338 GROWTH OF PLANTS 



combination of sunlight and artificial light so far as day lengths and flower- 

 ing are concerned. 



(3) Under artificial light the dry weight, percentage of assimilable car- 

 bohydrates, and size of plants increased and the nitrogen content decreased 

 with lengthening of day up to 17 or 19 hours. Increasing the day length 

 from 17 to 19 hours up to continuous illumination reversed these changes, 

 which probably means that all the species of plants grown under continu- 

 ous illumination with this particular artificial light showed incipient injury. 

 The tomato was especially sensitive to continuous illumination by this 

 light, the foliage being completely killed within a few days. Geranium, 

 coleus, and tobacco also showed foliar injury. 



(4) Buckwheat plants grew well for 8 to 1 1 days under continuous illumi- 

 nation from Mazda, neon, sodium vapor, and mercury vapor lamps (700 

 to 800 foot-candles) and sodium vapor showed much higher efficiency than 

 the other lights. Sodium vapor in this intensity killed all the foliage of the 

 tomato within two months under continuous illumination at a temperature 

 somewhat above 70° F (21° C). If the sodium vapor lamp was supple- 

 mented for two hours a day by a mercury vapor or Mazda light, no foliar 

 injury was produced by continuous illumination by the former light and 

 the geranium thrived indefinitely under this combination of light at a 

 temperature of 63° F (17° C). The light in the constant-light room cost 

 15 cents a square foot day (900 foot-candles) for current and the sodium 

 vapor combination 3 cents a square foot. The former gave foliar injury 

 under continuous illumination and the latter none. 



(5) Combination of daylight in winter with 6 hours of artificial light at 

 night from the gantry crane gave excellent conditions for the growth of 

 plants. The artificial illumination gave a great increase in size, dry weight, 

 and carbohydrate percentage. Under this condition all long-day plants 

 flowered promptly and the spring cereals headed much earlier while the 

 short-day plants failed to flower. Increasing the CO2 content of the air in 

 the gantry crane house ten-fold, to 0.3 per cent, gave a great additional 

 increase in size, dry weight, and carbohydrate content of plants, but had 

 no effect comparable to day length on flowering. It did seem to increase 

 the profusion of flowering in the rose and nasturtium. 



(6) The insulated greenhouse as well as other greenhouses were used for 

 growing plants in daylight supplemented by intermittent artificial light at 

 night. Because of lack of air exchange, the insulated greenhouse had to be 

 enriched with CO 2 to give maximum growth. Intermittent light at night 

 had the same effect as lengthening the day by a continuous period of artifi- 

 cial light. Calceolaria and many other long-day plants were forced to 

 bloom. The intermittent light also increased the dry weight, size, and 

 carbohydrate content of the plants. At favorable temperatures intermit- 

 tent artificial light 5 seconds on and 5 seconds off gave about one-half as 

 great increase in dry weight as the same light on continuously, although 

 the plants were nearly the same size. 



