530 PLANT GROWTH AND PLANT COMMUNITIES 



physiologically." He further postulated that if smoothed curves for the 

 five experiments were fitted on a common basis, then any departures 

 would represent the influence of external environmental factors on the 

 internal physiological processes. Combining all five experiments, Greg- 

 ory compared the variations in the environment within and between 

 seasons with the departures in relative growth rate, and from the cal- 

 culated multiple regression he concluded that the rate was primarily 

 dependent on a positive eflFect of the mean day temperature and a 

 negative effect of the mean night temperature while the term for the 

 light factor, expressed as total solar radiation per day, was very small 

 and negative. 



From the data for dry weight and leaf area Gregory also calculated 

 the net assimilation rate ( the unit leaf rate of Briggs et al. ) , and over 

 all the experiments he related the variation in rate to the seasonal 

 differences in the environment. In this instance the fitted multiple re- 

 gression showed a positive effect for solar radiation and day tempera- 

 ture and a negative linkage with night temperature. 



In 1928 Briggs criticized Gregory's treatment of his data in arriv- 

 ing at the effect of the environmental factors on the relative growth 

 rate. Briggs held that growth depends on the internal and external 

 complexes of factors and that the problem is to distinguish the influ- 

 ence of the environment at any given time from the drift in the internal 

 processes governing age and any previous effects of the environment. 

 He pointed out that "Gregory's methods attribute differences in rate 

 of growth in the nth period of existence (measured in days or as a 

 fraction of the total existence) solely to the difference in the environ- 

 ment of that period." There was also an unproved assumption in Greg- 

 ory's statistical treatment of the data: namely, that the effects of the 

 environmental factors— light and temperature— were the same at all 

 stages of development. In his reply Gregory ( 1928 ) admitted the 

 validity of Brigg's criticism that no proof had been produced that the 

 magnitude of the influence of individual environmental factors was 

 independent of the age of the plant. Gregory stressed, however, that it 

 was difficult to define the internal factor or tell what its precise role 

 was, and he maintained that the growth rate could be taken as a 

 quantitative measure of the internal factors when it came to determin- 

 ing the influence of the external factors. 



Later Williams (1946) commented on the "lumping together" of 

 the results of the experiments receiving either a low or a high nitrogen 

 level, since the nitrogen content would not only contribute to the in- 

 ternal factor but might equally well determine the order of the re- 

 sponse to an external factor. Again the statistical treatment of the cal- 

 culated net assimilation rates carried the assumption that the rate was 



