SUMMARY AND CONCLUSIONS. 107 



which the plant utilizes he does not regard as a necessary assumption. It may 

 be true that in many cases the amount of carbon dioxide and water produced 

 can be shown to balance with the amount of sugars consumed, but that is no 

 satisfactory proof that the sugar as such is actually split up in the processes of 

 respiration. It is far more likely that it forms combinations with the molecule 

 of the living protoplasm itself and in such connection the actual oxidations and 

 release of energy take place. The union must be an unstable one and with 

 our present incomplete knowledge of the structure of the protein molecule we 

 can hardly postulate more than the probable occurrence of a linkage of the 

 sugar or some derivative of it with the living protoplasm. Its union with the 

 active protoplasm, no matter how tenuous it may be, would afford the means 

 by which the chemical energy released by the oxidation processes is brought 

 in relation with the living substance. Such a picture, crude as it is, can not 

 lead us so far astray from what actually happens as to consider that the energy 

 release takes place simply in the sugars themselves and not in intimate con- 

 nection with the protoplasm. 



To return to the question of the role of deacidification in this regard, it may 

 be pointed out that the malic acid present can be broken down in vitro in the 

 presence of light and free oxygen. The acid is presumably present in the cell 

 in the vacuole and when the plant is exposed to the sunlight the deacidification 

 processes very probably take place there, quite removed from the sphere of 

 action of the living protoplasm. The writer is aware that Nathansohn takes 

 a wholly different view and regards the deacidification processes as simply a 

 second step in the katabolic changes, but to the writer such seems improbable 

 in view of all the evidence. The chief reason for assuming that the splitting 

 of the acid is a respiratory process seems to be that such a use of it would be 

 economical from the standpoint of the cactus. It is obviously not necessary 

 to make such an assumption, for the accumulation of acid seems, by general 

 agreement, to be the result of imperfect oxidation which in turn is due to the 

 morphological structure of the cactus. Consequently, whatever of energy 

 there may be from the final oxidation of the acid outside the sphere of proto- 

 plasmic activity is simply the result of anatomical peculiarities of the plant, 

 the advantages of which may well outweigh this loss. Any cause which leads 

 to the splitting of the malic acid or its derivatives, whether it be light, high 

 temperature, or prolonged darkness, increases the total evolution of carbon 

 dioxide faster than it does the absorption of oxygen. Consequently, the 

 C02/O2 ratio rises and approaches or even exceeds unity. What is considered 

 the normal ratio of these gases in katabolism is thus established, but in reality 

 the approach to the typical gas-relation is not real, for it is supplied simply 

 by the breaking down of the acid. It is to be observed at times when the 

 acidity is high and is falling and is brought about by processes which have 

 no immediate relation with the energy-releasing reactions of the living proto- 

 plasm. The true respiratory quotient of the cacti is that which is attained 

 when the acidity is stationary or is rising. At such times there is always less 

 and sometimes very much less carbon dioxide evolved than oxygen absorbed 

 and in consequence the CO2/O 2 ratio is less than unity. This primal fact was 

 established in the original observations of de Saussure, and has been supported 

 by subsequent investigators, but the interpretation adopted here presents the 

 question in a somewhat new light. 



