Polarity in cuticular ridge development and insect attachment on leaf surfaces of Schismatoglottis calyptrata (Araceae)

Deceptive Ceropegia sandersonii uses an arabinogalactan for trapping its fly pollinators

Many plant species have evolved surfaces that reduce insect attachment. Among such plants are deceptive trap flowers of Ceropegia. Their gliding zones consist of convex epidermal cells, each with a bristle-like central protuberance and a single small liquid droplet on its tip. So far, the molecular and physical mechanisms controlling the function of these droplets are unknown. We analyzed the droplets of Ceropegia sandersonii flowers by microscopic approaches, studied how they behave when getting in contact with the feet of fly pollinators, and analyzed their chemical composition. The droplets contaminate the insect feet, on which they solidify. As its main component, a negatively charged polysaccharide containing a β1,3-galactan backbone and Rha-α1,4-GlcA-β1,6-[Araf-α1,3-]Gal-β1,6 side chains or truncated versions of it was identified. The chemical structure represents a rudimentary version of an arabinogalactan, which is supported by its binding to β-d-glucosyl Yariv reagent. Candidates of arabinogalactan proteins were identified to which the polysaccharide might be connected. The high amount of GlcA in the polysaccharide helps to explain the unusual physical characteristics of the droplets, like viscoelasticity and hygroscopy. We add a new function to arabinogalactans and discuss why the identified polymer is well suited for catching and temporarily trapping pollinators.

Leaf unfolding and lamina biomechanics inSyngonium podophyllumandPilea peperomioides

In nature, leaves and their laminae vary in shape, appearance and unfolding behaviour. We investigated peltate leaves of two model species with peltate leaves and highly different morphology (Syngonium podophyllumandPilea peperomioides) and two distinct unfolding patterns via time-lapse recordings: we observed successive unfolding of leaf halves inS. podophyllumand simultaneous unfolding inP. peperomioides.Furthermore, we gathered relevant morphological and biomechanical data in juvenile (unfolding) and adult (fully unfolded) leaves of both species by measuring the thickness and the tensile modulus of both lamina and veins as a measure of their stiffness. InS. podophyllum, lamina and veins stiffen after unfolding, which may facilitate unfolding in the less stiff juvenile lamina. Secondary venation highly contributes to stiffness in the adult lamina ofS. podophyllum, while the lamina itself withstands tensile loads best in direction parallel to secondary veins. In contrast, the leaf ofP. peperomioideshas a higher lamina thickness and small, non-prominent venation and is equally stiff in every region and direction, although, as is the case inS. podophyllum, thickness and stiffness increase during ontogeny of leaves from juvenile to adult. It could be shown that (changes in) lamina thickness and stiffness can be well correlated with the unfolding processes of both model plants, so that we conclude that functional lamina morphology in juvenile and adult leaf stages and the ontogenetic transition while unfolding is highly dependent on biomechanical characteristics, though other factors are also taken into consideration and discussed.

Petals Reduce Attachment of Insect Pollinators: A Case Study of the Plant Dahlia pinnata and the Fly Eristalis tenax

In order to understand whether the petal surface in "cafeteria"-type flowers, which offer their nectar and pollen to insect pollinators in an open way, is adapted to a stronger attachment of insect pollinators, we selected the plant Dahlia pinnata and the hovering fly Eristalis tenax, both being generalist species according to their pollinator's spectrum and diet, respectively. We combined cryo scanning electron microscopy examination of leaves, petals, and flower stems with force measurements of fly attachment to surfaces of these plant organs. Our results clearly distinguished two groups among tested surfaces: (1) the smooth leaf and reference smooth glass ensured a rather high attachment force of the fly; (2) the flower stem and petal significantly reduced it. The attachment force reduction on flower stems and petals is caused by different structural effects. In the first case, it is a combination of ridged topography and three-dimensional wax projections, whereas the papillate petal surface is supplemented by cuticular folds. In our opinion, these "cafeteria"-type flowers have the petals, where the colour intensity is enhanced due to papillate epidermal cells covered by cuticular folds at the micro- and nanoscale, and exactly these latter structures mainly contribute to adhesion reduction in generalist insect pollinators.

Growing up in a rough world: scaling of frictional adhesion and morphology of the Tokay gecko (Gekko gecko)

Many geckos have the remarkable ability to reversibly adhere to surfaces using a hierarchical system that includes both internal and external elements. The vast majority of studies have examined the performance of the adhesive system using adults and engineered materials and substrates (e.g., acrylic glass). Almost nothing is known about how the system changes with body size, nor how these changes would influence the ability to adhere to surfaces in nature. Using Tokay geckos (Gekko gecko), we examined the post-hatching scaling of morphology and frictional adhesive performance in animals ranging from 5 to 125 grams in body mass. We quantified setal density, setal length, and toepad area using SEM. This was then used to estimate the theoretical maximum adhesive force. We tested performance with 14 live geckos on eight surfaces ranging from extremely smooth (acrylic glass) to relatively rough (100-grit sandpaper). Surfaces were attached to a force transducer, and multiple trials were conducted for each individual. We found that setal length scaled with negatively allometry, but toepad area scaled with isometry. Setal density remained constant across the wide range in body size. The relationship between body mass and adhesive performance was generally similar across all surfaces, but rough surfaces had much lower values than smooth surfaces. The safety factor went down with body mass and with surface roughness, suggesting that smaller animals may be more likely to occupy rough substrates in their natural habitat.

Groovy and Gnarly: Surface Wrinkles as a Multifunctional Motif for Terrestrial and Marine Environments

From large ventral pleats of humpback whales to nanoscale ridges on flower petals, wrinkled structures are omnipresent, multifunctional, and found at hugely diverse scales. Depending on the particulars of the biological system-its environment, morphology, and mechanical properties-wrinkles may control adhesion, friction, wetting, or drag; promote interfacial exchange; act as flow channels; or contribute to stretching, mechanical integrity, or structural color. Undulations on natural surfaces primarily arise from stress-induced instabilities of surface layers (e.g., buckling) during growth or aging. Variation in the material properties of surface layers and in the magnitude and orientation of intrinsic stresses during growth lead to a variety of wrinkling morphologies and patterns which, in turn, reflect the wide range of biophysical challenges wrinkled surfaces can solve. Therefore, investigating how surface wrinkles vary and are implemented across biological systems is key to understanding their structure-function relationships. In this work, we synthesize the literature in a metadata analysis of surface wrinkling in various terrestrial and marine organisms to review important morphological parameters and classify functional aspects of surface wrinkles in relation to the size and ecology of organisms. Building on our previous and current experimental studies, we explore case studies on nano/micro-scale wrinkles in biofilms, plant surfaces, and basking shark filter structures to compare developmental and structure-vs-function aspects of wrinkles with vastly different size scales and environmental demands. In doing this and by contrasting wrinkle development in soft and hard biological systems, we provide a template of structure-function relationships of biological surface wrinkles and an outlook for functionalized wrinkled biomimetic surfaces.