MORPHOLOGY. 



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largest bacteria are several thousand times as bulky as the smallest. Errera has 

 described a spirillum the largest specimens of which measured 23 to 28 by 3 to 3.4 

 micra ('02, Errera, Bibliog., X), and Schaudinn has described a bacillus the largest 

 forms of which are 24 to 80 by 3 to 6 micra ('02, Schaudinn, Bibliog., XI). 



In shape the bacteria vary according to genera and species and sometimes 

 within the limits of the species, from globose cells or very short straight rods, through 

 curved forms or spirals, to filaments which are many 

 times the diameter of the organism. To what ex- 

 tent does form vary under changed conditions ? With 

 the eye-piece micrometer make careful measure- 

 ments of unstained organisms taken from the host- 

 plant and from cultures of various ages and kinds. 

 There is frequently considerable variability in the size 

 of individuals of the same species. Is the breadth 

 more constant than the length? Does the size or Fig. 12.* 



shape as observed in the plant differ from that observed on culture media? How does 

 the living organism differ in size and general appearance from the dead, stained one? 



CAPSULES. 



The presence of capsules may be suspected whenever a bacterial 

 growth becomes viscid. They are often difficult to see because their 



index of refraction is so nearly 

 that of the fluid in which they 

 are usually examined. In ex- 



V~v ", ;,', I .. amining unstained material the 



field should be illuminated with 

 j a narrow pencil of rays, and the 

 effect of illumination with ob- 

 lique light should be tried. 

 Several methods of contrast 

 staining are in use. By one 

 '^xj. fc - l0v method the capsule remains un- 

 stained or nearly so, while the 

 central portion of the bacterium 

 and the slime lying on the cover 

 between the bacteria stain more 

 or less deeply. By another 

 method which has been spe- 



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Fig. I3.t 



Fig. I4.{ 



*Fir,. 12. A portion of the yellow ooze from the black spot of the plum, stained by ordinary 

 methods. X 2,000. 



tFic. 13. Cobwebby, sticky threads of Bacillus trachcifhilus drawn from the cut end of a 

 muskmelon stem, arranged on a slide and stained with carbol-fuchsin. About three times natural 

 size. Buzzards Bay, Mass., Oct. 8, 1903. Fig. 14 was drawn from the left-hand thread at the 

 point marked X. 



JFic. 14. Bacillus trachciphilus Erw. Sm. A portion of one of ithe threads shown in fig. 13. 

 The arrow indicates the direction of the thread, which was extremely tenacious. The distance be- 

 tween the bacterial rods indicates very clearly the extreme viscosity of he unstained substance 

 lying between them and holding them together. X 1,000. 



