456 EFFECT OF HEAT [Ch. XVIU 



As for the minimum we find that, while it varies somewhat 

 with the optimum, it never falls below 0° C. The reason for 

 this is clear ; for, as we have already seen (p. 241), 0° C. is the 

 minimum for most vital activities. The fact that the minimum 

 for phanerogams is given some distance above 0° is partly due 

 to the fact that, since growth becomes very slow towards 0°, the 

 absolute minimum for growth is hard to find. Kiechnbe ('83), 

 who paid particular attention to this matter, concluded that 

 the minimum temperature of both radicles and plumules of 

 many seedlings lies between 0° and 1° C. However, we cannot 

 ignore the fact that Cucurbita, for example, has a minimum for 

 growth considerably above the point of cold-rigor , nor that, 

 in pathogenic bacteria, the minimum is near the optimum of 

 some free-living organisms. These facts teach us that the con- 

 ditions for growth may be surpassed before metabolism has 

 wholly ceased. 



The maximum temperature tends to be rather constant ; 

 inside of the group of phanerogams the range is only from 37° 

 to 46°, or 9°. But 45° to 46° is a fatal temperature for most 

 plant protoplasm (p. 234), and 50° is the outside limit ; 

 hence the death-point (ultra-maximum) lies very close to — 

 only slightly beyond — the maximum temperature for growth. 

 It is probable that growth ceases where heat-rigor comes in. 

 The extraordinarily high resistance of Bacillus thermophilus 

 can create no surprise after our study of the capacity of organ- 

 isms for acclimatization to temperatures near the boiling-point 

 of water (p. 250). It is merely another striking case of the 

 capacity of protoplasm for self-adjustment. 



The range of growing temperatures varies, as we have seen, 

 with the species. The greatest range in our table is that of 

 Bacillus subtilis, 44°. It is tolerably uniform for phanerogams 

 (87° to 32°); but in bacteria we have a range of 38° in the 

 case of Bacillus phosphor escens to only 12° in the case of 

 Bacillus tuberculosis. Here, again, we see a relation between 

 the vital peculiarities and the environment of the organism. 

 B. phosphorescens lives on the surface of the sea, whose tem- 

 perature varies with that of the air ; whereas B. tuberculosis 

 lives in the mammalian body, whose temperature is nearly con- 

 stant. It is interesting that the temperature of 42°, which is 



