DESTRUCTIVE METABOLISM 407 
decomposition stops while yet the materials are complex, and hydrogen 
appears because no oxygen is available to combine with it. 
Why 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 CO 8 
and H 2 O ; 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 O 2 fixed to CO 2 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 C>2 : COg is affected by the food supplied 
to non-green plants. Thus, in Aspergillus it ranges from 0.43 with ro 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 CO 2 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 CO 2 and H 2 O 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 
