1 6 BACTERIA IN RELATION TO PLANT DISEASES. 



assumed cause, the different number of cases in the two groups of plants being 

 accidental variations. If, in such a locality, only a very few plants are inoculated 

 and a few held as checks, the evidence becomes still weaker and would not be con- 

 sidered entirely conclusive even though all of the inoculated plants should contract 

 the disease and all of the checks should remain free, since in a region subject 

 to a given disease five or six healthy plants may sometimes be found in prox- 

 imity to five or six diseased ones, although all may have appeared healthy earlier in 

 the season. The case is quite different if out of 100 control-plants and 100 inocu- 

 lated plants 95 per cent of the latter and only 2, 5, or 10 per cent of the former 

 contract the disease. It then becomes a question of probability which may be 

 converted into reasonable certainty by several repetitions of the experiment with 

 like results. Of course, the ideal experiment is one in which all the inoculated 

 plants contract the disease and none of the control-plants, and in which a large 

 number of plants has been used so as to exclude all possibility of the results being 

 due to anything but the organism used. 



Whenever the disease occurs naturally in the vicinity selected for the experi- 

 ments, too much emphasis can not be laid on the necessity of having numerous 

 inoculated plants and numerous controls, and on the desirability of repetitions of the 

 experiment in different years and under different local conditions. It is important 

 also that the inoculated plants should be under healthful conditions, /'. e., under 

 conditions as nearly natural as possible. For example, proper (natural) conditions 

 would be much more nearly attained by inoculating vigorous plants growing in the 

 open air or in well-kept greenhouses than by inoculating parts of the same plants 

 cut away from the stems and kept under bell-jars. It is conceivable that inocula- 

 tions which would succeed very well under the conditions last named, especially 

 at abnormally high temperatures, might entirely fail when under a more natural 

 environment. 



Not one of these four requirements can be omitted safely. A chain of evidence 

 is not stronger than its weakest link. Particular stress, therefore, is laid on being 

 able to produce at will the characteristic signs and lesions of the disease in healthy 

 plants by inoculation with pure cultures of a given sort; also on the -re-isolation of the 

 organism from the artificially-infected plants after they have become diseased ; on the 

 subsequent proper behavior of the organism in nutrient media ; and on its ability to 

 produce the disease when again inoculated. This is the whole thing in a nutshell. 

 The experiments must be continued until there is no doubt whatever as to the 

 pathogenic or non-pathogenic properties of the organism. "Almost certainly path- 

 ogenic" always leaves room for grave doubt in the mind of every thoughtful reader. 

 As a rule, the re-isolations should be made at a considerable distance from the point 

 of inoculation, particularly if there is any doubt whatever as to the identity of the 

 physical signs, since saprophytes have been known to live in plant tissues for a 

 considerable number of weeks near the place of inoculation, and, if abundant, 

 might cause various disturbances of nutrition without being the pathogenic organism 

 sought for. For example, one would be more likely to obtain the cause of the 

 disease in pure culture by attempting isolations from a plant in the stage shown in 



