292 BACTERIA IN RELATION TO PLANT DISEASES. 
The optimum reaction for growth in peptonized beef-bouillon is about +8 of Fuller’s 
scale. Growth on the acid side (acid of beef-juice) takes place up to about +28 (?); on the 
alkaline side (sodium hydrate) growth ceases at about —4(?). These statements are to be 
taken only as general indications, for here again much depends on the original reaction of 
the culture-fluid used for the inoculation. Growth may be pushed farthest on the acid side 
by inoculating from acid bouillons, and on the alkaline side by inoculating from alkaline 
bouillons, @. g.—2 bouillon will cloud when inoculated from +2 or o bouillon, but not when 
. inoculated from +25 or +20 bouillon. The 
strain used for these experiments was the 
one which did not infect squash (1905). 
In milk, growth continues for quite a 
long time but with no precipitation of casein 
or change in the appearance of the fluid. In 
litmus-milk there is little or no change of 
color, 7. e., no decided reddening or bluing 
of the fluid, or loss of color (reduction) not 
even after several months. It has seemed 
to me at times that I could distinguish a 
slight change in the color of the litmus-milk 
(bluing), but if any it is so slight as to be 
readily overlooked. Milk is, therefore, a 
good differential culture-medium. In old 
litmus-milk-cultures (dried out two-thirds), 
on the walls of the tubes above the fluid, 
very small branching fern-like crystals (fig. 
96) occurred after wetting the walls with 
the fluid, and these crystals did not appear 
in the 3 check tubes. On litmus-lactose-agar 
there is no change at first. After some time 
there may be a gradual deepening of the 
blue color, but never any reddening. This 
experiment was repeated in 1906 with the 
same result. 
Only one bacillus more sensitive to dry 
air is yet known, viz., B. carotovorus Jones. 
In the writers’ experiments, portions of solid 
cultures or fluid cultures were spread in thin 
layers and dried at room-temperatures on 
clean sterile cover-glasses and then tested 
by dropping from time to time into tubes of 
a bouillon known to be well adapted to the 
growth of the organism. In all cases the 
Fig. 91.* organism was found to be dead one-half 
hour to onehour after drying out, andinsome 
instances when taken from bouillon drying for so short a time as 15 minutes sufficed to kill 
it. Covers inoculated from the same bouillon and dried only for 10 minutes yielded cultures 
of the organism when thrown into bouillon. Much seemed to depend on the thinness of 
the layer. Possibly also the surface on which the bacillus is dried may exert an influence. 
The bacillus is also sensitive to sunlight (fig. 97.) ‘ 
*Fic. 91.—a, streak culture of Bacillus tracheiphilus on litmus-lactose agar after 7 days, showing frequent tend- 
ency of organism to grow in discrete colonies. Tube 15, June 30, 1904. Photographed July 7. 134. 6, Same 8 days 
old at 22° to 26°C. In this tube it is also growing in the form of astreak. Tube 4, June 16,1903. Photographed, 
June 22. X2. 
