RESPONSES TO EN'-s'IRONMENT BY PLANTS IN THE VEGETATIVE PHASE 531 



relatively independent of the stage of development and nitrogen 

 supply. 



At this point it is perhaps worthwhile to look back over the first 

 decade of research and summarize the position. At the end of the 

 period there was little doubt that the gain, either in dry weight or total 

 leaf area, was an accumulative process, and that, at least for the vege- 

 tative phase, the change with time followed an exponential or auto- 

 catalytic course. It had been established on theoretical grounds that 

 the relative growth rate was the product of the net assimilation rate 

 and the leaf-area ratio, but no experimental work had been undertaken 

 either to compare simultaneously the performance of different species 

 under the same conditions or to eliminate as far as possible ontogenetic 

 drifts in the anahsis of the effects of different environments on growth. 

 The concepts then current of the distinct roles of internal and external 

 factors in determining the pattern of development perhaps masked an 

 appreciation of the great variation in the plastic response of species to 

 a change in the environmental conditions and of the fact that the age 

 or physiological status of a plant was the resultant of the interactions 

 between the past external conditions 'and the internal processes. 



The next 15 years saw the further application of these concepts to 

 the study of plant development under either field or glasshouse con- 

 ditions. Crowther, one of Gregory's students, carried out pioneering 

 investigations on cotton in the Sudan and in Egypt, and by subsampling 

 techniques he obtained comprehensive records of the leaf production 

 and flower and boll formation. In two papers ( 1934, 1937) he was con- 

 cerned with the changes in the relative growth rate and net assimilation 

 rate induced by internal and external factors. It was demonstrated that 

 over the ranges of added irrigation water and nitrogen, nitrogen in- 

 creased the relative growth rate of the leaves and the shoot, but it did 

 not alter the net assimilation rate, while water caused no significant 

 effects. He also concluded that, leaving out of account environmental 

 factors, the net assimilation rate reached a peak value before leaf pro- 

 duction was complete and thereafter the rate fell. He ascribed this fall 

 not so much to an internal ontogenetic drift as to the increased self- 

 shading that resulted as the plants grew taller. 



It should be pointed out, however, that Crowther did not sample 

 the roots, and this omission might introduce considerable errors in the 

 estimates, since the ratio of shoot to root would vary both with the 

 stage of development and the external conditions. Then again, the 

 values for the net assimilation rate were based on data for leaf weight 

 and not for leaf area, and it will be shown later that the ratio of leaf 

 area to leaf weight is by no means constant. 



Between 1936 and 1946 a number of Australian workers, princi- 



y 



