in 1950. It is published with only very minor editorial changes from a manuscript which he left, to record a contribution by one of the two pioneers who discovered photoperiodism.It gives his interpretation on problems of light relationships to plant response, particularly photoperiodism in 1950, after a lifetime of research in this field.He discusses the role of visible and near-visible radiation and reviews the literature on effects of light duration, intensity and quality in relation to growth and physiologic process of plants, phototropism, phy tosynthesis , and photoperiodism.iii CONTENTS