METABOLIC GRADIENTS 63 



of highest carbon-dioxide production being the region of 

 highest respiratory rate. Since the oxidations are un- 

 questionably reactions of fundamental importance in 

 the metabolic reaction system, the estimations of 

 carbon-dioxide production lead to the same conclusions 

 concerning the existence of metabolic gradients as do 

 the results obtained by the susceptibility method. 



So far as technical and other sources of error can be 

 eliminated, the rate of oxygen consumption of different 

 parts of the body may be used like the rate of carbon- 

 dioxide production as a measure of respiratory activity. 

 The use of this method in animal physiology has been 

 such that the data, while of great value for various other 

 purposes, have in most cases no bearing upon the problem 

 of metabolic gradients. In the plants, however, the 

 rate of both oxygen consumption and carbon-dioxide 

 production have been found to differ in different parts 

 in such a way as to indicate very clearly the existence in 

 the plant-body of metabolic gradients. The growing 

 bud, for example, respires at a higher rate than the 

 full-grown stem or leaf. 



Differences in electrical potential indicating differ- 

 ences of some kind in chemical or physical activity are 

 known to occur very generally in different parts of 

 both animal and plant organisms and even in different 

 parts of the same organ or cell. The presence of these 

 electrical differences gives no clue to the exact nature 

 of the physical or chemical differences which produce 

 them, but it is becoming more and more evident that in 

 both animals and plants they are to a large extent 

 associated with differences in metabolic activity. So 

 far as this is the case, we should expect in general that 



