18 BACTERIA IN RELATION TO PLANT DISEASES. 
If all experimenters in plant pathology, even in recent years, had been careful 
to conform to these four rules of practice, the first three of which in essence were 
formulated by Robert Koch as long ago as 1882, some very deep chagrins might 
have been avoided. 
Owing to insurmountable difficulties many animal pathologists, especially those 
who study human diseases, now frequently rely on the first two rules as sufficient, 
but, if possible, one should comply also with cand d. Plant pathologists are under 
no such limitations, and should conform to each one of the above-mentioned require- 
ments, particularly if they desire their work to take high rank and to be generally 
accepted as conclusive. Material for plant-inoculation experiments is so cheap and 
easily procured that a writer who undertakes to describe a bacterial disease of plants 
has usually no good excuse for leaving any doubt whatever as to the pathogenic 
properties of the organism. ‘There is also no excuse for limiting the inoculations 
to mixtures of bacteria or to crude material taken directly from the diseased plant, 
since every tyro in bacteriology now knows how to separate one organism from 
another in nutrient agar or gelatin by means of poured plates or Petri-dish cultures. 
Fig. 11.* 
MorRPHOLOGY. 
SIZE, SHAPE, ETC. 
The smallest observed bacteria are only a small fraction of a micron in diameter. 
Migula states that the stained rods of Ps. zndigofera (Voges) Mig. from colonies 36 
hours old measured only 0.18 by 0.06 micron. Bacillus denitrificans (Amp. & Gat.) 
Mig. is also a very small rod—t.o to 1.5 by 0.1 to 0.3 micron, according to Migula. 
Micrococcus progrediens Schroter is said to be only 0.15 micron in diameter. ‘The 
organism of peri-pneumonia isolated by Nocard & Roux is more minute. It is 
probable also that still smaller organisms occur, 2. ¢., so small as to be invisible under 
the highest magnifications. In this way are interpreted the results obtained by 
animal pathologists in the foot-and-mouth disease and in some other diseases. 
Photographs with ultra violet light may in the end render some service here. ‘The 
*Fic. 11.—a, Capsule of organism obtained from black spot of the plum. Bacteria grown in 
Uschinsky’s solution and stained by Ribbert’s method; 0b, ropy Uschinsky solution from which 
a was made. 
