fungi, these substances may well be important factors in in- 

 creasing the growth of plants possessing mycorrhizae. 



Much has been written to indicate that mycorrhizal fungi are 

 parasitic and that the balance may be easily tipped toward one or 

 the other partner in the relationship. The observations of Masui 

 (1926, 1927) in Japan and of McDougall (1914) in this country 

 inclined them to regard the association as one of parasitism by 

 the fungus. 



Bernard's experiments (1909), summarized in her "L'evolution 

 dans la svmbiose des Orchidees," are fundamental to an apprecia- 



