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Abstract |
Street lights in the City of New York cause a retention of the leaves of certain trees: Carolina poplar (Populus canadensis), London plane (Platanus acerifolia), sycamore (Platanus occidentalis), and crack willow (Salix fragilis). Illuminated portions of a tree retain their leaves; shaded portions of the same tree do not. One side of a tree, or the lower part, may thus have numerous leaves, while the other side, and the upper part, may be entirely devoid of foliage. A relatively weak light, at a distance of as much as 45 feet from the tip of the nearest branch, may cause retention of numerous leaves. Light intensity as low as 1 foot candle, or less, may be effective. Some leaves may be retained at least a month, others more than that, beyond the normal season. The orientation of the light with respect to the tree – i.e., north, east, south, and west – is not significant. In Populus canadensis all of the leaves ultimately fall, abscission apparently taking place at the base of the petiole. In Platanus acerifolia and Platanus occidentalis some of the leaves are retained until killed by low temperature; then some of them break off above the base of the petiole. Leaves of the Populus and Platanus species discussed remain green unusually long when receiving additional illumination. Leaves of these same trees do not emerge from the buds earlier in the spring as a result of the additional illumination. |
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