[94 



METABOLISM 



If we express these results graphically we obtain a curve similar to that 

 obtained for many other physiological processes. Fig. 36 represents such 

 a curve, based on Rischavi's experiments on germinating wheat, where the 

 abscissae represent days and the ordinates the amounts of carbon-dioxide pro- 

 duced daily in mg. 



The examples quoted above furnish us with an approximate estimate of the 

 variations in respiration but are not adapted to exact comparative study, because 

 in some cases the oxygen absorbed is determined, in others the carbon-dioxide 

 produced, the calculations being based either on volume or on weight and be- 

 cause in some cases the fresh weight, in other cases the dry weight or the 

 volume of the parts are used to found estimates on. Strictly speaking none of 

 these methods are quite accurate, for we shall see that it is the living protoplasm 

 that is really the seat of respiration. The point of special interest is to determine 

 whether or not differences exist in the amount of respiration taking place in the 

 protoplasm of these organs, but unfortunately we have no data as to the amount 

 of protoplasm, either volumetric or gravimetric, to serve as starting-points 

 for such a comparison. All we know is that the amount of protoplasm present 

 in young organs is relatively much greater than in mature organs, and that so 

 far as we at present are aware, the respiratory variations at different develop- 

 mental stages maybe thus, at least partly, 

 accounted for. It is, however, also extremely 

 probable that a given amount of protoplasm 

 may respire with varying intensity accord- 

 ing to its condition. It may be sufficient 

 at present to draw attention to the two 

 chief vital conditions in which protoplasm 

 occurs, viz. the active and the passive, 

 the former being exhibited during the 

 vegetative period, the latter during the 

 summer or winter resting period. Under 

 constant external conditions the resting 

 protoplasm of tubers, bulbs, trees, &c , 

 exhibits marked differences to the same 

 protoplasm in the active state, evidenced 

 by the greatly diminished intensity of respiration, but so long as the necessary 

 external factors are present no protoplasm entirely ceases to breathe. 



Proof of this continuous respiration is not always easy to establish since it 

 may be completely masked by other processes. Thus, as we have already seen, 

 cells containing chlorophyll decompose carbon-dioxide in sunlight. Such cells, 

 even though they be respiring can still give off oxygen, or may, if respiration and 

 assimilation be equally active, fail to show any evidence of gaseous exchange. 

 In fact as the light decreases in intensity the amount of oxygen given off also 

 decreases ; later on, it ceases altogether, and finally an evolution of carbon- 

 dioxide manifests itself instead. This is, doubtless, most simply explained 

 by assuming that respiration and assimilation go on concomitantly and quite 

 independently of each other. Although there are no good reasons for assuming 

 that reduction and oxidation go on in the same cell, yet it is very difficult to 

 prove this exactly, since it is possible that respiration, which can be easily demon- 

 strated to occur in a green leaf in the dark, is masked when the leaf is exposed to 

 light. Observations made on non-green tissues and organisms do not aid us 

 much in this relation. For long, attempts have been made to study these two 

 antagonistic functions separately and not without a certain amount of success. 

 Cl. Bernard (1878) was the first to inhibit assimilation by means of chloro- 

 form vapour, and Ad. Mayer (1879) noted that a similar result might be ob- 

 tained by using prussic acid. It has been shown generally that assimilation may 



Fig. 36. Curve of carbon-dioxide excretion (in 

 mg.) of 40 wheat-seedlings at a temperature of 

 21^ C. After RiscHAVi (1876). 



