MORPHOLOGY. fe) 
largest bacteria are several thousand times as bulky as the smallest. Errera has 
described a spirillum the largest specimens of which measured 2 3 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 (’o2, 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 
L index of refraction is so nearly 
em . aemm that of the fluid in which they- 
“eee sa ms are usually examined. In ex- 
SS, se =f bg Ne _ amining unstained material the 
Fa, 478 s AR, ; field should be illuminated with 
= bay a Ea. fa narrow neil of d th 
ee a a al Tp a co) pe cil o rays, an the 
id A ¥ PF 6.) effect of illumination with ob- 
, “snk, x £ We nate : lique light should be tried. 
ao) * "¢ ~ eg os ? #3 Several methods of contrast 
Ns Sa é, The ,” staining are in use. By one 
: pre te UNS NE S| smethod the capsule remains un- 
ey ota ere : 
eee RS 5 sy stained or nearly so, while the 
x ad ir | ead ee aa central portion of the bacterium 
aoe. pk a) ed * and the slime lying on the cover 
We oe BIA, ST! Ss between the bacteria stain more 
— or less deeply. By another 
Fig. 13. Fig. 14. method which has been spe- 
*Fic. 12—A portion of the yellow ooze from the black spot of the plum, stained by ordinary 
methods. X 2,000. 
TF ic. 13—Cobwebby, sticky threads of Bacillus tracheiphilus 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. : 
tFic. 14.—Bacillus tracheiphilus Erw. Sm. A portion of one of the 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 the unstained substance 
lying between them and holding them together. X 1,000, 
