304 GROWTH OF PLANTS 



gated coleus. The response of nasturtium to extra CO2, both as to growth 

 and flowering, is especially striking, exceeding in this respect the response 

 of any other plant tested. There was relatively little response to the 6 hours' 

 extra illumination wthout extra COo except in the eggplant. This figure 

 shows the injury caused in geranium and coleus by continuous illumination 

 in the constant-light room. 



Experiments were also run in the gantry crane greenhouses using 12 hours 

 of artificial light and 12 hours of daylight. Even this proved injurious to 

 tomato plants. In fact, tomato plants receiving 12 hours of artificial illu- 

 mination at night are injured by daylight of more than 6 hours daily. The 

 injury was lessened by reducing the intensity of the light. The authors 

 suggest that it would be interesting to grow tomatoes well within the 

 Arctic Circle in greenhouses in summer so that growing temperatures 

 could be maintained and the plants subjected to continuous sunlight. We 

 have already seen that certain plants (geranium, variegated coleus, etc.) 

 that are injured by continuous illumination in the constant-light room 

 thrive under continuous illumination from sodium vapor lamps supple- 

 mented by mercury vapor or tungsten lamps. A proper balance in the 

 light spectrum seems to be demanded. It may be that daylight has a 

 properly balanced spectrum for continuous illumination of even the tomato 

 without injury. 



Insulated greenhouse and intermittent light effect. Impressed as they 

 were by the inefficient use of heat by conventional greenhouses, Arthur 

 and Porter ^^ designed an insulated greenhouse (8 ft. X 19 ft. inside) that 

 makes efficient use of the sunlight in mid-mnter and at the same time can 

 be heated economically by use of 10 500-watt tungsten lamps attached to 

 a single thermostat. 



Fig. 124 gives a general idea of the structure, but for a detailed descrip- 

 tion the reader is referred to the article cited above. During the winter 

 months the heat requirements for the house kept the lights on about 3.4 

 hours per day and this always at night except on cloudy days. With bright 

 sunlight in mid-day even in zero weather one of the windows had to be 

 opened slightly to prevent overheating. Since the house was so successful 

 in preventing loss of heat, it also prevented the entrance of CO 2 from the 

 outside air. Photosynthesis by the plants soon depleted the air of CO2. 

 As we shall see later, this deficiency was supplied in various ways. The 

 average cost of heating the house with lights during December, January, 

 and February was alDOut 36 cents per day. The three to four hours of 

 supplemental artificial light during the winter months produced gro%v'th 

 in plants equivalent to that in an ordinary greenhouse in March and April. 

 The artificial light furnished in the insulated greenhouse was of course 

 on and off intermittently during the night as the heat requirements de- 

 manded. Plants m nature receive intermittent light, but on longer day 

 and night periods. A further study s- ^ of the effect of short mtermittent 

 periods of light at night was made. To further this study and add more 



