Microorganisms and Plant Life 183 



and continued to grow and bear fruit with no watering 

 except that provided by the fog. 



Somewhat more is known of the effects on plants of 

 microorganisms in the rhizosphere, but the data are little 

 more than suggestive. The solvent effect of roots on marble 

 was shown by plant physiologists many years ago, and 

 it was found by Fred and Haas (10) that the solvent effect 

 was greater when microorganisms were present than when 

 they were absent. The effect was ascribed to carbon diox- 

 ide excreted from the roots. Of particular significance in 

 the same connection are some results of Gerretsen (11) 

 on the solubility of phosphate. It was found that more 

 tricalcium phosphate was absorbed by plants from sub- 

 strates containing microorganisms than from sterile sub- 

 strates, and that the phosphate underwent solution by 

 microbial development about plant roots. 



Solubility of iron, and also of manganese (12), and their 

 availability to plants are probably affected by the rhizo- 

 sphere population. Microbial development reduces the 

 redox potential and may increase the acidity, both of which 

 increase iron solubility (40). Recent results of Bromfield 

 (3) support this contention. 



Although reduction reactions play an important part 

 in the solution of iron and manganese, complex formation 

 may affect the availability of these elements to plants as 

 well. Among the diverse organic compounds formed dur- 

 ing development of microorganisms, presumably there will 

 be chelating materials which will persist long enough to 

 form complexes with iron and other metals, and these 

 complexes will enter the plant. Amino acids, citric acid, 

 and other microbial products could serve as chelating 

 agents. Penetration of chelating compounds should be no 

 problem and would be likely to aid in iron utilization. 

 Recent observations of Weinstein et al. (44) have shown 

 that, when part of a root system was placed in a solution 

 containing the chelating agent ethylenediaminetetraacetic 



