CONTROLLED ENVIRONMENTAL CONDITIONS 



307 



same illumination at night. The great advantage of the night-illuminated 

 houses over the control greenhouse shows up in the periods December 11 

 to January 14 and January 14 to March 4, and is far less marked in the 

 early fall and late spring periods. The reasons for this are simple: in fall 

 and spring the lights for heating the insulated house were on for much 

 less time during the night, and the daylight was brighter and of greater 

 duration in fall and spring. This table also shows that the dry weight 

 increase was less in the insulated house than in the artificially lighted 

 ordinary house. " This is due in the main to CO 2 deficiency in the insulated 

 house. This was later cared for by CO2 enrichment of the air, as mentioned 

 above. Probably the daylight is not quite as intense in the insulated house 

 because of the distribution of windows. 



Table 32. Dry Weight Production of Buckwheat in Greenhouse with Intermittent 



Light 



Lighting interval 



Sunhght plus 7.06 hours of light each night at 

 intervals of 6 min, 8 sec on, alternated with 

 4 min, 24 sec off 



Sunhght plus 7.14 hours of light each night at 

 intervals of 2 hr, 20 min on, alternated 

 with 1 hr, 31 min off 



Sunhght only. Same temperature as above, 

 55° to 60° F (13° to 16° C) at night 



Sunhght only. Temp. 75° F (24° C) at night 



No. of 

 plants 



51 



47 



45 

 47 



Total dry 

 weight 

 (grams) 



21.506 



19.704 



11.416 



10.032 



Dry weight per 



plant 



(grams) 



0.422 



0.419 



0.254 



0.213 



Table 32 shows the effect of clock-regulated intermittent illumination 

 of buckwheat plants at night in ordinary greenhouses. The intensity of 

 the artificial illumination was the same as that used for the data shown 

 in Table 31. Both the 7.06 hours of illumination at night in 6-minute and 

 8-second periods and the 7.14 hours of nightly illumination in 2-hour and 

 20-minute periods give great increases in dry weight as compared with 

 daylight alone. Night temperatures of 55° to 60° F (13° to 16° C) in the 

 control houses gave greater increases in dry weight than 75° F (24° C) 

 night temperatures. Lower respiration at the lower temperature at night 

 is in part the explanation of this effect. Low-temperature periods are of 

 great significance to plants, as we have already seen with seeds and as we 

 shall see in a later section of this chapter dealing with yarovization and 

 with the necessity of a low-temperature period to prepare the rosette of 

 Digitalis and other plants for flowering. In general, shorter periods of inter- 

 mittent illummation at night are better than longer periods because of the 

 rise in temperature caused by the long periods. The reader is referred to 

 the origmal ^' p-^^^^ for a discussion of the relative efl&ciency of alternate 



