230 PHYSIOLOGY OF NUTRITION 



exerted no accelerating influence in the case of the frozen and thawed tissues, 

 which were dead but still contained their enzymes. 



Respiration in living plants is thus accelerated, not only by certain sub- 

 stances that are necessary for life (such as co-enzymes), but also by unnecessary 

 and generally injurious substances (poisons, in the usual sense). Both kinds of 

 substances produce the same result, namely an acceleration of respiratory activ- 

 ity, but the chemical responses within the cells are quite different in the two 

 cases. In one case we have to do with phenomena of nutrition and in the other 

 case with those of poisoning. In living plants this difference is not apparent, 

 but Ivanov 1 has clearly demonstrated it with plants that were killed without 

 destroying their enzymes. Phosphates, which belong to the class of necessary 

 accelerators, produce a marked influence, both upon living plants and upon those 

 that have been killed but that still retain their enzymes. 



§11. Materials Consumed in Respiration. — Notwithstanding the fact that 

 respiration in plants is accompanied by a decrease in carbohydrates and fats, 

 which are non-nitrogenous, it was generally supposed until quite recently that 

 such nitrogen-free compounds were not directly consumed in the process and 

 that atmospheric oxygen acted directly to oxidize only proteins. The nitro- 

 genous residues left as products of protein decomposition were supposed to 

 combine with carbohydrates, thus regenerating the proteins. According to 

 this conception, as long as the supply of reserve carbohydrates is not exhausted 

 the amount of protein material in the organism remains unchanged, while the 

 non-nitrogenous reserve gradually diminishes; but as soon as the reserve of 

 carbohydrates has been exhausted then decomposition of proteins becomes 

 apparent and the nitrogenous products of this decomposition begin to accumu- 

 late. Evidence in favor of the idea that protein is directly oxidized in respira- 

 tion was found in the fact that the respiration process is especially active in 

 young, growing tissues, rich in protein. This conception has proved to be un- 

 tenable, however. 



The protein of the organism does not remain constant in amount as long as 

 carbohydrates are available; in the germination of seeds, for example, the de- 

 composition of proteins proceeds most rapidly in the earliest stages of germina- 

 tion, when the seeds are still very rich in carbohydrates. With decreasing car- 

 bohydrate content protein decomposition becomes less vigorous and finally 

 may even cease altogether. To give an illustration of this, 100 g. of wheat seeds 

 contained 0.0668 g. of protein nitrogen, and etiolated seedlings six days old, from 

 a similar lot of seeds, contained only 0.0554 g., so that 0.0114 g. of protein nitro- 

 gen had been lost during germination. When the seedlings were fourteen days 

 old their content in protein nitrogen was 0.0549 g., so that only 0.0005 g- na d 

 been lost during the last eight days. The progress of protein decomposition 

 in dark-grown wheat seedlings has been graphically shown in Fig. 88. 



Carbohydrates are necessary for aerobic respiration, even in the presence of 

 an excess of proteins. Etiolated bean leaves, which are rich in protein but con- 



1 Ivanov, N. N., Action des agents stimulants utiles et nuisibles sur la respiration des plantes. [Russian.] 

 Bull. Acad. Imp. Sci. St.-Petersbourg VI, 4: 571-581. 1910. This is also reported in the second refer- 

 ence given in note 1, p. 217.] 



