Mechanics without Muscle: Biomechanical Inspiration from the Plant World

Abstract

Plant and animal biomechanists have much in common. Although their frame of reference differs, they think about the natural world in similar ways. While researchers studying animals might explore airflow around flapping wings, the actuation of muscles in arms and legs, or the material properties of spider silk, researchers studying plants might explore the flow of water around fluttering seaweeds, the grasping ability of climbing vines, or the material properties of wood. Here we summarize recent studies of plant biomechanics highlighting several current research themes in the field: expulsion of high-speed reproductive projectiles, generation of slow movements by shrinking and swelling cell walls, effects of ontogenetic shifts in mechanical properties of stems, flexible reconfiguration and material properties of seaweeds under crashing waves, and the development of botanically-inspired commercial products. Our hope is that this synopsis will resonate with both plant and animal biologists, encourage cross-pollination across disciplines, and promote fruitful interdisciplinary collaborations in the future.

Self-organisation and motion in plants

“…The primary mechanism is adjustment of cell hydration in order to increase or decrease water pressure, the so-called turgor pressure, within the individual cells. Leaves and stalks can for instance bend toward the sun utilizing special motor cells in the hinge of the leaf petiole, the so-called pulvinus, such that if cells on the upper-side of the pulvinis dehydrates, and thus become less rigid, while cells on the under-side hydrates, and thus become more rigid, the leaf will bend upwards 16 . Similar, although more complex, mechanisms are responsible for other types of plant movement.…”

“…Similar, although more complex, mechanisms are responsible for other types of plant movement. In the Sphagnum moss mentioned above, dehydration of the capsule containing the spores causes it to decrease in volume, which leads to an increase in internal pressure up to 500 kPa, which eventually results in the cap breaking free and the spores violently being propelled upwards 16,12 . In the Venus fly trap, the modified leaves are bi-stable and can be in either an open concave or a closed convex shape.…”

Evolutionary optimization of material properties of a tropical seed

“…For an elastic solid, fracture occurs when the energy released by crack extension reaches a critical value. The energy release rate for a crack in the vessel wall is given by G ¼ ps 2 E f ðl; rÞ;…”

“…Research into the structure and properties of biological tissues benefits enormously from knowledge of ecological and evolutionary contexts because consideration of the selective pressures to which a tissue is exposed can give important insights into its design [1,2]. This is particularly true for seeds of angiosperms.…”

The fern cavitation catapult: mechanism and design principles

“…The small size of the spores (less than 50 mm) allows them to be carried by air currents over great distances. However, the same smallness prevents the spores from detaching easily from the mother plant; hence, the requirement for an active ejection mechanism [6]. In the case of the leptosporangium, a row of 12 -25 cells known as the annulus is responsible for spore ejection (figure 1b).…”