550 



PLANT GROWTH AND PLANT COMMUNITIES 



ratio for leaf area (3.2) than for leaf weight (11.4). That is, the 

 changes in area are more dependent on the induced variations in cell 

 number than on the changes in leaf weight. It cannot, however, be 

 concluded that the mean weight of the individual cells is unaflFected by 

 light and temperature. Figure 14 clearly demonstrates that it is affected 

 by both. At least, up to 1,500 foot-candles there is a progressive rise in 

 weight at each temperature, while over all light intensities there is a 

 negative influence of temperature. 



One other aspect of development requires comment, and that is 

 the rate of leaf formation, which in turn is dependent on the produc- 

 tion of new buds. It is evident from Figure 15 that the rate is tempera- 

 ture-dependent but the changes induced by light are linked with tem- 

 perature. At 20° C. light has a negligible effect, while at 30° C. the 

 effect is appreciable. Thus, from a comparison of Figures 13 and 15, 

 it is apparent that the factors that determine the rate of production of 

 new leaf primordia are different from those that control the ultimate 

 number of cells in the expanded leaf. This difference is particularly 

 striking in the case of temperature: the rate of new leaf formation goes 

 up and the number of cells per leaf goes down as the temperature rises. 

 When the trends for cell weight (Figure 14) also are included in the 

 comparison, it is to be observed that at the lowest temperature it is 

 only the cell weight that increases over the range of 600 to 1,800 foot- 



o 



X 



E 



U 



i_ 



Q. 



'a 



0-8 



Figure 14. The interacting effects of light and temperature on the mean 

 weight per cell in the leaves of Salvinia natans. 



