Minnesota Plant Diseases. 193 



Bacterial partnerships and antagonisms. Bacteria often 

 form dense colonies of individuals developing in gelatinous 

 masses. These bacteria may all be of one kind, but frequently 

 are different, and may then live in a partnership apparently ben- 

 eficial to each other. It is evident that such forms do not com- 

 pete with each other for food stuffs. Bacteria may also form 

 partnerships with other organisms as with yeast plants. In such 

 cases the waste products in the nutritive processes of one may 

 be food for the other plant and thus a beneficial partnership is 

 established. Such is the association of bacteria and yeast in 

 the English ginger beer "plant" and in the production of other 

 drinks as the Asiatic kephir. Such a partnership is also ex- 

 plained in the fact that organisms of this kind often form waste 

 products which, if allowed to accumulate, may prove detri- 

 mental to the organism producing them. This is a common 

 method by which bacterial growth is limited. These com- 

 pounds are in the case of bacteria often poisonous and form the 

 toxins which in disease germs are the poisoning agents of the 

 disease. The accompanying organism of a partnership may 

 use up and remove these detrimental substances and thus allow 

 the first partner unhindered development. Antagonism of bac- 

 teria in colonies may result from competition for food materials 

 or from the production of substances by one, which are poison- 

 ous to the other organism. 



Disease-causing bacteria. One of 

 the most useful classifications of bac- 

 teria is the arrangement of forms ac- 

 cording to their prominent physio- 

 logical effects. In such an arrange- 

 ment the disease-causing or patho- 

 logic forms are of great economic im- 

 portance. These are the forms which 

 give rise to most of the well-known 

 diseases of man and lower animals. 

 FIG. 97.-Bacteria of fire-biight of Cholera, tuberculosis, diphtheria and 



apples (Bacillus amylovorus). fv^l^JH Qr ~ K n f Q f~ 7 ^f fU P Hp^trnr 

 Highly magnified. After B. OP 11 a are DU t a ICW OI me aCstrUC- 



tive diseases of bacterial origin. By 



an accurate knowledge of the life-story, physiology, etc., of 

 these organisms preventive measures of sanitation and quaran- 



13 



