Session J05: Plant Biomechanics

Light exposure in seagrass meadows is modified by flow-driven plant reconfiguration

Seagrasses are foundation species in marine and estuarine ecosystems. They contribute biomass, provide habitat, and damp waves and currents. Globally, seagrass health and primary productivity are particularly threatened by factors that affect light availability, such as shading by algae, self-shading, and increased water column turbidity. This study focuses on how plant motion and reconfiguration lead to shading of an individual plant, and how flow conditions, plant flexibility, and plant density affect light availability along a seagrass blade. We use a simple ray-optics shading model with simulations of plant motion using the model of Zhu et al. (2020). Results show the effect of wave orbital excursion on plant shading, as well as the more complex dynamics that arise for blades with greater flexibility or asymmetry in their motion. We develop a parameterized model that can be used when field conditions, such as seagrass species characteristics and wave statistics, are known, in order to predict light exposure in seagrass meadows globally.   

Poroelastic zippering actuation of hygroresponsive Viola seed pods

Seed pods of some Viola species exhibit a remarkably efficient and unique seed-launching mechanism through a hygroscopic motion termed "zippering." In this poroelastic process, seeds are sequentially pinched and launched from the proximal to the distal end of the drying pod. This mechanism enhances Viola's probability of germination by ensuring robust seed launching and wide, yet even, dispersal patterns. Here, we theoretically show that the semi-circular cross-section of Viola seed pods, consisting of a hygroscopic porous inner layer and an external rigid shell, generates a force significantly stronger than a rectangular bilayer structure of the same volume. Furthermore, our combination of theoretical analysis and experimental measurements of both natural and artificial pods reveals that Viola seed pods possess an optimal geometrical design for sequential pinching or zippering. Inspired by the unique hygroscopic actuation of Viola seed pods, we have designed and fabricated a soft actuator capable of zippering, driven by the wetting or dewetting of its poroelastic interior.   

Intrinsic Curvatures of the Volvox carteri

Extracellular matrix (ECM) is common across living systems and is integral to many cellular processes, as well as a means of providing structure to an organism. The colonial green alga Volvox carteri is one such organism, where ciliated somatic cells produce ECM throughout their lifecycle to generate an expanding spherical colony that is capable of phototaxing through the water column. In these experiments where Volvox carteri is broken into pieces using a homogenizer, we discover that these thin sheets of cells embedded in ECM have an intrinsic curvature different to that of the initial spheroid, and depending on the characteristic shape and size of these broken-off pieces we can infer residual stresses in the ECM of a volvox colony as it expands over its lifecycle. We use confocal imaging techniques to observe how changes in the number of cells, boundary conditions, and age of the tissue changes the final shape of the piece, as well as how the cilia interact and synchronize with each other to propel a piece with a very different shape factor. Using our experimental data as a guide, we then propose a general theoretical framework that recovers these unique shapes and curvatures, and also determines the extent of influence the number of ECM producing neighbours has on the intrinsic curvature of a piece. The sum of this will help answer important questions regarding shape, structural integrity, and aging as it relates to ECM in general.   

Discrete Element Modeling of Buzz Pollination

Buzz pollination involves the release of pollen from flower anthers through vibrations generated by bees. Despite previous experimental and numerical studies, the intricacies of pollen dynamics within these vibrating structures remain elusive due to the challenges in observing these small, opaque systems. Discrete Element Method (DEM) is used to simulate the pollen expulsion process, establishing a baseline for understanding the correlation between the maximum jerk of the anther walls and the initial rate of pollen expulsion. Under purely translational oscillations, increased vibration intensity enhances pollen release, though this has diminishing returns beyond typical buzzing conditions. Further, pollen-pollen interactions, which can account for approximately one-third of total collisions, play a more significant role than previously considered. Building on this baseline, we explore the impact of material damping, for both anther walls and pollen particles, and non-translational bending shapes have on the pollen expulsion rate. Initial results suggest the restitution coefficient is a significant driver in how quickly pollen is released and the importance of characterizing the material properties of the system.