In an experiment conducted in England in 1913 mercury glass-tube arcs were used in one part of a hothouse and the other part was reserved for a control test.  The same kind of seeds were planted in the two parts of the hothouse and all conditions were maintained the same, excepting that a mercury-vapor lamp was operated a few hours in the evening in one of them.  Miss Dudgeon, who conducted the test, was enthusiastic over the results obtained.  Ordinary vegetable seeds and grains germinated in eight to thirteen days in the hothouse in which the artificial light was used to lengthen the day.  In the other, germination took place in from twelve to fifty-seven days.  In all cases at least several days were saved in germination and in some cases several weeks.  Flowers also increased in foliage, and a 25 per cent. increase in the crop of strawberries was noted.  Seedlings produced under the forcing by artificial light needed virtually no hardening before being planted in the open.  Professor Priestley of Bristol University said of this work: 

The light seems to have been extraordinarily efficacious, producing accelerated germination, increased growth, greater depth of color, and more important still, no signs of lanky, unnatural extension of plant usually associated with forcing.  Rather the plants exposed to the radiation seem to have grown if anything more sturdy than the control plants.  A structural examination of the experimental and control plants carried out by means of the microscope fully confirmed Miss Dudgeon’s statements both as to depth of color and greater sturdiness of the treated plants.

Unfortunately there is much confusion amid the results of experiments pertaining to the effects of different rays, including ultra-violet, visible and infra-red, upon plant growth.  If this aspect was thoroughly established, investigations could be outlined to greater advantage and efficient light-sources could be chosen with certainty.  There is the discouraging feature that the average intensity of daylight illumination from sunrise to sunset in the summer-time is several thousand foot-candles.  The cost of obtaining this great intensity by means of artificial light would be prohibitive.  However, the daylight illumination in a greenhouse in winter is very much less than the intensity outdoors in summer.  Indeed, this intensity perhaps averages only a few hundred foot-candles in winter.  There is encouragement in this fact and there is hope that a little light is relatively much more effective than a great amount.  Expressed in another manner, it is possible that a little light is much more effective than no light at all.  Experiments with artificial light indicate very generally an increased growth.