Quantitative Analysis of the Groivth 0/ Helianthus annuus. 369 



tinder field conditions. In addition, the results afford information as to the 

 effect of age upon respiration. Furthermore, it is thus possible tol compare 

 the effect of progressing age upon the rate of respiration with the effect of 

 age upon the relative rate of increase in dry- weight. 



For each plant process or group of processes, such as respiration or growth, 

 there is a group of factors within the plant which will affect the rate of the 

 particular process or group of processes. Each group we will call the 

 " internal " factor for that process, it being clearly understood that groups 

 may have factors in common or may even be identical. This quantitative 

 study of plant growth may enable one to elucidate the interconnection of 

 these " internal " factors. 



2. The Respiratory Index as a Measicre of the Effect of the " Internal " Factor 

 upon the Rate of Respiration. 



In any attempt to determine the effect of the " internal " factor upon the 

 rate of respiration throughout the life cycle of the plant the other factors 

 affecting respiration should be standardised, that is, kept constant or not 

 limiting throughout the experiments. 



In view of our present knowledge of respiration the factors which may 

 affect the rate of respiration per unit of dry- weight* can be classified as 

 follows: — 



1. Concentration of respirable material. 



2. Effective amount of respiring cell-matter per unit of dry-weight. This 

 we term the " internal " factor for respiration. 



3. Concentration of oxygen. 



4. Temperature. 



Our measurements of respiration have been made per gramme dry-weight. 

 In order to make the results strictly comparable from the point of view of 

 the " internal " factor, the respiration should be measured when the amount 

 of respirable materia^l is in excess, the internal concentration of oxygen 

 constant and the temperature at a definite constant value. Under such 

 conditions the respiration per gramme dry-weight will be a measure, for 

 purposes of comparison from time to time, of the effective amount of 

 respiring cell-matter per gramme dry -weight, that is to say, of the " internal " 

 factor. 



It has been generally assumed that the concentration of oxygen in the 

 atmosphere {i.e., about 21 per cent.) provides an excess of oxygen for 



* Having already decided to express growth per unit of dry-weight, it is necessary to 

 use the same unit when dealing with respiration, in- order to make the results comparable. 



