290 GROWTH OF PLANTS 



illumination. This kept all space in the light room occupied all the time. 

 When the 5-hour-day plants were getting their night, the 19-hour-day 

 plants were occupying their space in the light room. The same was true 

 of the 7-hour- and 17-hour-day plants, and two shifts were run in the 12-hour 

 day. This economical use of light was desirable even in an experiment, for 

 the cost of lighting this room was great. With the maximum illumination 

 used, the current for the lights alone at two cents a kilowatt hour cost $18 

 a day. The cost for air-conditioning, CO2 enrichment, etc. must of course 

 be added to this. To illuminate an acre of land with the same light system 

 and intensity would cost $6480 a day. These costs put in doubt the feasi- 

 bility of economically growing plants exclusively in artificial light, even if a 

 much more economical light than the Mazda is found. 



Now let us look at the plants groA\Ti in these rooms. Fig. 114 shows 

 lettuce (upper left), salvia (upper right), buckwheat (lower left), and 

 tomatoes (lower right) grown in these rooms for 32 days, each with six 

 different daily periods of exposure to light (5, 7, 12, 17, 19, 24 hours). 

 Lettuce did not flower with 5, 7, or 12 hours of daily illumination, but it 

 did so with 17, 19, and 24 hours. It is a long-day plant. The cereal 

 grains also flower earlier on long days, as shown in Fig. 115 for barley. 





Figure 115. Barley grown during the period February 28 to March 24, 1926, at 

 68° F (20° C), in the constant hght room on 5, 7, 12, 17, 19, and 24-hour days. The 

 plants marked control at right were grown under ordinary greenhouse conditions. 



In this plant the amount of growth increases as the day increases from 5 

 to 17 hours. Heading has already occurred in the 17-hour day and has oc- 

 curred still earlier on the 24-hour day. Note in this figure that the control 

 in the ordinary greenhouse during February and March showed a little 

 less growth than the plants in the constant-condition room on the 12-hour 

 day but more than the plants in the shorter days. In contrast to lettuce, 



