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RESPONSE OF PLANTS TO ARTIFICIAL LIGHTS 



RESPONSE OF PLANTS TO ARTIFICIAL 

 LIGHTS 



By G. E. Stone 



Light constitutes one of the most important 

 external factors affecting vegetation, and plays a 

 prominent role in modifying the configuration of 

 plants. Photosynthesis, or the assimilation of car- 

 bon, is one of the most fundamental processes in 

 the vegetable kingdom, and is dependent on light. 

 The activity of this process increases proportion- 

 ally to light intensity. 



Except in the polar regions, plants are exposed 

 to the influence of light during only half their life 

 period ; the other half is spent in darkness. So far 

 as is known, plants do not assimilate carbon during 

 bright moonlight nights, although sensitive appli- 

 ances for determining light intensity are capable 

 of registering the comparatively feeble illumina- 

 tion of even bright nights, which would tend to 

 show that the minimum amount of light necessary 

 for photosynthesis is comparatively strong. Pho- 

 tosynthesis takes place under the influence of elec- 

 tric and artificial lights, as has long been known, 

 but the activity of the process depends on the in- 

 tensity and the spectrum of the particular kind of 

 light. 



In glasshouse and other intensive cultures, it is 

 important to know whether artificial lights of any 

 kind can be used economically to supplement sun- 

 light and thereby produce an earlier or better 

 crop. It is also important to know what effect 

 artificial lights have on plants in exhibition halls. 

 This subject has been the theme of considerable 

 experimenting, but little very practical agricul- 

 tural result has yet been secured. In the winter, 

 particularly in cloudy climates, artificial light may 

 very likely come into prominence in the growing 

 of some kinds of crops. The following account 

 gives a brief survey of what has been accom- 

 plished. 



Electric arc light. 



Many experiments have been made relating to 

 the infiuence of electric light on vegetation, more 

 particularly with the stronger lamps, such as the 

 arc light. The spectrum of the ordinary electric 

 arc light is that of carbon, with a slight addition, 

 in some cases, of the spectra of certain gases. It 

 is especially rich in the rays of high intensity, 

 lying in the ultra-violet or actinic part of the 

 spectrum, beyond the range of vision. It is well 

 known that electric light possesses more of the 

 ultra-violet rays, with probably less of the orange 

 rays, than sunlight; therefore it would not be ex- 

 pected that electric light would possess the same 

 value to plants as sunlight, even if the intensity of 

 each were the same, since the rays which are the 

 most valuable for photosynthesis are those located 

 in the yellow and orange bands of the spectrum. 

 On the other hand, the highly refrangible or ultra- 

 violet rays of the spectrum stimulate growth of a 

 spindling nature, which would be, undesirable to 

 most crops. 



Herve-Mangon was one of the first to demon- 



strate that electric light was capable of producing 

 chlorophyll in plants as well as inducing heliotrop- 

 ism, and as far back as 1869 Prillieux showed that 

 electric light is capable of promoting assimilation. 



The first recorded horticultural experiments with 

 electric light were made by Dr. C. W. Siemens, 

 an English physicist. He experimented with a 

 variety of plants, such as strawberries, tomatoes, 

 grapes and melons, and found that an arc light 

 produced decided effects on the growth of these 

 crops, sometimes producing beneficial, and, at other 

 times, injurious effects. He ascertained very early 

 in his experiments that a naked or unscreened light 

 was injurious at short range, but that the inter- 

 position of a glass globe or ordinary window-pane 

 prevented such injury. He demonstrated that an 

 arc light could be placed over a greenhouse with 

 good results, the glass in such cases screening off 

 the injurious rays, and found that plants developed 

 easier under screened lights than otherwise. As a 

 result of his experiments he became very sanguine 

 that electric light could be used to advantage in 

 horticulture, and he was the first to employ the term 

 " electro-horticulture " to designate this new appli- 

 cation of electrical energy. He showed that growth 

 can be hastened by the addition of electric light to 

 daylight, and that injury does not necessarily fol- 

 low continuous light through the twenty-four hours ; 

 that electric light often intensifies the green color 

 of leaves, producing a deeper color in flowers and 

 modifying the flavor of fruits. Siemans maintained 

 that the addition of electric light enables plants to 

 stand a higher temperature in a greenhouse. 



At the time Siemens, in England, was conducting 

 his experiments, Deherain was making investiga- 

 tions in Paris along the same line. He attempted 

 to grow plants by continuous electric light, that is, 

 with no daylight whatsoever. He found, as Sie- 

 mens did, that an unscreened light injured plants, 

 although it promoted assimilation more effectively 

 than a screened light. He found that barley, flax, 

 chrysanthemums, pelargoniums, roses and others 

 were severely injured after seven days of continu- 

 ous exposure to electric light, and that this injury 

 was manifested by the dropping off or turning 

 black of the foliage. In the case of lilacs, when 

 the leaves were screened or protected by the upper 

 leaves, no injury took place. Plants which received 

 sunlight by day and electric light by night were 

 injured in the same manner, but to a less degree. 

 He found that electric light was far inferior to 

 bright sunlight in its effects on photosynthesis, and 

 that electric light was particularly injurious to seed- 

 lings, as most of them died before forming leaves. 

 Deherain's conclusions are briefly as follows : Elec- 

 tric light contains rays harmful to vegetation. 

 These, however, can be modified or eliminated by the 

 use of transparent glass. It contains enough rays 

 to maintain full-grown plants 2i months, but is too 

 weak to enable seedlings to reach maturity. 



Among those in America who have experimented 

 with electric light are L. H. Bailey, of Cornell 

 University, and F. W. Rane, formerly of the West 

 Virginia Experiment Station. Bailey made exten- 

 sive experiments with the arc light, covering a 



