GROWTH IN CONTINUOUS ILLUMINATION 721 



back. Tliis point was usually a 17- or 19-hr. day, although in case of 

 tomato at high intensity the point is reached on an even shorter day. 

 Some plants grown under artificial light, such as the tomato, geranium, 

 and coleus, were especially sensitive to long days of 17, 19, and 24 hr. 

 At low intensities, in case of the tomato, the weight of tissue produced 

 increased with increasing day length up to a 17- or 19-hr. day, while at 

 higher intensities the peak was reached on a 12-hr. day. Cabbage plants 

 were found to increase in weight of tissue produced and in total carbo- 

 hydrate with length of day up to 17 or 19 hr., followed by a decrease on 

 continuous illumination. The dry weight of tissue produced increased 

 regularly with day length in many of the common grains and forage crops 

 such as barley, clover, wheat, and buckwheat up to a 17- or 19-hr. day 

 and decreased again on continuous illumination. While many of these 

 plants were able to withstand continuous artificial illumination, they 

 were always found to produce at a maximum on a shorter day length, 

 usually on 17 to 19 hr. The best growth and dry-weight production was 

 obtained in the greenhouse with 12 hr. of sunlight supplemented by 

 6 hr. of artificial light each night and with carbon dioxide at about 10 

 times the normal concentration. That is, an 18-hr. day, 12 hr. of which 

 was sunlight, was found to be the most effective of all the lighting com- 

 binations studied. Continuous illumination which was made up of 

 12 hr. of sunlight supplemented by 12 hr. of artificial light each night 

 was not so effective. The tomato was greatly injured by this combina- 

 tion, while geranium and coleus, the two other species which did not 

 withstand continuous artificial illumination, withstood the combination 

 successfully. In this work it is evident that long continuous exposure of 

 plants to the incandescent filament lamp produces a foliar or other injury 

 on many plants which is reflected in less dry weight production. Sun- 

 light is a much better light source for growing plants than the incan- 

 descent filament lamp and has a definite balancing effect when used in 

 combination with supplementary artificial light. 



Since the tomato plant was found to be the most sensitive to con- 

 tinuous illumination of all plants studied by Arthur, Guthrie, and Newell, 

 it furnishes a good test plant for further work on the effects of quality of 

 light on plants. These authors used carbon arc lamps and mercury- 

 vapor lamps in combination with filament lamps in further tests with 

 tomato under continuous illumination. Arc lamps w^ere found to 

 retard the rate of development of the injury but the final result was 

 the same as under the filament lamps. Lower light intensities produced 

 the injury at a much slower rate but produced greater height growth, 

 weaker stems, and other etiolation effects. Recent correspondence with 

 parties in both northern Canada and northern Sweden has established 

 the fact that tomatoes have been grown successfully, to the production of 

 ripe fruit, in both of these places at points within the Arctic circle on the 



