SUPRA-MAXIMAL TEMPERATURES 225 



can withstand desiccation are extremely resistant to heat when dry, and 

 may indeed be uninjured by heating to 90 or to 100 C. for several hours, 

 although when moist a temperature of 45 to 50 C. may soon prove fatal 1 . 

 The moist spores of certain bacteria are even able to withstand from a half 

 to several hours' immersal in boiling water, which kills most plants almost 

 instantaneously. Death usually, indeed rapidly, ensues at 70 C., which 

 temperature permits active growth in the case of certain bacteria, while a few 

 organisms may even grow at 85 C. 



All turgid plants ultimately die when the temperature surpasses 

 the maximum by one or two degrees, although growth may at first 

 still continue. Hilbrig 2 found that the radicle of Vicia Faba ceased to 

 grow after sixty to ninety minutes' immersal in water at 35 C., and died 

 in another fifteen minutes, whereas under favourable conditions growth 

 was resumed after five or six hours when the heat-rigor had lasted half 

 an hour, and after about a day when the exposure lasted an hour. In 

 such fungi as Penicillium glancum, Mncor stolonifer, and Cladosporium 

 herbarum, the heat-rigor may last a long time before death ensues. Thus 

 when Penicillium, whose maximum temperature for growth lies at 34 C., 

 was kept in a nutrient solution at 35 C., the mycelium died only after 

 thirty-one days and the spores after fifty-four. Previously to this recovery 

 is possible, and the longer the exposure, the longer will be the time of 

 recovery. Thus after exposure to 35 C. for fifty-one days, the spores of 

 Penicillium glaucum took eleven days to germinate at 22 C., whereas after 

 two days' exposure, only two days were required for germination. 



Certain micro-organisms may be capable of enduring heat-rigor for 

 a still greater length of time, whereas others may be rapidly killed. 

 Thermo-bacteria appear to be rapidly killed when the temperature rises 

 above their maximum, but it does not follow that a plant with a low 

 maximum will be able to withstand prolonged heat-rigor. Hilbrig found, 

 for instance, that a water-bacterium whose maximum lay between 

 34 to 35 C. was killed by five days' exposure to 35 C. 



When the temperature lies much above the maximum death is 

 naturally more rapid. Thus the mycelium of Penicillium is killed in 

 two days by a temperature 6 C. above the maximum. All flowering 

 plants, and most others as well, seem to be rapidly killed by a temperature 

 10 C. above the maximum. Sachs and de Vries found, for example, that 

 ten to thirty minutes' immersal in water at 51 to 52 C. killed most flowering 

 plants whose maxima in air lay between 40 to 45 C. 3 Indeed prolonged 



1 For instances of this in mosses and lichens cf. Ewart, Journ. of Linn. Soc., 1896, pp. 369, 374, 

 378, 388. 



2 Hilbrig, Ueber d. Einfluss supramaximaler Temperatur a. d. Wachsthum d. Pflanzen, Leipziger 

 Diss., 1900, p. 15. Similar results were obtained with the aerial parts of seedlings. 



3 Sachs, Flora, 1864, p. 33 ; de Vries, Materiaux p. 1. connaissance d. Tinfluence d. 1. tempera- 

 ture s. 1. plantes, 1870, p. 2 (repr. from Archives Neerlandaises, T. v). 



PFEFFER. II Q 



