CONTROLLED ENVIRONMENTAL CONDITIONS 337 



for the flowering response brought about by periods of low-temperature 

 exposure. 



Arthur '■ ^'"' ""Published work confirms the findings of Slogteren and others 

 that exposing the planted bulbs of hyacinth and daffodil to low temperatures 

 for a few weeks induces flowering. Tulip bulbs and many other mature 

 plants show similar responses. A large plant of Crassula ruhicunda, which 

 had been growing in the greenhouse for many years without flowering, 

 was induced to flower profusely by exposure to low temperature followed 

 by a period in the greenhouse. In many cases low temperatures during the 

 night are sufficient to induce flowering. Thermoperiodism seems to ap- 

 proach photoperiodism in importance as a flower-inducing factor. 



Summary 



(1) When the Institute was built, three major pieces of equipment were 

 installed for growing plants on a large experimental scale under a rather 

 wide range of controlled conditions, with special emphasis on light of vari- 

 ous intensities, quality, and daily duration. The two constant-condition 

 rooms, one continuously illuminated and the other dark, provided for 

 gro^ving plants entu^ely in artificial light with various day lengths, with 

 controlled temperatures and humidities, and with increased concentrations 

 of CO2. The two gantry crane houses provided means of growing plants 

 under sunlight during the day, supplemented by artificial light at night. 

 The five spectral greenhouses provided for growing plants under a full 

 solar spectrum and under various portions of the solar spectrum. Finally, 

 the insulated greenhouse provided means of growing plants under sunlight 

 during the day with intermittent artificial light at night as the sole source 

 of heat. Several other types of small-scale equipment were used to grow 

 plants under different kinds of light with other environmental conditions 

 controlled. The plants produced under the several conditions were studied 

 as to rate and nature of growth, dry and wet weight increase, chemical 

 composition, pigment development, and content, size, form, and anatomy 

 of the several organs, and as to vegetative or reproductive development. 



(2) Several runs of a number of different kinds of plants were made in 

 the constant-condition rooms with 5, 7, 12, 17, 19, and 24 hours of daily 

 illumination. Some runs were made at 68° F (20° C) and some at 78° F 

 (26° C). The humidity was held mainly at 80 per cent and the CO2 at 0.3 

 per cent. Salvia, a short-day plant, flowered on 5, 7, 12, and 15-hour days 

 and did not flower on the 19 or 24-hour days. Lettuce and radish, long-day 

 plants, flowered on the 17, 19, and 24-hour days but not on the shorter days. 

 The spring cereals were also hastened greatly in heading by longer days. 

 Buckwheat, which is indifferent to day length in flowering, flowered on all 

 day lengths. Tomato, another day length-indifferent plant, flowered on 

 all day lengths except 5 and 24 hours. Continuous artificial light soon 

 killed all foliage of the tomato. Artificial light acted like sunlight or a 



