424 The Physiology of Plants BOOK in 



It was not difficult to perceive on studying the different 

 items of information yielded by these researches, that a 

 considerable share of protoplasmic activity is indicated. 

 Attention was consequently drawn by the more thoughtful 

 of the physiologists of the time to the living substance of 

 both animals and plants, and a new theory was put forward 

 by Pfliiger in 1875. 



Pfliiger was at the time advancing the views we have 

 already alluded to as to the constitution of living sub- 

 stance and its relations to dead proteins, coming to the 

 conclusion that in the former the nitrogen is linked directly 

 with carbon, forming cyanogen groupings. His theory of 

 respiration was that the entering oxygen combines or unites 

 in some way with the cyanogen radicals of the living 

 substance. These cyanogen groups already contain a large 

 quantity of internal energy, all their compounds showing 

 a ready tendency to decomposition. The approximation 

 of oxygen to such a labile group, in which already there 

 is active intramolecular vibration of the carbon and nitro- 

 gen atoms, brings about a readjustment, carbon dioxide 

 and water appearing, together with more complex sub- 

 stances, among the products of the decomposition. 



If we consider this process as proceeding simultaneously 

 with the process of nutrition or repair of the living substance, 

 we see that on Pfliiger's hypothesis the protoplasm is the 

 seat of constant changes of building up and breaking down 

 so long as life lasts. The instability of some of the groupings 

 of its constituent atoms causes the decompositions to be 

 of an explosive character. The latter constitute the respira- 

 tion of the living substance. The entry of the oxygen 

 increases the instability and provokes the explosive decom- 

 positions. 



Pfliiger's hypothesis receives considerable support from 

 observations made before and since it was advanced. Long 

 before, DeSaussure had found that when he subjected to the 



