258 Higher Plants and Soil Microorganisms 



formation of the CO^ during the growing season is different for each 

 of the plants. There is a parallel influence of the plants on the 

 formation of CO2 and changes in the bacterial population in the 

 soil. This is expressed by a slight effect in the early stages of growth 

 of the plants, and by a greater effect with advance in vegetative 

 development and fruiting; the effect becomes less when the plants 

 begin to degenerate and die. The oxidation of the soil nitrogen to 

 nitrate was affected in a somewhat similar manner. 



The fact that the roots of plants are surrounded by a film of bac- 

 teria actively respiring was taken as explaining the formation of COj 

 about the roots. 



Grass growing in the vicinity of trees is usually observed to have 

 a harmful effect upon the trees. This was explained by the fact that 

 the surface roots of the trees are deprived of combined nitrogen by 

 the grass roots. Furthermore, by producing a soil atmosphere rich 

 in CO2, the grass causes the surface roots to grow down and thus 

 suffer from lack of oxygen. It has frequently been suggested that 

 the injurious effect is due to the formation of a toxin by the grass; 

 however, no evidence of this has been presented. 



The root residues, in the form of sloughed-off portions and finer 

 rootlets, may influence greatly the nature of the population develop- 

 ing in their neighborhood. This is likewise true of the excretion 

 products of the plants. Plants also produce a variety of gases which 

 greatly influence the nature of the organisms developing in the par- 

 ticular area. Further evidence of the marked influence of growing 

 plants on the microbiological population of the soil is found in the 

 fact that a given soil decomposes cellulose with varying rapidity 

 according to the nature of the plants that have been growing in it. 

 The nature of the organisms taking part in the decomposition of 

 cellulose in a given soil varies with the plants grown in the soil. 



It thus seems to be definitely established now that larger numbers 

 of microorganisms find a more favorable condition for their develop- 

 ment in close proximity to plant roots than at a distance. Nitrogen- 

 fixing and cellulose-decomposing bacteria are particularly prominent. 

 This may be because the plants excrete or leave in the form of 

 residues a certain amount of available energy; this would favor the 

 development of the nitrogen-fixing organisms. There is no evidence, 

 however, that fixation of nitrogen is increased around the roots. The 

 cellulose-rich residues would naturally favor the development of 

 cellulose-decomposing bacteria and fungi in the soil. The nature of 



