RESPONSES TO ENVIRONMENT BY PLAxNTS IN THE VEGETATIVE PHASE 



543 



TABLE II 



The Effects of Seasonal Changes in the Solar Radiation Per Day 



and the Mean Daily Temperature on the Net Assimilation 



Rate, Leaf-Area Ratio, and Relative Growth Rate, as 



Determined by Regression Analysis 



Locality 



and 

 Species 



Oxford, England 

 Helianthus annuus 



Nature of Correlation 

 Net Assim. Leaf-Area Rel. Growth 



Rate Ratio Rate 



Radiation Temp. Radiation Temp. Radiation Temp. 



+ 







+ 



+ 



+ 



Invergowrie, 

 Scotland 



Helianthus annuus + + 



Vicia faba + + 



Adelaide, Australia 



Trifolium suh- 



terraneum + 







+ 



+ 



+ 

 + 



+ 



+ 

 + 







that, under the experimental conditions, environmental factors other 

 than those investigated could not have played a significant part. From 

 the multiple regressions, and given the mean seasonal changes in diur- 

 nal radiation and temperature, it can be shown with clarity how the 

 growth parameters change with the season. As an example, these trends 

 for H. annuus under the conditions at Oxford are illustrated in Figure 

 9. Inspection shows tliat from May to the end of June the rise in the 

 relative growth rate was closely matched by the gain in the net assimi- 

 lation rate, but from midsummer onward the net assimilation rate fell 

 faster than the relative growth rate. This failure to keep step after mid- 

 summer can be attributed to the variation in the leaf-area ratio which 

 changes but little up to midsummer and then steadily rises. The pattern 

 is imposed by the differences in the seasonal trends for light and tem- 

 perature. Solar radiation reaches a maximum at the end of June, but 

 temperatures do not until the latter part of July, and as a consequence, 

 for a given level of solar radiation the days are warmer in the autumn 

 than in the spring. Thus, since the leaf-area ratio is temperature-sensi- 

 tive but the net assimilation rate is not, w^iile the net assimilation rate 



