Phototropism of Whole Trees: Effects of Habitat and Growth Form

The hypothesis that phototropism applies to entire trees as well as to apical meristems was tested. The hypothesis was correct but with important modifications: (1) canopy emergent species exhibit very little phototropic response; (2) streamside species exhibit significantly more response than upland species. Costs and benefits of learning are shown to provide an evolutionary explanation for these differences. Extrapolation of the cost/benefit model yields predictions about what habitats are likely to select for phototropic response.