Biomechanics of selected arborescent and shrubby monocotyledons

Multiple stressors drive convergent evolution of performance properties in marine macrophytes

Extreme environments have driven the evolution of some of the most inspiring adaptations in nature. In the intertidal zone of wave-swept shores, organisms face physical forces comparable to hurricanes and must further endure thermal and desiccation stress during low tides, compromising their physiological and biomechanical performance. We examine how these multiple stressors have influenced the evolution of tissue properties during desiccation using eight phylogenetically independent pairs of intertidal and subtidal macrophytes. Intertidal species generally lost water more slowly than their subtidal counterparts, presumably as an adaption to regular emersion. Under partial desiccation, breaking force, strength, and extensibility of intertidal species generally exceeded those of subtidal species, although important differences existed among phylogenetic pairs. This was often true even when subtidal relatives resisted greater forces or were more extensible under full hydration. The interacting effects of mechanical forces and desiccation during low tide are likely a major selective agent in determining macrophyte performance and fitness. Overall, we found that lineages that have independently evolved to occupy the wave-swept intertidal have converged on performance metrics that are likely to be adaptive to the interacting stressors associated with their extreme niches.

Branching morphology and biomechanics of ivy (Hedera helix) stem-branch attachments

PREMISE

Hedera helix is a striking example of a plant with morphological traits and growth habits that vary between juvenile and adult phases. The present study focuses on its branching morphology and variations with age and change in growth habit, based on conspicuous stem-branch attachments previously described in related Araliaceae species.

METHODS

We decorticated and morphologically analyzed 300 samples of ramifications from prostrate, climbing and self-supporting axes of H. helix. For bending experiments, 103 specimens with the self-supporting growth habit were collected.

RESULTS

Ramifications of H. helix exhibited a so-called "finger-like" branching morphology with abaxial branch lobes and varying degrees of fusion of woody strands. Three categories of woody strand coalescence were defined. Biomechanical experiments in which the branches of stem-branch attachments were bent revealed two main modes of failure, breaking failure in (1) the attachment region and (2) the side branch.

CONCLUSIONS

Coalescence of woody strands in H. helix ramifications results from accumulation of secondary xylem with age, influenced by mechanical stimuli causing specific loading situations during different growth habits. Mechanical experiments showed the tendency toward failure in the side branch with increasing fusion of woody strands, affected by the diameter ratio of the side branch to the main axis. Of specific interest is the comparison of H. helix branching with tropical Araliaceae, which do not show the described coalescence of woody strands to this extent. Fracture toughness of self-supporting H. helix axes with merged stem-branch attachment regions are comparable to other self-supporting plant species, despite anatomical and ontogenetic differences.

Resolving Form–Structure–Function Relationships in Plants with MRI for Biomimetic Transfer

In many biomimetic approaches, a deep understanding of the form–structure–function relationships in living and functionally intact organisms, which act as biological role models, is essential. This knowledge is a prerequisite for the identification of parameters that are relevant for the desired technical transfer of working principles. Hence, non-invasive and non-destructive techniques for static (3D) and dynamic (4D) high-resolution plant imaging and analysis on multiple hierarchical levels become increasingly important. In this study we demonstrate that magnetic resonance imaging (MRI) can be used to resolve the plants inner tissue structuring and functioning on the example of four plant concept generators with sizes larger than 5 mm used in current biomimetic research projects: Dragon tree (Dracaena reflexa var. angustifolia), Venus flytrap (Dionaea muscipula), Sugar pine (Pinus lambertiana) and Chinese witch hazel (Hamamelis mollis). Two different MRI sequences were applied for high-resolution 3D imaging of the differing material composition (amount, distribution, and density of various tissues) and condition (hydrated, desiccated, and mechanically stressed) of the four model organisms. Main aim is to better understand their biomechanics, development, and kinematics. The results are used as inspiration for developing novel design and fabrication concepts for bio-inspired technical fiber-reinforced branchings and smart biomimetic actuators.