212 



Dr. C. W. Siemens. 



[Mar. 4, 



Experiments on Effect of Electric Light on Plants. 



My object in making these experiments was to ascertain whether 

 electric light exercised any decided effect npon the growth of plants. 

 For this purpose I placed the regulator in a lamp with a metallic 

 reflector, in the open air, about 2 metres above the glass of a sunk 

 melon house. A considerable number of pots were provided, sown 

 and planted with quick-growing seeds and plants, such as mustard, 

 carrots, swedes, beans, cucumbers, and melons. The plants could then 

 be brought at suitable intervals under the influence of daylight and 

 electric light, without moving them, both falling upon them approxi- 

 mately at the same angle. The pots were divided into four 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. 

 The electric light was supplied for six hours, from 5 to 11 each 



evening, all the plants being left in darkness during the remainder of 

 the night. 



In all cases the differences of effect were unmistakable. The plants 

 kept in the dark were pale yellow, thin in the stalk, and soon died. 

 Those exposed to electric light only showed a light-green leaf, and 

 had sufficient vigour to survive. Those exposed to daylight only were 

 of a darker green and greater vigour. Those exposed to both sources 

 of light showed a decided superiority in vigour over all the others, 

 and the green of the leaf was of a dark rich hue. 



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 2 to 1, but all allowance made, daylight appeared 

 to be about twice as effective as electric light. It was evident, how- 

 ever, that the electric light was not well placed for giving out its 

 power advantageously. 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.* Notwithstanding 

 these drawbacks, electric light was clearly sufficiently powerful to 

 form chlorophyl and its derivatives in the plants. It was interesting 

 to observe that the mustard-seed stem, when placed obliquely, turned 

 completely towards the light in the course of two or three hours, and 

 that cucumber and melon plants were affected in the same way, though 

 at a slower rate. The cucumber and melon plants which have been 



* Professor Stokes lias shown, in 1853, that the electric arc is particularly rich in 

 highly refrangible invisible rays, but that on passing the rays through glass, all 

 those of high refrangibility are found to have been absorbed. 



