222 



CIVIC BIOLOGY 



saprophytic bacteria as it would be to condemn all higher 

 plants because a few of them are poisonous. In general, sap- 

 rophytic bacteria do no more harm than dust if breathed, or 

 than vegetables if eaten. 



Nitrifying bacteria. Certain bacteria of the soil are symbi- 

 otic upon the roots of leguminous plants, such as clover, alfalfa, 



beans, and peas, and cause 

 tubercles to form. These 

 bacteria gain entrance 

 through the root hairs 

 of the plant and cause 

 smooth young roots to as- 

 sume a nodular appear- 

 ance (Fig. 104). 



Experiment shows that 

 if a legume, notably clo- 

 ver, is grown upon soil of 

 known composition, a part 

 of which has been ster- 

 ilized (baked), the crop 

 upon the unsterilized soil 

 will be notably larger and 

 the soil will have nitro- 

 gen added to it. These 

 bacteria are important, 

 since they can fix the free nitrogen of the air and give it to 

 the soil in the form of nitrates. The benefit to the soil result- 

 ing from clover cropping was discovered and practiced by 

 farmers long before the cause was known. 



Much experimental work is being done with these nitrifying 

 bacteria, and pure cultures are being sold to inoculate soil 

 that does not contain them. To prevent extravagant and mis- 

 leading claims of dealers, the United States government has 

 issued the following statements : "No beneficial results can be 



FIG. 104. Clover plant with many 

 bacterial nodules on roots 



