A drip tip is a pointed projection that extends out from the end (apex) of a leaf. Its primary function is to help the leaf quickly and efficiently shed water from the leaf surface. To work effectively, drip tips are accompanied by:
- glabrous (smooth) surface
- waxy cuticle
- curved leaf blade (decreases capillary resistance to water moving – source)
- grooved veins, which help channelize water
And it’s not just the tips of the leaves that help shed excess water. The teeth on the margins can also help facilitate getting rid of excess water, as in the jewelweed leaf below (though this is not to be confused with guttation – link).
Drip tips are most prominent on leaves of plants growing in the understory of humid rainforests in the tropics though plenty of our native trees in the northeast have them. The tips range in size and shape, though certainly they reach their most exaggerated form down in the tropics while are of a more modest stature here in the northeast. While tropical plants tend not to have teeth or lobes, our temperate trees have these in spades and you can even find drip tips on the lobes of leaves (they’re particularly prominent on the striped maple).
Drip tips at home and abroad are not hard to find. In one study in the Amazon, about a third of trees surveyed had leaves with drip tips (source). The pressure to shed excess water is significantly lower up here in the relatively drier, colder northeast, but you can still find drip tips here in the Vermont understory (e.g. basswood, birches – especially gray birch, elms, cherries – especially pin cherry, alternate-leaf dogwood, and many many more). Above I mentioned that many leaves are curved at the tip. And while this is true, I also find that many species have leaves with more of a twist than a curve (e.g. common buckthorn).
What’s the function of a drip tip?
Water that stays on a leaf can create a great little microclimate for pathogens to grow. This is in part why there’s less pressure for canopy and open grown trees to have drip tips – being exposed to the wind and sun both dries the leaves and shakes off potential threats (think of those wagging leaves from the quaking aspen). There are also dust particles and organic matter that accumulate on the leaf surface, effectively reducing the photosynthetic efficiency of the leaf. Additionally, down in the tropics, epiphytes (mosses, algae, lichens, bromeliads, orchids, etc) are ubiquitous. A drip tip helps shed these as well.
Drip tips (combined with that water-phobic leaf surface), make water flow faster off of a leaf. The faster the water flows, the more energy it has pluck seeds, spores and other debris from off the leaf surface and send them hurtling down to the forest floor below. Having glabrous (smooth) and waxy leaves reduces the drag on the surface of the leaf and helps move water towards the drip tip. Grooved veins act like gutters to channel all the water to a single point on the leaf.
A different type of drip
Out in California, farm edges are lined with Eucalyptus planted as windbreaks. The long elliptical waxy leaves droop down off the branches and act like one big drip tip. As moist air blowing off the ocean washes over the leaves, water droplets condense on the waxy leaves and drip down from the tips into the soil below, effectively collecting water for the roots in an otherwise arid environment. Gardeners are likely familiar with a similar phenomenon – the dripline for fruit and ornamental trees (and anyone seeking shelter from the rain) is the outermost ring where water drips down from a tree’s canopy. During a rain storm, the soil under the canopy of the tree stays much drier than on the edge. Trees tend to concentration their finer roots right along the edge of the dripline (this is called the critical root zone).