192 Minnesota Plant Diseases. 



Influence of external forces as light, temperature, etc. It is a 

 well-known fact that most bacteria do not thrive in sunlight 

 but that the direct rays of the sun are fatal to them. Too much 

 stress cannot, therefore, be placed upon the necessity for sun- 

 shine in thorough processes of sanitation. Waters of lakes and 

 rivers are largely purified by direct sunshine which can fatally 

 affect bacteria to a depth of several feet below the surface. Of 

 course this is not the only agent of "purification but is one of 

 the most important. It is to be expected, therefore, that when 

 bacteria cause disease in plants it is in the underground por- 

 tions, as bulbs or roots or stems, or in situations where the illu- 

 mination by the sun's rays is always poor. Excessive moisture 

 may, moreover, aid in bacterial dissemination. The bacteria 

 are easily carried about in the water currents especially if they 

 have the whip-like swimming apparatus which is common 

 among so many forms. 



Some bacteria can live and even reproduce in temperatures 

 near the freezing point, while the resting, inactive cells are often 

 capable of resisting very much lower temperatures. On the 

 other hand, heat-loving bacteria are known which thrive in 

 comparatively high temperatures even fifty degrees Fahr. 

 above blood heat. The bacteria are thus seen to enjoy wide 

 extremes of temperatures. A given species, of course, has 

 usually a much more limited range and always possesses a 

 favorite temperature at which it grows best. In general, bac- 

 terial cells in the dry state are capable of enduring higher 

 temperatures than those in the moist condition, facts which 

 are used to advantage in combating bacterial disease germs. 

 In the canning of fruits, for instance, boiling for a short time 

 will destroy most germ cells though spores will often re- 

 sist even such harsh treatment. The power of resistance in the 

 dry state is an important feature, for bacterial germs may live 

 for months and even years in such conditions, germinating 

 again as soon as favorable conditions of moisture and nutrition 

 present themselves. Thus germs of various diseases of man 

 may lurk in the air or soil, becoming evident only upon the ad- 

 vent of suitable conditions. Strong electric currents usually 

 destroy bacteria and this fact has led some to believe that elec- 

 tricity may be utilized in the purification of municipal water 

 supplies. 



