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 o 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 1 1 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, f 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- 

 phorcum 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. 



tVery 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.) 



