300 GROWTH OF PLANTS 



Growing Plants under a Combination of Sunlight and 



Artificial Light 



Gantry crane greenhouses. Two of the greenhouses (Fig. 119), each 

 approximately 20 X 25 feet, were equipped for supplementing daylight 

 with artificial light at night. The equipment consisted of a large gantry 

 crane electrically driven so it could be easily moved over the greenhouse at 

 night and be removed in the daytime. The crane carried 48 1000-watt 

 lights, so spaced as to give uniform intensity over the center area of the 

 greenhouses. This enabled the investigator to lengthen the daily period of 

 natural illumination at will, with an intensity of light amounting to 400 or 

 500 foot-candles. In most of the experiments one of the houses was illumi- 

 nated from 6 P.M. to midnight and the other one from midnight to 6 a.m. 

 and the experiments were run during the winter months. One of the houses, 

 G-2, was piped for CO2 enrichment and CO2 was run at 0.3 per cent when 

 needed ventilation did not interfere. The other house, G-1, was run with 

 normal CO2 concentration. 



From Fig. 120A and B, it is evident that red clover grows much faster 

 when the daylight in winter is supplemented by 6 hours of artificial illumi- 

 nation at night; if this is accompanied by ten times the normal concentra- 

 tion of CO 2 there is another great addition in growth. The plants with 

 extra light, and with extra light and CO 2, are in full bloom after 40 days 

 of growth from seeds, while those without extra light are not in bloom at 

 the end of the experiment. Under continuous artificial illumination red 

 clover shows fine growth and flowering. In Fig. 120C the cucumber shows 

 considerably more growth with 6 hours' extra illumination and very marked 

 response to increased CO 2. Barley (D) responded as do the summer cereals 

 generally by early heading on the long day. The extra CO 2 gave great 

 additional growth over the 6 hours' extra illumination alone. 



Fig. 121 shows that Clydesdale oats grown at 78° F (26° C) are headed 

 after 40 days of growth in 6 hours extra light both with and \vithout extra 

 CO2, and that CO2 does not increase the growth at this temperature. The 

 same oats gro^^^l under a 12-hour day are not headed and are taller than 

 the plants with 6 hours' extra illumination. At 68° F (20° C) the 18-hour- 

 day plants are headed whether with or without extra CO 2. The CO 2 

 under this condition gave much bigger plants. In this case plants were 

 grown with extra CO2 but no extra illumination. Extra CO2 alone does 

 not induce earlier heading. Blue Stem, a spring wheat, grown at 78° F 

 (26° C) is headed after 47 days from seed when grown ^\dth 6 hours' extra 

 illumination both \Yiih and without extra CO2; extra CO2 gives noticeable 

 increase in growth. The plants grown under the 12-hour day are not 

 headed. A similar condition holds for Blue Stem wheat gro\\Ti at 68° F 

 (20° C). Here again extra CO2 ^^^th a 12-hour day does not cause heading. 

 This wheat shows heading and great growth in height under a 24-hour day 

 of artificial light. 



