CONTROLLED ENVIRONMENTAL CONDITIONS 



309 



68° to 72° F (20° to 22° C) the half-time Ulumination gives very little more 

 than one-half the increased dry weight of full-time illumination. At 85° 

 to 90° F (29° to 32° C) the half-time illumination gives less than half the 

 increased dry weight produced by continuous illumination. At 95° to 

 100° F (35° to 38° C), because of the injurious effect of the high tempera- 

 ture, the half-time illumination gives 80 per cent as great increased dry 

 weight as continuous illumination. The 68° to 72° F (20° to 22° C) temper- 

 ature is better than either of the higher temperatures. 



Table 33. Dry Weight Production of Buckwheat Plants under Artificial Light 



* Five seconds on, alternated with 5 seconds off. 



Supplementing greenhouse light in the winter with intermittent artificial 

 light at night hastened the growth and flowering of many plants such as 

 Calceolaria, lily, gladiolus, and carnation, as is shown by Figs. 125 and 126. 

 The beneficial effect on flowering was especially great on long-day plants or 

 plants that flower sooner on the long day. Several varieties of large flower- 

 ing gladiolus could be brought to flower by late January from bulbs planted 

 in early October when grown at low temperature with intermittent light 

 for an average of 3.4 hours each night. At 2 cents a kilowatt hour the cost 

 for this supplemented light amounted to about 5 cents per stalk of gladiolus. 

 This includes all the heating cost in the insulated house and part of that 

 in the ordinary greenhouse. 



Garner ^^ mentions that, except for the onion, bulb or tuber formation 

 occurs on short days. Zimmerman and Hitchcock ^^ find that dahlia pro- 

 duces fibrous roots on long days and tuberous roots on short days. The 

 same investigators ■*- confirm the earlier work of Garner and Allard that the 

 Jerusalem artichoke produces underground stems on long days and tubers 

 on short days, and the work of Knott that the growing stem tip is the region 

 of light perception. As already stated in this chapter, Arthur and co- 

 workers have found that potatoes tuberize on a long day and even under 

 continuous illumination if the temperature is sufficiently low. Gladiolus 

 forms corms on short and on long days if flowering has ceased or is pre- 

 vented. 



