546 PLANT GROWTH AND PLANT COMMUNITIES 



temperatures (20, 25, and 30° C.) and six levels of continuous light 

 provided by daylight fluorescent tubes, the light ranging in intensity 

 from 300 to 1,800 foot-candles (equivalent to 38 to 204 cal/cmVday). 

 Since a large number of growth parameters has been measured, only 

 some of the results, at times in a condensed form, will be presented. 



The general trends for the relative growth rate, net assimilation 

 rate, and leaf-area ratio can be seen in Table III. Taking first the net 

 assimilation rate, it is evident that there is a marked interaction be- 

 tween temperature and light. At the lowest light intensity the tem- 

 perature effect is small, while at the higher intensities there is a consid- 

 erable rise in the net assimilation rate as the temperature is increased 

 from 20 to 25° C; little further increase takes place when the tempera- 

 ture is raised to 30° C. At this juncture it is necessary to point out that 

 the net assimilation rates have been calculated on the assumption that 

 the floating leaves are the only photosynthetic tissues. Such an assump- 

 tion leads to some degree of overestimation, since the "roots" ( actually 

 submerged filiform leaves ) contain chlorophyll. 



Similarly because of the morphology the leaf-area ratio is some- 

 what of a misnomer, since it is the ratio of the area of the floating 

 leaves to the combined weight of the floating leaves, internodes, and 

 submerged leaves. Nevertheless, Table III shows that the changes in- 

 duced fall into line with those observed in the previous field studies; 

 namely, that the ratio is temperature-dependent but negatively cor- 



TABLE III 



The Effects of Temperature and Light Intensity on the 



Net Assimilation Rate, Leaf-Area Ratio, and Relative 



Growth Rate of Salvinia nutans 



Temperature Light Level (foot-candles) 



(degrees C.) 300 900 1500 



Net Assimilation Rate (g/dmVweek) 

 20 0.33 0.61 0.78 



25 0.28 0.82 1.06 



30 0.29 0.73 1.19 



Leaf -Area Ratio (cmVg) 

 20 400 219 163 



25 607 295 237 



30 679 397 273 



Relative Growth Rate (g/g/day) 

 20 0.18 0.21 0.19 



25 0.27 0.34 0.37 



30 0.31 0.43 0.45 



