Rub-A-Dub-Dub

Characterization and Distribution of Water-repellent, Self-cleaning Plant Surfaces
by C. Neinhuis and W. Barthlotti. Annals of Botany 79:667-677. 1997.

Lotus leaves are round and impressively large, with strong, dark green color, enveloped in a blue sheen. More often than not, they look so clean and perfect! And what happens when you splash water on them? They remain completely dry. Pour as much water as you want to on lotus leaves, water bounces right off--it's impossible to get them wet.

What makes plant leaves water-repellent and why? ("Water-repellent" means that water applied to the leaf surface forms spherical droplets that roll off the surface immediately.) How does having water-repellent leaves help plants? Take a guess?

People who work in wetland environments know of many plant species that have water-repellent leaves: water fern (Salvinia spp.), Azolla spp., wild taro (Colocasia esculenta), water lettuce (Pistia stratiotes), golden club (Orontium aquaticum), blue flag (Iris spp.), parrot feather (Myriophyllum aquaticum), saw-grass (Cladium mariscus), common reed (Phragmites communis), wild rice (Zizania aquatic) and lotus (Nelumbo spp.). And many others. (Interestingly, plants whose leaves float directly on the water are not water repellent: water lily (Nymphaea spp.), duckweed (Lemna spp.), banana lily (Nymphoides spp.), cow lily (Nuphar lutea) and others...)

For this research, the scientists microscopically examined 200 species of water-repellent trees and herbaceous plants, both terrestrial and aquatic. Among other things, they found that water-repellency was concentrated in herbaceous plants, especially wetland grasses, and that water-repellent plants are often recognized by the dull blue appearance of their leaves.

Their main findings, however, were that water-repellency in leaves is determined primarily by two things: 1) whether or not the plant produces and exudes wax crystals onto the epidermal cells; and to a lesser extent, 2) the shapes of the epidermal cells. With an electron microscope, they found that wax crystals come in three basic shapes: "rodlets", "tubules" and "threads". Wax crystal density seems to determine how water-repellent a leaf is. They also found that in some water repellent leaves, the epidermal cells have convex surfaces; in other plants, each epidermal cell had one or more "papillae" or bumpy protrusions; in still other plants, the epidermal cells had long hair-like structures (trichomes): all the better to make the leaf surface rough. A rough surface structure covered with wax crystals makes it impossible for water to stick.

Why would plants evolve water-repellent leaves? To prevent sickness, say the scientists. They found that water-repellent leaves are also self-cleaning leaves: dirt particles on the leaves adhere more strongly to the water droplets than to the leaf surfaces, and so are washed away when water hits the leaves. The scientists conjecture that dirt, which also contains the spores and conidia of disease fungi, is also washed away. And because water cannot adhere to the leaves, any disease organisms that are on the leaf, since they require water for germination and growth, simply cannot survive to infect the leaf.

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