Many experiments have been carried out in the forcing of plants by means of artificial light.  Some of these were conducted forty years ago, when artificial light was more costly than at the present time.  Of course, it is well known that light is essential to plant life and in general it is reasonable to believe that daylight is the most desirable quality of light for plants.  In greenhouses the forcing of plants is desirable, owing to the restricted area for cultivation.  It has been established that some of the ultra-violet rays which are absorbed or not transmitted by glass are harmful to growing plants.  For this reason an arc-lamp designed for forcing purposes should be equipped with a glass globe.  F. W. Rane reported in 1894 upon some experiments with electric carbon-filament lamps in greenhouses in which satisfactory results were obtained by using the artificial light several hours each night.  Prof.  L. H. Bailey also conducted experiments with the arc-lamp and concluded that there were beneficial results if the light was filtered through clear glass.  Without considering the details of the experiment, we find some of Rane’s conclusions of interest, especially when it is remembered that the carbon-filament lamps used at that time were of very low efficiency compared with the filament lamps at the present time.  Some of his conclusions were as follows: 

     The incandescent electric light has a marked effect upon
     greenhouse plants.

     The light appears to be beneficial to some plants grown for
     foliage, such as lettuce.  The lettuce was earlier, weighed more
     and stood more erect.

Flowering plants blossomed earlier and continued to bloom longer under the light.  The light influences some plants, such as spinach and endive, to quickly run to seed, which is objectionable in forcing these plants for sale.

     The stronger the candle-power the more marked the results,
     other conditions being the same.

     Most plants tended toward a taller growth under the light.

It is doubtful whether the incandescent light can be used in the greenhouse from a practical and economic standpoint on other plants than lettuce and perhaps flowering plants; and at present prices (1894) it is a question if it will pay to employ it even for these.

     There are many points about the incandescent electric light
     that appear to make it preferable to the arc light for
     greenhouse use.

     Although we have not yet thoroughly established the economy and
     practicability of the electric light upon plant growth, still I
     am convinced that there is a future in it.

These are encouraging conclusions, considering the fact that the cost of light from incandescent lamps at the present time is only a small fraction of its cost at that time.