696 BIOLOGICAL EFFECTS OF RADIATION 



growth in hemp, but aided growth and development of spring wheat. 

 Tincker found that by use of supplementary illumination of 2 to 5 foot- 

 candles intensity to increase the light period from 12 to 17 hr., much the 

 same effects were obtained as with a natural daylight period of approxi- 

 mately the same duration. Oden (52) employed supplementary electric 

 illumination in forcing various plants in the greenhouse and concludes 

 that as a rule the gas-filled incandescent lamp supplying a preponderance 

 of light of the longer wave-lengths is a desirable source, although for 

 certain species best results were obtained with light relatively rich in the 

 shorter wave-lengths. 



As regards continuous illumination as such, generally speaking the 

 effects on reproductive activity in long-day and short-day plants is similar 

 to that of the long day, that is, flowering and fruiting are accelerated in 

 the former and retarded in the latter. In many plants, though by no 

 means in all, continuous light produces maximum growth, at least with 

 the sources and intensities of light that usually have been employed. 

 Apparently in no case has uniform illumination of the intensity and 

 composition of strong sunlight been used in such tests. Results obtained 

 by several investigators with a combination of daylight and artificial 

 light already have been considered. With electric light of moderately 

 high intensities as the only source of illumination Harvey (31) was able 

 to grow a number of species from seed to seed, though no controls were 

 provided for direct comparison with responses to regvilated day lengths. 

 Ramaley (54) grew several caryophyllaceous species from seed to seed in 

 continuous light derived from daylight and artificial night illumination 

 and obtained earlier flowering and taller plants with the uninterrupted 

 illumination. In the far North Smith (69), utilizing the natural con- 

 tinuous daylight of summer, daylight supplemented with artificial light, 

 and the latter alone, found that with several species the maximum mean 

 rate of development was obtained with the continuous illumination, 

 though, in some cases, the rate of development per hour of illumination 

 was greatest with short day lengths. 



On the other hand, continuous light has been shown by several investi- 

 gators to produce definite injury to certain plants, and growth may be 

 restricted even though no pathological symptoms are evident. Reding- 

 ton (60) finds that for a number of species studied a daily period of dark- 

 ness is not essential, although, in most cases, the largest and best plants 

 were obtained with a 16-hr. day. Arthur and others (5) grew numerous 

 species in different day lengths and in continuous light supplied as artifi- 

 cial illumination alone or daylight supplemented with the artificial 

 illumination. The carbon dioxide supply also was varied and the 

 conditions were accurately controlled. In general the continuous 

 illumination was more effective when the carbon dioxide supply was 

 increased. Tomato was unable to tolerate continuous illumination. 



