GROWTH IN CONTINUOUS ILLUMINATION 717 



the day, apparently the faster the rate, and further, that there is no 

 definite indication of injury from exposure to continuous sunlight. This 

 is not true of plants grown under artificial light in more carefully con- 

 trolled environments. 



PLANT GROWTH IN SUPPLEMENTAL AND CONTINUOUS ARTIFICIAL 



LIGHT 



The first study of the effects of electric light on plant growth was 

 recorded in 1861 by Herve Mangon (16). Seedlings of rye grown in 

 darkness were exposed to an arc lamp for 11 to 12 hr. each day for 

 4 days. The plants developed chlorophyll and bent toward the light, 

 reacting in the same way as when exposed to sunlight. Prilheux (21), 

 in 1869, observed that green shoots of the water plant Elodea canadensis 

 gave off bubbles of oxygen gas when exposed to the light from either 

 arc lamps or gas lamps. He concluded that photosynthesis took place 

 in artificial light the same as in sunlight but to a lesser degree on account 

 of the lower intensity of artificial light. Siemens (26), in 1880, was 

 probably the first to grow plants under continuous artificial light. He 

 used a carbon-arc lamp with an output of 1400 candle power. His 

 comments regarding the experiments are of interest: "I was induced to 

 look for interesting results in these experiments on account of the great 

 abundance of blue and actinic rays in the electric arc, upon which its 

 value in photography depends. In experimenting with powerful electric 

 lamps for illuminating purposes I have been struck, moreover, by the 

 action produced upon the skin, which is blistered, without the sensation 

 of excessive heat at the time, an effect analogous to that produced by 

 solar rays in a clear atmosphere." 



Siemens first used the arc lamp over a greenhouse for a few hours each 

 night. He found that even though the light had to travel through the 

 glass wall of the globe enclosing the arc and the glass roof of the green- 

 house, it still produced an effect on plant growth equal to about one- 

 half that of sunlight. Believing that the open arc would be more 

 effective when placed directly over plants in the greenhouse, he made 

 tests with cucumber and melon plants exposed at a distance of 1 meter. 

 Siemens observed that the plants developed considerable leaf injury 

 at 1 meter when exposed to the naked arc, but when the distance was 

 increased to 1.5 to 2.3 meters no further injury developed, and those 

 plants which were injured produced new leaves and subsequently 

 recovered. Plants were then exposed for 11 hr. each day to daylight and 

 again to 11 hr. each night to electric light for a 4-day period. The 

 plants after this treatment he observed far surpassed the control plants 

 receiving only daylight in both general appearance and amount of green 

 pigment produced. His further conclusion is of special interest: "These 

 experiments are not only instructive in proving the sufficiency of electric 



