544 



PLANT GROWTH AND PLANT COMMUNITIES 



May 



June 



July 



Aug 



Sept 



Figure 9. The interrelationships between seasonal changes in the diurnal 

 solar radiation and mean temperature at Oxford and the net assimilation rate, 

 leaf-area ratio, and relative growth rate of Helianthus annuiis in the early 

 vegetative phase. 



may be the same in August as in May the leaf-area ratio will be higher 

 in August, and this in turn will bring about a higher relative growth 

 rate. 



For H. annuHS at Oxford the statistical analysis was extended to 

 determine the effects of light and temperature on the growth of the 

 different parts of the plant ( Blackman et al., 1955 ) . The relative growth 

 rate of the stem was positively dependent upon both light and tempera- 

 ture; that of the leaves was positively linked only with light; and the 

 root growth was affected only by temperature. The stem-weight ratio 

 was positively correlated with temperature and negatively with light, 

 while the root-weight ratio was negatively linked with temperature. 

 On the other hand, the leaf-weight ratio was not significantly affected 

 by either factor; the ratio of leaf area to leaf weight, however, was re- 

 lated positively to temperature and inversely to light. 



I hope that by this time my illustrations have provided convincing 

 proof of the value of growth analysis in assessing the whole plant's 

 reactions to environmental factors. To me it is not only the relative 

 simplicity of the basic concepts that appeals but also the fact that much 

 can be done with the simplest of laboratory facilities. Indeed, in the 

 laboratory a bare minimum of a drying oven, a balance, blueprint 



