722 BIOLOGICAL EFFECTS OF RADIATION 



continuous sunlight which obtains in these regions during the short sum- 

 mer season. Attention has been previously directed to the records of 

 the growth of tomatoes in Alaska (8). This is further evidence that 

 sunlight is a better light source than the artificial light which has so far 

 been used in the growth of plants. Much more work needs to be done on 

 both the quality and intensity of light which is best suited to plant growth. 

 When this information is available, it will be possible to choose a light 

 source, or combination of light sources, which is much nearer the ideal for 

 plant growth. It is possible that sunlight is not ideal for the growth of 

 plants; on the other hand, it is established that sunlight is a better source 

 than most of our common electric lamps. 



Roodenburg (23) has tested the effects of neon gas discharge tubes on 

 the growth of plants and compared the rate of development with plants 

 grown under both filament and mercury-vapor lamps. Because of the 

 output of neon tubes in the red region near the main absorption band of 

 chlorophyll, Roodenburg believes this light source will have some prac- 

 tical significance in plant production. Intensities produced by the 

 consumption of 75 watts per square meter he found sufficed for forcing 

 several varieties of plants as compared with an energy consumption of 

 300 to 400 watts per square meter when incandescent filament lamps 

 were used. He found the mercury-vapor-arc lamp useful in supplying 

 sufficient blue light to prevent excessive stem elongation but questions 

 its practical value as a high intensity is required which means higher 

 current consumption. 



Oden (18) has studied the growth of a number of plants under incan- 

 descent filament lamps on various day lengths up to and including con- 

 tinuous illumination. His studies were designed to work out (intensity X 

 time of illumination) relations for commercial plant production. The 

 time was varied from 3 to 24 hr. and the highest intensity used was 

 120 gm. cal. per day. This intensity was not sufficient for many of the 

 plants studied but was sufficient for sweet peas and certain varieties of 

 beans. He found that a current consumption of 200 to 500 watts per 

 square meter was necessary to grow plants from seed but a lower intensity 

 could sometimes be used for forcing flower production. He has included 

 an extensive bibliography of 422 references on light relations in general. 

 Ramaley (22) has also published a general bibliography on this subject 

 and Schratz (24) has published a review and bibliography on the effects 

 of artificial light on higher plants. 



EVIDENCE OF INJURY OF CONTINUOUS SUNLIGHT 



Smith (27) has published a bibliography including many citations on 

 the effects of day lengths and has studied the growth of plants in various 

 day lengths including continuous illumination in a series of experiments 

 extending from Aas, Norway, at latitude 59° 40' through Tromsoe, 



