252 Higher Plants and Soil Microorganisms 



a phenomenon of symbiosis or of commensalism, whereby the fungi 

 favor in some way the growth of the plants. It has even been sug- 

 gested that fungi play a role in tuberization in plants as well as in 

 protein formation in certain seeds, as in legume association or my- 

 corrhiza formation. 



Various groups of bacteria, fungi, and actinomycetes are capable 

 of penetrating the roots of plants and developing there, or of living 

 in close proximity, giving rise to a rhizosphere effect. It has been 

 suggested that certain soil organisms are capable of bringing about 

 a decided stimulation of the development of plant roots, either 

 through the production of hormones or through some other mech- 

 anism. 



The formation by microorganisms of CO:., and various organic 

 acids results in a greater solubility of the soil minerals, particularly 

 the carbonates and phosphates, and some of the zeolites. The vari- 

 ous inorganic acids produced as a result of bacterial action, includ- 

 ing nitrous, nitric, and sulfuric acids, are also known to exert marked 

 effects in the solubilization of minerals in the soil. Since microorgan- 

 isms influence the concentration of gases in the soil atmosphere, 

 notably oxygen and CO^, root development of higher plants may 

 thus be appreciably affected. 



Gerretsen found that, in sterile sand cultures with Cas(P04)2 as 

 the source of phosphorus, more of this element was absorbed when 

 the sand was inoculated with 1 per cent of garden soil. Various 

 experiments indicated that microorganisms play a highly important 

 role in making phosphorus available to the plants. When plant roots 

 were placed on an agar medium containing tricalcium phosphate, 

 the microorganisms developing about the roots caused the solution 

 of the phosphate. The factors affecting this process are the nature 

 and quantity of root excretions, presence and number of phosphate- 

 dissolving microorganisms in the soil, chemical composition of the 

 phosphate, and pH and temperature of the soil. 



Microorganisms have often been reported to exert a highly favor- 

 able effect upon the germination of plant seeds and their subsequent 

 growth. This relation was even said to be symbiotic in nature. 

 More careful studies, however, established the fact that the normal 

 microflora of the seed is essentially that derived from the soil, and 

 that none of the organisms are specific for the seeds themselves. 

 Seeds and soil microorganisms are associated in an indirect way, 

 but they are physiologically unrelated. The growth and activities 

 of the two do not depend on one another, although some studies seem 



