DESTRUCTIVE METABOLISM 407 



decomposition stops while yet the materials are complex, and hydrogen 

 appears because no oxygen is available to combine with it. 



AVhy carbohydrates disappear. — The end products, however, prob- 

 ably do not represent in any case the whole of the protein molecule. 

 Certain fragments of it, under suitable conditions, go down into CO2 

 and H2O ; but others are not so far split up that they cannot be rebuilt, 

 with necessary additions, into protein again. It seems to be the com- 

 ponents of the protein molecule derived from carbohydrates, which are 

 particularly liable to complete decomposition. If this nucleus alone were 

 broken up, the ratio of free O2 fixed to CO2 produced should have a 

 value of unity. This is not by any means true; the average is below i 

 and the value varies from 0.3 to 5.0; so it is probable that the process 

 is complicated by the interaction of other substances. The repair 

 of the proteins requires chiefly carbohydrates, because the nitrogenous 

 losses in the plant are quite inconsiderable as compared with those of 

 an animal. So a marked effect of respiration is a disappearance of the 

 accumulated carbohydrates. 



The assumption that carbohydrates are directly decomposed in respiration rests 

 largely on the fact that the value of the ratio O2 : CO2 is affected by the food supplied 

 to non-green plants. Thus, in Aspergillus it ranges from 0.43 with 10 per cent 

 tannin, to 1.78 with 10 per cent glucose, indicating that not composition alone but 

 other and unknown factors are concerned. And composition, as well as these un- 

 known factors, may produce this result indirectly, through their influence on assimi- 

 lation, quite as effectively as by directly modifying the " combustion " of foods. 



Loss of weight. — The transformation of carbohydrates in the repair 

 of proteins can have little effect on the weight of the plant; but the 

 escape of CO2 as a gas and the evaporation of the water produced does 

 result in a loss of weight. If the total dry weight of seeds be calculated 

 (the percentage of water in like seeds having previously been determined), 

 and these seeds be grown for some weeks in the dark, plants of consider- 

 able size can be raised. But on drying them, the residue will be found 

 to weigh less than the calculated dry weight of the original seeds. This 

 difference corresponds to the combined CO2 and H2O produced and lost 

 in the course of respiration. 



Production of heat. — The heat produced by respiration is often not 

 observable at all, unless some means are used to prevent its radiation and 

 its transfer to the air by the evaporating water. If a mass of wheat 

 seeds be germinated, a thermometer thrust into the mass will show a 

 temperature considerably higher than that of the air ; but this is due 



