562 



PLANT GROWTH AND PLANT COMMUNITIES 



50 



LIGHT INTENSITY, 7. OF FULL DAYLIGHT 



I 



100 



Figure 2. The growth rates and final wet weights of tomato plants grown 

 under the same conditions as in Figure 1. Standard error indicated. Abscissa: 

 the percentage of daylight passing through the filters. 



mercury-vapor lamp on plant growth. Both tomato and pea plants were 

 grown either in a greenhouse (average light intensity: 4,000 foot- 

 candles), or under warm-white fluorescent lamps (intensity: 1,500 foot- 

 candles), or under high-pressure mercury- vapor lamps (H400-Ri Gen- 

 eral Electric) at different intensities, with or without a glass plate to 

 screen out excessive ultraviolet light. For both plants the responses 

 were the same: they showed essentially equal growth in daylight and in 

 fluorescent light, in spite of the big difference in intensity. This shows 

 that at 1,500 foot-candles these young plants are almost saturated with 

 light, and that the mercury-vapor lamp causes a strong inhibition of 

 growth. This inhibition is not due to the high intensity as such, as the 

 growth in daylight proves; nor is it a result of too much ultraviolet, 

 since interposition of the glass pane did not produce more growth. 

 Therefore the conclusion seems justified that the high intensity of the 

 green rays emitted by this lamp is responsible for the poor growth. 



An experiment was performed to test this conclusion. Instead of 

 the H400 high-pressure mercury lamp, one with the same light element 

 but with a fluorescent coating (H400-RCi) was used. This lamp pro- 

 vides a better distribution of light over the visible light range, although 

 it still emits proportionately much more green light than warm-white 

 fluorescent lamps or sunlight. Under two of these lamps, frames with 



