26 



BACTERIA IN RELATION TO PLANT DISEASES. 



the contrary, as much as possible should be made out of morphology, particularly 

 that of the living organism, and in this connection the recent efforts of Migula and 

 Fischer are especially deserving of commendation. 



MOTILITY. 



If motile, determine kind of motion and rapidity 

 (margin of small hanging drops on thin covers sus- 

 pended over hollow-ground slides).! The cover may 

 be prevented from sliding by touching one edge with 

 a very little vaseline or cedar oil ; if too much is 

 used it runs under, mixes with the hanging drop, and 

 spoils the mount, and possibly in the end the objec- 

 tive is ruined, if the student continues to search for a 

 clear field. The beginner is very apt to mistake 

 Brownian movement for self-motility. It sometimes 

 requires very careful observation to be quite certain. 

 Rods which appear to be motionless will sometimes 

 be seen to dart away quite suddenly if watched. In 

 some species young cultures are much more apt to 

 be motile than old ones ; in others motility appears 

 to be an almost constant characteristic. The move- 

 ments of bacteria are sometimes quite characteristic 

 for particular sorts. They may be slow or rapid 

 tumbling motions centering in the shorter axis, or 

 straight or sinuous slow or rapid darting move- 

 ments in the direction of the longer axis, with 

 rotation on this axis. The media of Hiss ('97, Bib- 

 Hog., XVI) and of Stoddart ('97, Bibliog., XVI) are 

 sometimes useful for distinguishing macroscopically 

 between motile and non-motile forms. The former 

 spread as a thin layer over the whole surface, the 

 latter pile up in restricted areas around the points 

 of inoculation. The student should not remain con- 

 tent with merely determining motility, but when this 

 has been settled he should turn his attention to 

 staining the organs of motion. 



Fig. 24* 



*Fic. 24 Upright Zeiss camera for photomierographic work. The cup (a) slips over the end 

 of the microscope and forms a light-tight connection with the bellows without touching it. The 

 microscope rests on the table independent of the camera. The stout rod turns freely in the socket 

 X and is locked in place by a set-screw on the side opposite the observer. The height is about 

 45 inches. 



tLehmann and Fried (Arch. f. Hyg., Bd. XL VI, 1903, p. 3") found the swiftest movement of 

 bacteria to be I mm. in 22 seconds ; the slowest I mm. in 222 seconds ; average : cholera, I mm. in 

 3414 seconds; typhoid, I mm. in 56 seconds; B. vulgare, I mm. in 73 seconds; B. subtilis, I mm. in 

 40 seconds ; B. megaterium, I mm. in 2 minutes 1 1 seconds. 



