SUN RAYS AND PLANT LIFE— JOHNSTON 359 



the other to a long-day length. Flowers were formed in the one case 

 but not in the other. 



Attention should be called to two other interesting plant responses 

 to the length of the light period. These are illustrated by two ex- 

 periments also taken from the work reported by Garner and Allard 

 (1925). In one of these the upper and the lower sections of a yellow 

 cosmos plant were exposed to 10 hours of light. The middle section 

 received light during the entire long summer days. Both the top 

 and bottom sections of the plant responded to the characteristic short- 

 day light exposures and soon bloomed, while the middle section re- 

 mained vegetative to the long-day exposure. This would indicate 

 a localized response. 



In another set of experiments (Garner and Allard, 1931) artificial 

 light was used, and the plants were exposed to this illumination for 

 a total of 12 hours per day. One group received 12 hours of con- 

 tinuous light and 12 hours of darkness. The other groups were alter- 

 nately illuminated and darkened for periods of the following dura- 

 tions : 1 hour, 30 minutes, 15 minutes, 5 minutes, 1 minute, 15 seconds, 

 and 5 seconds. Again using the yellow cosmos as an example, the 

 interesting growth response is shown in plate 1, figure 2. 



A progressive decrease in height, size, and weight of the plants, 

 and an increase in etiolation was noticed down to the 1-minute in- 

 terval. Further shortening resulted in marked improvement in 

 growth and general appearance. All exposure intervals less than 1 

 hour were equally unfavorable for flowering. In one of the long-day 

 plants tried (rocket larkspur) none of the shorter alternations showed 

 a retarding action in flowering, although the general growth responses 

 were similar to those of the short-day plant. These are exceedingly 

 interesting growth responses to the duration of light and to date no 

 satisfactory explanation has been given. 



A most interesting method of forcing greenhouse crops has been 

 found by Dr. R B. Withrow (1933, 1934, 1936), of Purdue Univer- 

 sity. Lamps of very low wattage used as supplementary lighting 

 sources for a number of greenhouse-grown plants produced responses 

 whicli were seemingly out of all proportion to the treatment applied. 

 The plants were illuminated for several hours each night in addi- 

 tion to the natural light they received during the day. The intensi- 

 ties varied from less than 1 foot-candle to over 100 foot-candles. 

 In plate 2, figure 1, very little difference in flowering is noted be- 

 tween the aster (Heart of France) receiving 100 foot-candles and 

 the one receiving 0.3 foot-candle. Flowering even occurred with 0.1 

 foot-candle. This was an intensity about double that of moonlight 

 on a bright winter night at Lafayette, Ind. 



Dr. Withrow was next interested in determining what wave lengths 

 of his Mazda lights were effective in bringing about the flowering. 



