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Published online by Cambridge University Press: 12 June 2017
The minimum red-light (> 590 mμ.) intensities from fluorescent tubes at which growth or maintenance of original fresh weight were obtained at 20 C in a growth chamber for 28 days were 11–20 foot-candles for waterstargrass (Heteranthera dubia [Jacq.] Mac M.) and 12–45 foot-candles for coontail (Ceratophyllum demersum L.). Duckweed (Lemna minor L.) frond number increased significantly at 45 foot-candles red light, and the fresh weight probably increased at 45 foot-candles. The intensites of 11–45 foot-candles constituted 1.0–3.8 percent of the red portion of full sunlight.