SCIENTIFIC NEAVS. 



[March ist, 1887. 



THE INFLUENCE OF ELECTRIC LIGHT 

 ON VEGETABLE GROWTH. 



AMONG the many subjects which engaged the attention 

 of the late Sir William Siemens was the influence of 

 the electric arc light on vegetable growth, and his communi- 

 cations to the Royal Society, in March, 18S0, and to the 

 British Association, in September, 1S81, were extremely 

 interesting. 



In the former, he stated that he had been struck with the 

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

 value in photography depends ; and he had noticed also, 

 when experimenting with powerful electric lamps, that the 

 skin was often blistered without the sensation of excessive 

 heat ; an effect similar to that produced by solar rays in a 

 clear atmosphere. This led him to make experiments to 

 ascertain whether electric light exercised any decided effect 

 upon the growth of plants, and for this purpose he placed 

 an arc lamp with a metallic reflector in the open air, about 

 two metres above the glass of a sunk melon house. Pots 

 containing quick-growing seeds and plants, such as mustard, 

 carrots, beans, cucumbers, and melons, were arranged so 

 that thej' could be brought at suitable intervals under the 

 influence of daylight and electric light, without moving them, 

 both lights falling approximately at the same angle. The 

 pots were divided into the following groups : — ■ 



1. One pot of each group was kept entirely in the dark. 



2. One was exposed to the influence of the electric light 

 only. 



3. One was exposed to the influence of daylight only. 



4. One was exposed successively to both day and electric 

 light. 



For groups 2 and 4 the electric light was supplied for six 

 hours, from 5 to 1 1 each evening, all the plants being left 

 in darkness during the remainder of the night. 



In all cases the differences of effect were said to be unmis- 

 takable. No. I plants were pale yellow, thin in the stalk, 

 and soon died. No. 2 shewed a pale green leaf, and had 

 sufficient vigour to survive. No. 3 were of a darker green 

 and greater vigour. No. 4 shewed a decided superiority in 

 vigour over all the others, and the green of the leaf was of 

 a dark rich hue. Sir William adds: " It must be remembered 

 that, in this contest of electric against solar light, the 

 time of exposure was in favour of the latter in the 

 proportion of nearly two to one, but all allowance made, 

 daylight appeared to be about twice as effective as 

 electric light. It was evident, however, that the electric 

 light was not well placed for giving out its power advan- 

 tageously. The nights being cold, and the plants 

 under experiment for the most part of a character to 

 require a hot, moist atmosphere, the glass was covered 

 very thickly with moisture, which greatly obstructed the 

 action of the light, besides which, the electric light had to 

 pass through the glass of its own lamp." The loss of effect 

 being, therefore, considerable, experiments were made with 

 the electric lamp placed inside the glass-house, and some of 

 the general conclusions arrived at were as follows : — ■ 



1. Under the influence of electric light, chlorophyl [the 

 green colouring matter] is formed in the leaves of plants, 

 and their growth is promoted. 



2. An electric centre of hght, equal to 1,400 candles, 

 placed at a distance of two metres from growing plants, 

 appeared to be equal in efiect to average daylight in the 

 winter season, but more economical effects could be attained 

 by more powerful light centres. 



3. Plants do not appear to require a period of rest during 

 the twenty-four hours of the day, but make increased and 

 vigorous progress if subjected during daytime to sunlight, 

 and during the night to electric light. 



4. While under the influence of electric light, plants can 

 sustain increased stove heat without collapsing, a circum- 

 stance favourable to forcing by electric light. 



In the same year a new set of experiments on a larger 

 scale were commenced on Oct. 23rd, and continued till 

 May 7th, 1881. Two electric lamps of 5,000 candles each 

 were used, one being suspended at a height of 12ft. to 14ft. 

 over some sunk greenhouses, and the other placed inside a 

 glasshouse of about 2,300 cubic feet capacity. The effects 

 produced by the outside light over the sunk houses were 

 similar to those observed during the previous winter, but 

 the plants in the house with the naked arc light soon mani- 

 fested a withered appearance. This gave rise to various 

 interesting experiments, but as Sir William Siemens 

 remarks, the plants did not take kindly to these innovations 

 in their mode of life. Finally, it was found that the effect 

 of merely interposing a thin sheet of clear glass between 

 the plants and the arc light was most striking. On placing 

 such a sheet of clear glass so as to intercept the rays of the 

 electric light from a portion only of a plant — for instance, a 

 tomato plant — it was observed that in the course of a single 

 night the line of demarcation was most distinctly shown 

 upon the leaves. The portion of the plant under the direct 

 influence of the naked electric light, though at a distance 

 from it of g to loft., was shrivelled, whereas that portion 

 under cover of the clear glass continued to show a healthy 

 appearance. Experiments were then made with plants 

 covered by glass of different colours, and it was found that 

 the largest and most vigorous growth was induced under 

 clear glass ; yellow glass came next in order, then red, and 

 finally blue. Having established the importance of sur- 

 rounding the arc light with clear glass, some very good 

 results were then obtained. Thus, peas sown at the end of 

 October produced ripe fruit on i6th February, when con- 

 tinuously under the influence of light, with the exception of 

 Sunday nights. Raspberry canes put into the house on the 

 i6th December produced ripe fruit on the ist March, and 

 strawberry plants planted about the same time produced ripe 

 fruit of excellent flavour and colour on the 14th February. 

 Vines which broke on the 26th December produced ripe 

 grapesofstrongerflavourthan usual onthe loth March. Many 

 other examples are mentioned by Sir Williams Siemens, 

 and he was sanguine that when the best conditions of tem- 

 perature in the house, and of proximity to the electric light, 

 had been thoroughly investigated, still better results would 

 be obtained. Unfortunately his death put an end to these 

 most interesting experiments, and so far as we are aware 

 they have not since been pursued in a systematic way by 

 any one else in this country. 



During the months of August, September, October, and 

 November, 1881, Professor Deherain, of the Natural History 

 Museum, Paris, conducted a series of experiments in a 

 glasshouse erected inside the Electrical Exhibition then being 

 held there. One half of this house was rendered completely 

 opaque to sunlight, and the inside of this half was painted 

 white to reflect the light from the electric lamps. The 

 other half of the house was covered with clear glass, and 

 various tests were made in each division. No heating 

 apparatus was used, and the temperature of the house rose 

 and fell with that of the external air, and was occasionally 

 as low as 40"^ F. It should, also be mentioned that the 

 sunlight, which first passed through the Exhibition 

 roof, and then through the roof of the testing house, 

 had a very different effect on the plants from that obtainable 

 in a house receiving the sun's rays direct. It will thus be 

 seen that these trials were made under somewhat un- 

 favourable conditions. 



The first experiments were made with naked arc lights, 

 and they confirmed what Sir William Siemens had pre- 



