308 



GROWTH OF PLANTS 



periods of lighting of various lengths. This involves the effect of the light- 

 ing on the opening of stomates as well as the latent period between the 

 beginning of illumination and the beginning of photosynthesis. 



Figure 125. A, Buckwheat plants grown under artificial light at 68° to 72° F (20° 

 to 22° C). Left: continuous illumination; right: intermittent illumination, 5 seconds on, 

 alternated with 5 seconds off. B, Same as A except at a temperature of 95° to 100° F 

 (35° to 38° C). C, Calceolaria, two at left grown with intermittent light each night, two 

 at right control greenhouse without additional Hght. D, Lily (L. harrisii) planted Sep- 

 tember 17, photographed December 24; four at left grown at 55° to 60° F (13° to 16° C) 

 control greenhouse, five at right same temperature but given intermittent light each 

 night. 



Fig. 125 shows the growth of buckwheat under continuous illumination 

 with artificial light and half-time intermittent (5 seconds on and 5 seconds 

 off) illumination -\^^th the same light intensity. From the height of the 

 plants at 68° to 72° F (20° to 22° C) one might think that they grew as well 

 when illuminated half the time as continuously. At the high temperature 

 of 95° to 100° F (35° to 38° C), they seem to do better. Continuous illu- 

 mination at this temperature added to the injurious effect of the high 

 temperature. Table 33 shows the increases in dry weight of the plants 

 grown under continuous illumination and half-time illumination. At 



