294 BACTERIA IN RELATION TO PLANT DISEASES. 
The thermal death-point is 43° C., the lowest yet recorded for any organism infesting 
plants, and until recently the lowest known. 
In an experiment made in June, 1896, an exposure of one hour to 41° C. killed all, 7. ¢., 
no colonies developed on the agar plate poured from the heated bouillon (11 days incubation) 
whereas the check-plate, i. e., the plate inoculated from the tube before heating yielded 
several thousand colonies per square centimeter. 
Exposure in bouillon for one hour at 40° C. killed three-fourths or more, as determined 
by agar poured-plates. Exposure to 40° C. for one-half hour destroyed about half. 
In many ways B. tracheiphilus is a very sensitive organism, 
and consequently it is difficult to work with. It is hard to plate out 
owing to its viscidity. It is very sensitive to heat, to dry air, and 
to direct sunlight. Freezings are also harmful. It is sensitive also 
to its own decomposition products, especially acids. It is not a 
rapid grower nor a very copious one on culture-media and vigorous 
organisms crowd it out. It does not, however, lose virulence readily 
by cultivation on artificial media. 
_ On most media, transfers must be reasonably frequent to keep 
it alive. It was alive once on sweet potato after 33 days. It was 
dead on carrot after 33 days. In one instance on steamed Irish 
potato, parts of a culture were alive after 26 days. In another 
instance a potato culture (which did not gray) was dead at the end of 
16 days (see also plant inoculations, pages 275, 283). It has lived, 
however, in some of the writer’s slant agar-cultures for several 
months and in litmus milk for 3 months; it may be kept alive for 
several months in peptonized beef-bouillon with addition of cane- 
sugar, if calcium carbonate is also added so as to neutralize the acid 
as fast as it is formed. Often from agar-stab-cultures a few months 
old it is recoverable, if at all, only by pouring sterile bouillon into the 
tubes and incubating for a week or more at 25° C. It is often recov- 
erable from the interior of the plant only by direct streaks on potato 
or other suitable medium, or by putting the viscid slime into bouillon 
for 6 to 24 hours before attempting plates. In this case great care 
must be used to avoid surface intruders and the second or third 
dilutions, if they cloud, are best for the plates. In experiments 
made in May-June, 1895, the organism was dead in the upper part 
of agar-stabs at the end of six weeks at room temperature. The 
organism was alive in saccharose-peptone-bouillon after 10 days, 
but not after 24 days (acid was detected prior to the tenth day). 
RESUME, OF SALIENT CHARACTERS. 
POSITIVE. 
Fig. 93.* 
A bacillus in the vascular bundles of cucurbits causing a wilt- 
disease; short rods (single, paired, in fours end to end, or in small clumps); motile, peri- 
trichiate; capsules; pseudozoogloeae; involution-forms; stains readily; smooth; white; 
viscid; glistening; slow grower on media; surface colonies small, round, discrete; no 
growth at 37° C. or at 6° C. (16 days); aerobic; facultative anaerobic (with grape-sugar, 
cane-sugar or fruit-sugar); from these sugars a non-volatile acid, soluble in ether; grows 
only in open end of F-tube with dextrine or glycerin, acid from glycerine; slime on steamed 
*F 1G. 93.—Method of making a culture of Bacillus tracheiphilus on slant agar in hydrogen. After thoroughly 
displacing the air, tubes were sealed in an open flame at the constricted parts. These tubeswere about 9 inches long 
and 1 inch or more indiameter. In later experiments Novy jars were used. 
