286 GROWTH OF PLANTS 



Three rather large installations -^ for modifying and controlling gro^i^h 

 conditions for plants were built as a part of the Institute : (A) the constant- 

 condition dark and light rooms where plants could be gro^^'n under artificial 

 light exclusively with day length, temperature, humidity, and carbon 

 dioxide concentration of the air regulated; (B) the gantry crane house 

 where plants were gro^vn during the day in daylight and 6 or 12 hours 

 during the night under light from tungsten lamps attached to a gantry 

 crane; (C) the spectral glass houses in which plants were gro^^Ti under dif- 

 ferent regions of the solar spectrum, as determmed by the solar transmis- 

 sion of the glasses on the several houses. Later an insulated greenliouse was 

 built which was Ughted by sunlight through a southern ex-posure of glass 

 during the day and wliich was further illuminated by tungsten lights 

 attached to a thermostat. These Hghts were the sole heat source of the 

 house, and at the same time they supplemented the sunlight, largely at 

 night or on cloudy days, for when the sun was shining little additional heat 

 was required. Besides these larger pieces of equipment, much other appara- 

 tus was purchased or made at the Institute for regulating, measuring, and 

 recording various growth conditions. The several pieces of equipment 

 will be briefly described later m connection with the experunents carried 

 out with them. 



Plants Grown Entirely in Artificial Light 



To attempt to grow plants under artificial light in competition \dth sun- 

 light is very expensive. Arthur '" figured the value of the radiant energy 

 falUng on an acre of land at Yonkers, New York, during 1936 under the 

 assumption that it could be converted quantitatively to electricity and 

 the electricity sold at two cents a kilowatt hour. The figures in Table 29 

 are calculated from New York meteorological data. 



On this basis the annual value of sunlight falling on an acre at Yonkers 

 is about S106,000. This makes it very clear why artificial light cannot 

 compete ^^'ith sunlight in growing plants, and why men like Abbott of the 

 Smithsonian Institution spend their time working up apparatus for trapping 

 sunlight as a source of energy. "\Mio ever heard of a crop being worth a 

 considerable fraction of -1106,000 per acre year? 



Constant-condition rooms. The two constant-condition rooms ^' ^ 

 (Fig. 113) were built in the basement under the greenhouses, each having 

 a floor space of 11 X 11 feet, or 121 square feet. Both rooms were attached 

 to the same air-conditioning and carbon dioxide-enriching systems so that 

 during any given period both had the same temperature, humidity, and CO2 

 concentration in the air. One room was dark and gave the plants their night ; 

 the other was iUuminated by 25 1000- or 1500- watt Mazda lights. The 

 lighting made it difficult to air-condition the room. This was accomplished 

 by placing a plate-glass ceiling between the lights and growing room, over 

 which a wen-regulated layer of water flowed. A large fan also aided in cool- 



