From Tree Life 213  November 1997

The specially shaped leaves of plants growing in rainforests may help conserve the forest soil from erosion. The tips of the leaves often taper into long, elegant spokes pointing downwards, and many botanists believe these so-called drip-tips help to drain off rainwater from the leaves, rather like the tips of an umbrella’s spokes. But Celeste Rebelo from the University of Sao Paulo and G Bruce Williamson at Louisiana State University, Baton Rouge USA, think that drip-tips may be more important in reducing the size of rainwater drops falling off the leaves and splashing more gently onto the ground underneath, thereby saving the soil from erosion (Biotropica, vol. 28, pp. 159-63).
They measured leaf drip-tips on plants growing in three neighbouring locations in the heart of the Amazon. The drip-tips of plants growing on clay soils were largest, whereas plants growing on sandy soils had much smaller drip-tips. As all the plants grew in the same climate, it is most likely that soil holds the key difference.

Erosion may lie at the heart of the drip-tip differences. Clay soils are so poorly drained that the ground becomes easily saturated, and the rain can wash off surface particles in a miniature flood, whereas sandy soils can soak up more water and accommodate heavy rainfall. So, by shedding rainwater in a softly-softly approach, drip-tips are easing the drainage problems of clay soils. But this erosion theory of drip-tips may only apply to the Amazon region, because scientists in other parts of the tropics have found that climate is the vital factor with the longest drip-tips occurring in the wettest regions.

-Paul Simons.
Leaf cuttings
If leaves of rainforest plants don’t get rid of rainwater quickly, it can build up a film that reflects sunlight, blocks the leaf pores, leaches out nutrients and encourages the growth of parasite infections and epiphytic plants. It’s thought that the leaf drip-tips are one way of draining that water off, helped by the leaf veins, which siphon off water rather like the treads of car tyres.
This interesting article is acknowledged with thanks to author and BBC Wildlife Oct.1996.