THE EFFECT OF STREET LIGHTS IN DELAYING LEAF‐FALL IN CERTAIN TREES

Abstract

THE EXTENSIVE research on photoperiodism in recent years has demonstrated that the life cycle and activities of the plant are closely interwoven with the duration of the daily illumination. Most of the studies on this subject have been concerned especially with growth, flowering, and fruiting. There is a close interrelationship, however, between these phenomena and leaf-fall, in certain plants at least. A simple method of studying response to prolonged illumination is afforded by certain trees growingf in proximity to street lights. Such trees often show retention of the leaves beyond the normal season. This is characteristic of a number of common genera and species. In their pioneer work on the general subject of photoperiodism Garner and Allard (1920, 1921) early recognized that leaf-fall and ripening of the pods in Peking and Biloxi soy-beans (Soja max) were accelerated by exposure to short day lengths. Later the same authors (1922, 1923) demonstrated that in smooth sumnac (Rhus glabra), dwarf sumac (Rhus copallina), and the tulip tree (Liriodendron tulipifera) the leaves were retained longer when the plants were given additional illumination. These results are also incorporated in the reviews by Kellerman (1926), Schick (1932), and Garner (1936). Mochkov (1929-1930), studying Robinia pseudoacacia, Phellodendron amurense, Salix lanata, and Salix babylonica, concluded that in longer days the periods of growth and vegetative activity increase, while in shorter days they decrease. Oden (1929) found that with seventeen hours of daily illumination in autumn the normally rather rapid fall of leaves continued all through the winter, in Acer campestre, Lonicera periclymenum, and TViburnum opulus. Similarly buds could be made to unfold in December by means of additional light. Kramer (1936) has demonstrated that Fraxinus americana, Liriodendron tulipifera, Liquidambar styraciflua, and Quercus stellata become dormant earlier if grown under short day-lengths. All of these, among others, excepting Fraxinus americana, grew later into the autumn when the days were artificially lengthened. The renewal of growth early in the year (beginning in January) could be hastened or retarded by lengthening or shortening the period of illumination. This recent work supplements the somewhat generalized but nevertheless significant observations on leaf-fall reported in the older literature. It is well known that the falling of leaves, especially in autumn, is not the result of one simple cause, but may be due to the interaction of various factors, some internal, some external to the plant. Similarly, the retention of leaves on certain trees in winter is again not to be attributed to a single cause. The literature of this subject has been well reviewed by Pfeiffer (1928). The part played by relative illumination in the cyclic