204 Quantitative Analysis of Plant Growth 



assimilatory activity of the seedling leaves would result in an initial rise 

 in the Unit Leaf Rate curve. With regard to respiration, there are two 

 types of changes going on in the plant which tend to alter the respiration 

 of the plant per unit leaf-area as the plant increases in age. First, the 

 dry- weight per unit leaf -area falls in the early stages and then rises until 

 it attains a value about nine times that of its lowest value. The effect 

 of this change, if respiration per unit dry-weight is constant, is to cause a 

 corresponding fall and subsequent rise in the respiration of the plant per 

 unit leaf-area. Secondly, the evidence available is that the respiration 

 per unit dry- weight of the whole plant at constant temperature decreases 



on the assumption that assimilation and respiration 



per unit leaf area are constant throughout. 



modification allowing for changes in assimilation. 



. allowing for changes in respiration. 



. . allowing for both. 



Fig. 2. Ideal Curves for Unit Leaf Rate. 



with age 1 . The effect of this is to accentuate the initial fall and to decrease 

 the subsequent rise just mentioned. The net result of these changes in 

 the rate of respiration per unit leaf-area would therefore be expected to 

 make the Unit Leaf Rate curve still more concave to the time axis (cf. 

 Fig. 2). No quantitative significance is to be attached to these curves 

 which are purely diagrammatic. 



The actual curves for maize given in Figs. 3-6 show that our expecta- 

 tions are in fact only definitely realised in so far as there are low initial 

 values in all cases. With regard to the main part of the curves the 

 fluctuations are so large that we cannot say more than that the weekly 



1 Work carried out by the authors shows that such is the case in Helianthus. 



