100 BACTERIAL DISEASES OF PLANTS 



reactions (acid, neutral, alkaline), using neutral litmus and phe- 

 nolphthalein, etc. — and here a medium generally neglected by 

 bacteriologists may be just the one needed. The student should 

 not regard the chart sanctioned by the Society of American 

 Bacteriologists as in any sense a finality — there are no last words 

 in science, at least not in bacteriology — and he must be always 

 on the lookout for simple and effective means of differentiation. 

 Often, when the colonies of two organisms look exactly alike on 

 + 15 peptone-beef agar, we try potato agar, prune agar, string- 

 bean agar, starch agar, whey agar, or some kind of gelatin 

 medium, and find a difference. In this connection see Fig. 184. 



For the fourth purpose, to avoid complications, one would 

 naturally select, first of all, well-aerated, simple synthetic media. 



Carefully considered (tested) formulae for the preparation 

 of various culture-media will be found in ''Bacteria in Relation 

 to Plant Diseases," Vol. I. For Meyer's mineral sollition con- 

 sult Ibid., Vol. Ill, p. 250. 



PREPARATION OF CULTURE-MEDIA 



It is impossible to pursue the study of bacterial diseases 

 of plants without the use of at least some forms of culture media, 

 and the student should know how to prepare with his own hands 

 all necessary substrata. It is a useful training of the hands 

 and of the judgment even for those who are not to use it later 

 in research. 



Culture-media may be divided into two classes: (1) complex 

 organic substances and (2) simple synthetic preparations. The 

 first, though they are harder to prepare, are still used more 

 generally than the second, and are in the main better adapted 

 to the growth of parasitic micro-organisms than are the more 

 exact media compounded out of simple chemical substances. 

 The latter, however, may be expected eventually to take the place 

 largely of the more or less variable animal and plant compounds 

 now in general use. At the same time the writer would like 

 to register an objection against the discontinuing of any of the 

 cultural substances now in use until such time as we have well- 

 recognized suitable substitutes. At present we need all of them, 



