250 



SCIENCE. 



[N. S. Vol. XXI. Xo. 529. 



plant be fed on one food or another,* this 

 dissociation resulting in the formation of 

 carbon dioxid and of various other prod- 

 ucts. 



THE MECHANISM OF RESPIRATION. 



Let us now focus the light coming from 

 the chemistry of proteids, the mechanism 

 of combustion, and the physiology of res- 

 piration, to form a picture of what goes 

 on in the body. 



First: We should conceive of the respira- 

 tory dissociation as taking place in the 

 living material of the body and not in a 

 food still unassimilated. Experiments 

 with a wide range of foods have shown 

 that they affect the intake of oxygen and 

 the output of carbon dioxid in the most 

 diverse ways, whence it has been assumed 

 that the r&spiratory ratio varies because of 

 the way in which the given food is oxidized. 

 I do not say that it is not possible for the 

 protoplasm to decompose a sugar directly 

 or to oxidize a fat. But it must be remem- 

 bered that in no case has it been experi- 

 mentally proved that the food is directly 

 attacked, and that all the facts can be ex- 

 plained on the other assumption, and some 

 of them very much better than on the 

 theory of direct oxidation. Moreover, the 

 lability of proteids which have been raised 

 to the life-level is their most striking char- 

 acteristic as contrasted with their ordinary 

 stability. 



In such labile material the second step is 

 ea.si]y conceivable. There occurs a shift- 

 ing of the atomic groups within the mole- 

 cule, perhaps as a result of the last step in 

 their anabolism— the addition of hydroxyl 

 groups from the water everywhere present. 



* See a paper by Kostytsehew which has just 

 come to hand ('Ueber die normale und die anae- 

 robe Atmung bei Abwesenheit von Zucker ' 

 Jahrb. Wiss. Bot. 40 : 563-.592. 1904), showing 

 the erroneousness of Diakonow's idea that anae- 

 robic respiration is only possible when sugar is 

 supplied. 



Dissociation follows necessarily ; very slow 

 perhaps, at ordinary temperatures and 

 with a scanty supply of water, yet suffic- 

 ient evidently for the maintenance of life. 

 Such conditions may very Avell be those ob- 

 taining in resting organs, spores and seeds. 

 But normally this cleavage may go on at 

 a measurable rate, without anji:hing more 

 than the inevitable dissociation when hy- 

 droxylation has progressed to a certain 

 point. It seems, however, that there is 

 generally— perhaps always — a hastening of 

 this process, and that the highly unstable 

 protoplasm is dissociated so rapidly that it 

 liberates not only the energy immediately 

 utilized in growth, movement, etc., but 

 also an excess sufficient to be easily meas- 

 ured by so coarse an instrument as the 

 tliermometer. Catalytic agents like the 

 enzymes are certainly (I think I may be 

 permitted so strong an assertion) the usual 

 accelerators. And it is highly probable 

 that an enzyme identical with zymase or at 

 least analogous to it, is an active though 

 secondary agent in this acceleration. It 

 may very well be also that those changes 

 outside the protoplast (whether without 

 the organism or not) that are called stimuli 

 accelerate still further the katabolism, even 

 to an explosive speed in some cases. 



This primary dissociation may plainly 

 be independent of free oxygen, though it 

 is hardly conceivable that there will not be 

 some oxygen present unless the plant has 

 grown under most unusual conditions, 

 which one can scarcely realize experi- 

 mentally. The products of this decom- 

 position are not sufficiently known, nor is 

 their precise character important for our 

 discussion. Among them are certainly the 

 more complex amido-acids, carbon dioxid 

 and alcohol. 



Third: Up to this point the respiratory 

 processes are quite alike whether the plants 

 grow in the air or apart from it. If suffi- 

 cient oxygen be not present the disruptive 



