Some Further Experiments on the Relation of Light to Growth

In a previous publication (I) I stated that "in summer the higher slopes of mountains and places in higher latitudes have a longer period of daylight, and this greater duration of daylight even with a lower temperature may produce as great an effect on plants as a shorter period of light with a higher temperature in places lying near the equator."In another and later publication (3) the following statement occurs: "From the experiments above described and from many others the conclusion is drawn that light and heat are to a certain extent interchangeable in a plant's economy."The experiments of which the details follow were undertaken mainly with a view to confirming, or, at any rate, supplying some further information bearing on, the above statements.At the same time some additional tests were made of the value of electrical illumination as a substitute for or an addition to ordinary daylight; finally an experiment was made out of doors to determine the effect on plant growth of the interposition of a screen of ordinary window glass.The details of the experiments are given in the following order: (I) experiments with electric light; (2) experiments on duration and intensity of light; (3) experiments on relation of growth to duration of light and temperature; (4) effect on growth of a screen of colorless glass.Discussion of the results obtained under each of these four heads follows farther on.