568 RESPIRATION AND FERMENTATION 



heat is less than might be expected from the respiratory activity, it does 

 not follow that the difference represents the amount of chemical energy 

 which has been directly converted into work, for the consumption of heat 

 in transpiration may lower the plant's temperature below that of the 

 surrounding air, and when heat is converted into work by an osmotic 

 manifestation, this is an endothermic process which would be produced 

 equally well by a supply of energy derived from the external world, and 

 without any need of the assistance of the heat evolved by respiration. 



The amount of the respiratory energy converted into work probably 

 differs in different plants and varies in the same plant according to circum- 

 stances : thus as the temperature rises respiration continually increases, 

 whereas growth and movement diminish beyond a certain optimum. 

 Similarly when rotation is inhibited by the action of ether, the undiminished 

 respiratory activity is no longer required to supply energy for this purpose, 

 and this energy is therefore manifested in the form of heat. Rapid growth 

 is usually but not always accompanied by active respiration, but certain 

 plants undoubtedly work more economically than others. Even when the 

 respiratory activity is apparently altogether out of proportion to the needs 

 of the organism, the latter may nevertheless possess a certain special 

 importance in the economy of nature in virtue of this seemingly superfluous 

 activity. This may be the case as regards many fermentations in which 

 disproportionately large amounts of material are decomposed and energy 

 liberated. It is, however, uncertain whether other anaerobes exhibit a less 

 marked production of heat, and hence work in a more economical manner. 

 Obligate aerobes produce much less heat in the absence of oxygen than 

 in its presence, but this is simply because intramolecular respiration does 

 not afford a supply of energy sufficient to satisfy the normal requirements. 

 To obtain the same amount of energy by anaerobic metabolism requires 

 a greater consumption of a given substance than when it undergoes perfect 

 aerobic combustion. 



The chemical energy latent in a given substance is not necessarily 

 decisive as to its nutritive and respiratory value (Sect. 66), for certain 

 bodies, such as sulphuretted hydrogen, &c., can be utilized by a few 

 organisms only, and others cannot be assimilated in any form whatever, 

 as is the case for example with regard to the amorphous or crystalline 

 varities of carbon, &c. Moreover, substances may be utilized in respira- 

 tion which by themselves alone are unable to yield an adequate supply 

 of energy. Complete isodynamic decomposition does not appear to be 

 of general occurrence in plants, but in animals it probably happens to 

 most of the actual nutrient materials, for here the production of heat by 

 katabolism is of especial importance l . 



1 Cf. Pfeffer. Jahrb. f. wiss. Bot, iSpj, Bd. \xvni, p. 258. 



