540 PLANT GROWTH AND PLANT COMMUNITIES 



before and after transfer. At the time of transfer the area-to-weight 

 ratios of the leaf blades were measured, and these were again deter- 

 mined four days later on comparable leaves of a second sample. The 

 data of Table I demonstrate that even within this short period there 

 have been major adjustments in the ratio: for leaves moved to a lower 

 light intensity the ratio has gone up, while for a move in the opposite 

 direction the ratio has gone down. Thus the adaptation to a "sun" or 

 "shade" type is not confined to the early primordial stage but can take 

 place during the course of leaf expansion. 



Interrelationships between light and nutrient supply 



The eflFects of varying light level have been dwelt on in some 

 detail because there is a significant corpus of experimental data to 

 illustrate the extent to which growth analysis can elucidate and classify 

 the nature of the plastic responses of different species to shading. It is 

 self-evident that the light factor cannot be considered in isolation, and 

 it is now proposed to discuss more briefly interactions with the level 

 of nutrient supply and then to go on to review some interactions be- 

 tween light and temperature. 



A number of experiments have been carried out with soils of low 

 nutrient status to examine the interrelationships between the light level 

 and the responses of H. anniius to additions of nitrogen, phosphorus, and 

 potassium. The results of two selected experiments are given in Figure 

 8. In both experiments the increase in the relative growth rate due to 

 the addition of nitrogen, phosphorus, and potassium (NPK) is in- 

 versely related to the light level, but the trends for the net assimilation 

 rate and the leaf-area ratio are dependent on the experiment. In one, 

 the primary effect of altering the nutrient status is to improve the effi- 

 ciency of assimilation at the higher light intensities, while in the sec- 

 ond, the additional nutrients have augmented both the net assimilation 



TABLE I 



The Effects of Transfer from One Light Level to 

 Another on the Ratio of Leaf Area to Leaf Weight of 



Helianthus anmius 



Ratio at Time Ratio after Transfer to the 



Light Level of Transfer Indicated Light Levels 



before Transfer (cmVg) 10 0.5 0.24 daylight 



1.0 daylight m I25 160 183 



0.5 " 169 145 181 202 



0.24 " 196 161 175 208 



