THERMAL RELATIONS. 87 
Some bacteria grow well only in the cool box, others only in the thermostat 
at blood-heat or at higher temperatures,—temperatures elevated enough to quickly 
destroy the unprotected protoplasm of the higher plants and animals. Few of the 
bacteria commonly studied will grow at temperatures much above 40° C., but this 
by no means expresses the whole truth. 
The lowest temperature at which growth will take place ranges in different 
species all the way from 0° C., and probably a few degrees below (certain salt-water 
bacteria) to + 40°, + 50°, + 56°, and even + 60° C. (certain thermophilic species 
found in dung-heaps, hay-mows, silos, hot springs, etc.), ‘The highest temperature 
at which growth will take place ranges from as low as 30° C. (and probably lower*) 
to as high as 75°, or 80° C., or even 89° C., according to Setchell. Higher temper- 
atures have been recorded, but I have here used only those determined with care in 
the exact places frequented by the bacteria. This will be better appreciated if it is 
remembered that a temperature of 60° C. (140° F.) can be endured by the fingers only 
a few seconds, while 70° C. (the optimum for some of these species) is intolerable 
to the hand even for the shortest period. It seems incredible, on first thought, it is 
so opposed to our customary observations, that any organism whatsoever should 
be able to live at a temperature only 11 degrees below the boiling point of water 
Nevertheless, protoplasm is an extremely adaptable substance, and it is conceivable 
that some organisms might grow at a temperature considerably higher. 
The thermal death-point (10 minutes exposure) ranges from 43° C. for Bacillus 
tracheiphilus, the lowest yet recorded, to temperatures only a few degrees under 
the boiling point (100° C.). For many species the thermal death-point lies between 
50° and 60°C. Russell & Hastings have recently discovered in milk a Micrococcus 
whose thermal death-point is 76° C. 
As the upper and lower thermal boundaries of growth are approached some 
functions are extinguished in advance of others; e. g., pigment production, patho- 
genicity, and sporulation disappear considerably in advance of loss of power to 
reproduce by fission. 
OTHER HOST PLANTS. 
Plants of related species, genera, and families should be tested. If the disease 
appears to be new to literature, it is also especially important to inoculate those 
plants which have been reported to be subject to bacterial disease and the nature of 
which disease is still in doubt. Many facts of scientific and economic interest will 
be brought to light in this way, and now and then the experimenter may be able to 
clear away some of the fog which, owing to the uncertain and contradictory state- 
ments of a majority of our plant pathologists, still hangs over the origin and nature 
of most of these diseases. 
Some plant pathogenes appear to be quite narrowly restricted. ‘They attack 
only one host plant, or at most a few hosts belonging to related species or genera. 
Others, particularly some of the soft-rot bacteria, attack many kinds of plants belong- 
ing to widely different families. The history of pear-blight, however, shows us that 
*Since this was written Molisch states (1. c., p. 93) that gelatin cultures of his Bacterium phos- 
phoreum were dead at the end of 48 hours when exposed to a temperature of 30° C. The maximum 
temperature of this organism is said to be about 28° C, 
Very recently Marsh has found a fish parasite which is said to have a thermal death-point of 
42° C. (See VI, Bibliography of General Literature.) 
