526 PLANT GROWTH AND PLANT COMMUNITIES 



weight will be proportional to the size of the plant." This rate there- 

 fore reflected the performance of the plant under a given set of con- 

 ditions, and Blackman contended that this "efficiency index" could be 

 employed as a critical yardstick for comparisons between either species 

 or habitats. 



A year later Brenchley (1920-21) published an account of her 

 investigations on the pattern of change in weight of Pisum sativum 

 grown under the conditions of water culture in a greenhouse at difiFer- 

 ent seasons of the year. In each experiment, the efficiency index be- 

 tween consecutive weekly samples was calculated as recommended by 

 Blackman, and an appraisal was made of the influence of temperature 

 and hours of bright sunshine on the variation in the index. At this 

 point it is of some interest to note that the statistical treatment of the 

 data was undertaken by R. A. Fisher, who had recently joined the staflF 

 at Rothamsted Experimental Station, and he was the first to calculate 

 multiple regressions linking the index with other factors— namely, in 

 this instance, age, maximal and minimal temperature, and hours of 

 sunshine. Dividing the data into two groups of young (four weeks) 

 and older plants, he showed that for young plants the efficiency index 

 was positively correlated with age and maximum and minimum tem- 

 perature, whereas for the second phase the correlation for maximum 

 temperature and hours of sunshine was positive and that for age 

 negative. 



The next year saw the publication of three papers by Briggs, Kidd, 

 and West ( 1920-21 a, b, and c ) with a varying order of authorship for 

 the individual contributions. Like Blackman, for their basic field data 

 they drew on the extensive records accumulated, but not interpreted, 

 by German botanists in the last quarter of the nineteenth century, 

 particularly the findings of Kreusler, Prehn, and Becker ( 1877 ) on 

 Zea mais and of Hornberger (1885) on Sinapis alba. From the original 

 data it was possible to calculate on a weekly basis the change in dry 

 weight of both species from the seedling stage to maturity and to com- 

 pare the seasonal trends with those derived for H. annuus from the 

 data of Gressler cited by Blackman. These authors observed that the 

 efficiency index, or the relative growth rate, as they preferred to call 

 it, fluctuated considerably from week to week, and they took Black- 

 man to task for considering that this measurement could be regarded as 

 a physiological criterion of the plant's performance. They were also 

 critical of Blackman and Brenchley for their assumption that growth 

 was a continuous exponential process and that the best method of 

 estimating the rate over a given time interval was obtained by the 

 change in weight on a logarithmic scale. Briggs et at. argued that "as 

 we have no exact knowledge of the way in which the relative rate of 

 growth varies over a given period, we have adopted the following 

 purely conventional methods of defining relative growth rate"— i.e., as 



