184 Perspectives in Microbiology 



acid (EDTA), and part in a solution containing iron, the 

 iron was mobilized, whereas in the absence of EDTA it 

 remained largely immobilized in the roots as ferric iron. 

 The production of chelating agents by microorganisms in 

 soil and the significance of these compounds in the metal 

 nutrition of plants are still in the area of speculation. 



Although plants can absorb certain organic compounds, 

 the extent of this absorption under natural conditions is 

 uncertain. But soil organic matter may be a source of 

 organic as well as mineral substances for plants. Miller 

 (24) found that tomato plants could satisfy their entire 

 sulfur requirements from the amino acid methionine. 

 Various antibiotics are absorbed by higher plants, as are the 

 auxin indolylacetic acid, various plant growth regulators 

 and herbicides, fungicides, and insecticides. The uptake 

 of certain of these compounds is cause for concern because 

 such compounds may affect the quality of the plant and its 

 acceptability as food for man and domestic animals. 



The possibility that many organic substances of mi- 

 crobial origin in soil, particularly in the rhizosphere, are 

 absorbed and affect the plant, or its food value, favorably 

 or unfavorably is still to be evaluated. 



There is still another manifestation of the effects of 

 organic materials of microbial origin on plant develop- 

 ment. Cholodny (4) reported that plant roots, when ex- 

 posed to soil gases, showed abnormalities indicative of 

 effects produced by some volatile organic substance of 

 microbial origin. The possibility that this was due to gase- 

 ous hydrocarbons gains support from recent experiments 

 of Davis and Squires (8). They grew mixed cultures of 

 microorganisms from cow manure and from sewage sludge, 

 as well as a culture of the fungus Penicillium digitatum, 

 in substrates containing carbohydrates or salts of organic 

 acids. In addition to methane and ethylene, the products 

 contained the hydrocarbons ethane, propane, and pro- 

 pylene, not previously reported as being formed by micro- 



