Petals Self-Clean Without Being Slippery

close up photograph of purple flower with green leaves

Biological Strategy

Wild pansy

Eleanor Banwell

Image: Helen@littlethorpe / Flickr / Some rights reserved

Protect From Excess Liquids

While water is essential to life, too much water or other liquids can overwhelm living systems. Excess liquids can, for example, decrease a living system’s access to oxygen, promote excessive bacterial or fungal growth, or strip away soil and nutrients. To prevent the accumulation of excess liquids, living systems must control the movement of liquids across their boundaries or surfaces. They do so using waterproofing materials or structures, slowing flow, and/or facilitating flow to move the liquid away. Plant leaves, for example, commonly have waxy surfaces comprised of water-repelling chemicals to keep water from engorging the leaves or facilitating bacterial and fungal growth.

Search this Function

Flowering Plants

Clade Angiosperms (“receptacle seed”): Dandelions, oaks, grasses, cacti, apples

With 416 families containing some 300,000 known species, angiosperms are the most diverse group of plants, and they can be found around the globe in a wide variety of habitats. They are characterized by seeds that grow enclosed in ovaries, which are enclosed in flowers. The floral organs then develop into fruits of myriad kinds and dimensions, from simple seed casings on maples to elaborate fleshy growths like papayas. The oldest flower known from fossils, Montsechia vidalii, appeared during the Jurassic Period 130 million years ago. They are the primary food source for herbivorous animals, which in turn makes them the indirect food source for carnivores as well.

Search this Living System

Petals of pansy flowers maintain self-cleaning without sacrificing grip because of cone-shaped cells.

Self-cleaning is a very useful surface property for plants as they are unable to clean themselves mechanically. The most famous example of plant self-cleaning is the sacred lotus, which uses micro-scale bumps coated in tiny wax needles to form very stable air bubbles between droplets of water and the leaf surface. When the droplets roll off the surface, they pick up dirt and contaminating microbes and wash them away.

Flowers are also often water-repellent (hydrophobic), however, their main function is to attract pollinators, and waxy surfaces like those on lotus leaves are slippery, making it more difficult for visiting insects to grip them. As flowers are short-lived organs and consequently contamination doesn’t matter as much, many flowers may have sacrificed self-cleaning for grip. Petals on flowers like roses have bumps similar to those found on lotus leaves, but they are covered with a smooth layer of wax instead of needles. This makes them less slippery, but also less repellent. Rose petals are still hydrophobic, enabling the flower to avoid becoming waterlogged, but water droplets stick to the petal surface instead of rolling off. As a result, contaminating particles are not washed away.

Image: Orangeaurochs / CC BY - Creative Commons Attribution alone

Image: Schulte AJ, Droste DM, Koch K, Barthlott W, 2011. Hierarchically structured superhydrophobic flowers with low hysteresis of the wild pansy (Viola tricolor) – new design principles for biomimetic materials. Beilstein J. Nanotechnol. 2:228–236. http://dx.doi.org/10.3762/bjnano.2.27 / CC BY - Creative Commons Attribution alone

ted from Schulte et al. 2011

Pansy petals, on the other hand, do show self-cleaning properties comparable to those of lotus leaves, but they do so with a smooth wax coating like that of rose petals, ensuring that visiting pollinating insects are still able to grip them.

Pansy petals have unusually tall and pointed bumps on their surfaces that are 45 microns in height: nearly three times the height of similar bumps on rose petals. The bumps of both rose and pansy petals have a wrinkled surface. In roses, the wrinkles are wide enough that water can enter by , holding droplets in place and preventing them from rolling. But on pansy petals, the wrinkles are too narrow for water to enter. In addition, the tips of the bumps are smaller and closer together on pansy petals than they are for rose petals or lotus leaves. This keeps the total contact area between the petal and the water droplet small and prevents the air bubble from collapsing. The result is enhanced hydrophobicity and a self-cleaning surface that insects can still get a grip on.

Image: Schulte AJ, Droste DM, Koch K, Barthlott W / CC BY - Creative Commons Attribution alone

Adapted from Schulte et al. 2011

Surface Allows Self-Cleaning

Sacred lotus

Leaves of the sacred lotus are self-cleaning thanks to hydrophobic microscale bumps.

Last Updated July 2, 2020

References

“Over millions of years of co-evolution, different morphological adaptations have evolved in petals. Scanning electron microscopy studies also revealed large structural variations in petal microstructures. These surface microstructures cause optical signals [30,31] or function as a tactile cue for bees [32]. For us the “petal effect” or the repellence of petals seems to be a side effect and not the primary aim of the flower. A petal is a relatively short lived organ of plants, developed for pollinator attraction, but the short duration of petal lifetime makes a self-cleaning property for pathogen defence expendable. The last point might explain why water repellence is not widespread in petals” (Schulte et al. 2011:233)

Journal article

Hierarchically structured superhydrophobic flowers with low hysteresis of the wild pansy (Viola tricolor) – new design principles for biomimetic materials

Beilstein Journal of Nanotechnology, 2: 228–236 | 04/05/2011 | Schulte AJ, Droste DM, Koch K, Barthlott W

embedly preview

AskNature