404 Quantitative Analysis of Plant Growth 



The Methods of calculating Secondary Data. 



The question then arises, are the methods put forward by us for 

 calculating the above secondary relations inaccurate? This seems to be 

 the second possible interpretation of Mr Fisher's summary and is, more- 

 over, what he literally states. In the first place, in spite of the con- 

 clusion quoted above, Mr Fisher deals only with the methods proposed 

 for calculating Relative Growth Rate, R, and says nothing about the 

 methods for calculating the other secondary data mentioned above. 

 We defined R ( (5), p. 204) as the weekly percentage rate at which the 

 dry weight increases, and stated that if the rate were continuous com- 

 pound interest the equation for calculating R would be 



IFo = ,oge w 2 ~ Iog * w « (1) ' 



and if the rate were simple interest the equation would be 



R W 2 - W ! 



(2). 



100 w x 



Both these methods of calculation were put forward (5) 1 . Mr Fisher 

 urges the use of the first, and states the second to be inaccurate. As a 

 matter of fact both methods of calculation are perfectly in concordance 

 with our definition and neither of them can be in itself characterised as 

 inaccurate. 



The Use of Relative Growth Rate Values. 



A third possible explanation of Mr Fisher's criticism is that, instead 

 of meaning the charge of inaccuracy to apply to our methods of calcula- 

 tion, he means it to apply to the use we have made of the values of the 

 Relative Growth Rate calculated by means of equation (2). So far we 

 have published a paper (4) in which the fact has been recorded that the 

 Relative Growth Rate, calculated by either of the two methods described 

 above, follows the generalised form of curve shown in Fig. 9 of the 

 paper in question, and that the Leaf- Area Ratio curve follows a closely 

 similar course. It was pointed out at the time that, for the purpose of 

 demonstrating this fact, it is immaterial which of the two methods of 

 calculation is utilised, and indeed, in Fig. 1 a comparison was made 

 of the results obtained by the two methods. Mr Fisher has only 



1 The definition of Relative Growth Rate given in (4, p. 105) is for the rate calculated 

 on the simple interest basis. It must be understood that we do not suggest that the 

 growth of a plant is a process of accumulation of dry-weight at either "continuous 

 compound" or "simple" interest. On the one hand, the whole of the new material is not 

 put out as new capital, and on the other hand there is an unknown time interval before 

 any new material can become new capital. 



