RESPONSES TO ENVIRONMENT BY PLANTS IN THE VEGETATIVE PHASE 



547 



related with the Hght level. For the relative growth rate the interactions 

 between temperature and light are very apparent. At 20° C. the light 

 level has little effect, but at the higher temperatures the rate of growth 

 is accelerated, particularly as the intensity is changed from 300 to 900 

 foot-candles. 



Turning to the development of the leaf, which will be examined 

 in somewhat greater detail, for a range of species it has already been 

 shown that ratio of the leaf area to leaf weight rises as the light level 

 falls, and that for H. anniius there is evidence of a positive temperature 

 response. Figure 10 demonstrates very clearly the interacting effects of 

 light and temperature on the ratio: the ratio is greatest when the high- 

 est temperature is combined with the lowest intensity; conversely the 

 ratio is least when the plant is subjected to a combination of the lowest 

 temperature and highest intensity. This plastic response can be analyzed 

 further by comparing Figures 11 and 12, which shows the changes in- 

 duced in the area per leaf and the weight per leaf. Basically the trends 

 are the same up to an intensity of 1,200 foot-candles, since both the 

 weight and the area increase as the level of light rises and the tem- 



E 

 u 





o 







^o ^^o^ 



Figure 10. The interacting effects of light and temperature on the ratio of 

 leaf area to leaf weight of Salvinia uatans. 



