THE EVOLUTION OF HEAT BY AEROBES 377 



with moisture was shown between 10 a.m. and 3 p.m., and usually became imper- 

 ceptible towards evening and remained so until morning. 



Dutrochet has shown that the daily periodicity continues for a few days in 

 darkness with decreasing amplitude, and is reinduced on re-exposure to periodic 

 illumination, the other conditions remaining constant. This periodicity, therefore, 

 closely corresponds to the periodicity of growth and movement induced by the 

 intermittent daily illumination. It is, however, uncertain whether the periodicity of 

 heat-production in the spadices of Aroids is produced in this way or not, nor has it 

 been determined whether the rise of temperature may not be due to a decreased loss 

 of heat as well as to an increased production of heat. No daily periodicity in respira- 

 tion has yet been determined *, although in fleshy plants the daily accumulation of 

 organic acids in the tissues indicates a periodic diurnal alteration of respiration. 



The spadices of Aroids have but little chlorophyll, and Hymenomycetes have 

 none, so that the daily periodicity of heat-production in these forms can hardly be 

 dependent upon photo-synthesis. Nor is it due to the fact that the lessened growth 

 in the daytime consumes less of the energy of respiration, for the spadices of 

 Aroids have ceased to grow when the production of heat is most active ; and a daily 

 periodicity of heat-production is shown, according to Dutrochet, by adult Cactus 

 stems 2 . The rise of temperature is certainly not the direct result of the absorption 

 of heat from the radiant light-rays, although the latter by favouring transpiration 

 may cause an increased loss of heat, and hence lower the temperature. A periodicity 

 of temperature is also shown by man, the maximum at evening being about i2C. 

 higher than in the morning. 



SECTION 82. The Production of Heat by Anaerobic Metabolism. 



Anaerobic metabolism probably always involves a liberation of heat, 

 and alcoholic fermentation, in the absence of free oxygen, always produces 

 a distinct rise of temperature. No detailed researches on the production 

 of heat by anaerobic metabolism have been performed 3 ; and although 

 the heating of dung and of fermenting fluids is mainly due to anaerobic 

 metabolism, it is not impossible that anaerobic organisms may exist 

 whose normal metabolism involves an absorption of heat, just as that 

 of green plants involves an absorption of light. In such cases the 

 temperature of the plant will be continually below that of the surround- 

 ing medium. An absorption and extinction of the dark heat-rays does 

 actually occur in the purple bacteria, the energy of these rays being used in 

 photo-synthesis ; but in this case the supply of energy precedes the endo- 

 thermic chemical change, whereas in the other the endothermic chemical 

 change is supposed to take place first, the subsequent inflow of heat from 

 without following as a natural consequence of the fall of temperature 4 . It 



1 Cf. Kolkwitz, Jahrb. f. wiss. Bot., 1898, Bd. xxxni, p. 128. 



2 G. Kraus, I.e., 1884, P- J 7- 



3 Popoff (Bot. Jahrb., 1875, p. 286) observed a slight warming during marsh-gas fermentation ; 

 Rubner, Hygienische Rundschau, 1903, Bd. xin, p. 753. 



4 Cf. Pfeffer, Studien zur Energetik, 1892, p. 189. 



