718 BIOLOGICAL EFFECTS OF RADIATION 



light alone to promote vegetation, but they also go to prove the important 

 fact that diurnal repose is not necessary for the life of plants, although 

 the duration of the experiments is too limited perhaps to furnish that 

 proof in an absolute manner. It may, however, be argued from analogy, 

 that such repose is not necessary, seeing that crops grow and ripen in a 

 wonderfully short space of time in the northern regions of Sweden and 

 Norway, and Finland, where the summer does not exceed two months, 

 during which period the sun scarcely sets." 



In 1891, Bailey (4) reviewed the literature on the growth of plants as 

 related to artificial light and added further experiments on the growth of 

 lettuce, radish, and other plants under a combination of daylight supple- 

 mented by artificial light. He observed that great injury developed on 

 plants exposed to the naked arc, but when exposed to an arc enclosed in a 

 glass globe there was little injury. Lettuce heads developed fully two 

 weeks earlier with supplementary lighting. A second report was pub- 

 lished in 1892 (5). 



In 1894, Bonnier (6) studied the structure of leaves and stems of 

 Arctic plants and compared these with Alpine plants growing at approxi- 

 mately the same temperature but the first under continuous sunlight, the 

 other on a normal day of high intensity. The leaves of Arctic plants 

 were thicker but less differentiated in structure. The palisade cells 

 were not well developed but the mesophyll tissue was of open structure 

 and well developed in Arctic plants. The epidermis and cuticle were 

 thinner in Arctic plants. The question of light intensity as well as day 

 length, however, should be considered. Although the Arctic day is 

 continuous, the intensity is much lower than the Alpine day and the 

 characteristics found in the Arctic plants are similar to those of shade 

 plants. In 1895, Bonnier (7) made a series of very critical experiments 

 aimed at separating the two effects of light intensity and continuous 

 illumination. He used, as a light source two 8-amp. arc lamps in glass 

 globes having a low ultra-violet transmission. Plants were exposed 

 continuously for several months in one series and were compared with a 

 series exposed for 12 hr. and then held in darkness for 12 hr., and again 

 compared with a series growing in a greenhouse in normal sunlight. 

 Intensity was increased by bringing the lamps closely together, arranging 

 reflecting mirrors around the plants, and shortening the distance from 

 the plants to the lamp. The shortest distance used was 0.5 meter, the 

 the greatest distance was 6 meters. Continuous illumination at low 

 intensity as compared with an alternation of 12 hr. light with 12 hr. 

 darkness, produced plants with more chlorophyll and less differentiation 

 of leaf tissue, that is, with a poorly defined palisade layer and thin 

 epidermis. Higher intensities produced the same effect. He concluded 

 that the structural changes produced by continuous illumination were due 

 to the continuity of the light and were independent of both its nature and 



