Adaptive function of drip tips:: a test of the epiphyll hypothesis in Psychotria marginata and Faramea occidentalis (Rubiaceae)

Drip tips are thought to resist the growth of epiphyllous fungi, lichens, bryophytes and algae on leaf surfaces by promoting the drainage of water following rain. This hypothesis was tested by altering the tip of one leaf in each of 50 pairs of opposite leaves on plants of each of two woody species in the understorey of Barro Colorado Island, Panama. After 11 mo in situ, there were no statistical differences between the mean epiphyll cover on leaves with and leaves without drip tips. Epiphyll cover was, however, strongly correlated between the opposite leaves at a node, even though one had a drip tip and the other had the tip removed. This correlation could be due to small-scale heterogeneity of insolation, temperature and humidity in the crowns of plants. A relationship between epiphyll cover and growth of the branch containing the leaves provides qualitative support for this interpretation. It seems clear that drip tips do not have a large, consistent, easily detectable effect on epiphyll cover. However, because of the tiny cost of drip tips, reductions in epiphyll growth that are small and transitory and occur in only a portion of a plant's leaves may provide sufficient advantage to explain the evolution of drip tips. No experiment to date has been capable of detecting such effects.