532 



PLANT GROWTH AND PLANT COMMUNITIES 



pally Ballard, Petrie, Tiver, and Williams, examined the eflPects of such 

 factors as phosphorus, nitrogen, and water supply on the growth of 

 oats, wheat, tobacco, Linum tisltatissimum, Sorghtim sudanense, and 

 Phalaris tiibcrosa. These results, together with further information de- 

 rived from data published by workers outside Australia, were reviewed 

 from the point of view of growth analysis by Williams ( 1946 ) . One 

 aspect he discussed was the magnitude of the ontogenetic drift in the 

 net assimilation rate and what was the best index of the internal fac- 

 tors governing the photosynthetic efficiency of the leaves. Comparisons 

 of the change in the rate' were made when the rate was expressed in 

 terms of leaf weight, leaf area, and leaf protein. The general conclu- 

 sions reached for an annual plant grown under constant environmental 

 conditions and with a relatively limited nitrogen supply are given in 

 Figure 1. It is seen that the maximum rates were achieved soon after 

 the seedling phase and that subsequently the period of relative con- 

 stancy was dependent on the parameter selected. For some species, 

 such as S. sudanense, the net assimilation rate on a leaf -weight basis 

 might be held constant for a very short period, while for others, such as 

 P. ttiberosa, the period might be seven or eight weeks. In contrast, if 

 the criterion was leaf protein, the rate remained constant during most 

 of the period of active growth for both these two species and for oats 

 and wheat. 



lOO 



o 

 cr 



c 

 o 



< 



Z 



so 



I I 



II 



I I 



Seedling 



Senescence 



Figure 1. Diagram illustrating the interrelationship between the stage of 

 development and the net assimilation rate, based on the criteria of (a) leaf 

 weight, (b) leaf area, and (c) leaf protein. A constant environment and a 

 limited nitrogen supply are assumed. (Redrawn from data of Williams, 

 1946.) 



