Plant Tissues

Cell walls, the ECMs of plants, are hygroscopic composites of cellulose, hemicelluloses, pectins and often lignin. In the living plant, they are in a fully hydrated state, even though a large proportion is dead material without metabolism providing essential functions such as water transport, mechanical stability and protection against environmental challenges. The cell walls are not just passive scaffolds but can be active. They are able to generate stresses or perform environmentally-triggered motions. Prominent examples are the maturation stresses of wood cells causing tensional stresses at the stem periphery, while compressive stresses in the inner parts of the stem occur.

In addition to maturation stresses, drying- and temperature-induced stresses play an important role, e.g. in seed pods which open upon environmental triggers. The functionality of dead plant materials is based on their cell wall composition, cell geometry and their arrangement in the plant. While wood properties and activities are comparably well described, only little is known about seed pod materials and hardly anything about properties and stress states during development, life time and finally seed release.

Left: Banksia attenuata. Yellow inflorescence with numerous flowers, some pollinated flowers develop into follicles (seed pods) arranged on the infructescence (cone). Right: Spruce wood ECM.

A key research goal is to understand the spatial and temporal development of material properties and internal stresses during seed pod development and to follow how the protection of the seeds and internal stress-states change with time. This understanding is essential to predict seed and seed pod development, for seed storage in the light of conservational aspects and changing environmental conditions due to anthropogenic pressures. Furthermore, the knowledge contributes to an understanding how biological organisms cope with changes in stress states.

Western Australia is blessed by an extremely old ecosystem and a large biodiversity. We are investigating a broad range of Banksia species with different geometries, tissue organization and opening behaviour. In addition to pyriscent (fire triggered) species we are investigating necriscent (death triggered) seed releasing species, in particular species of the related plant genus Hakea. Hakea follicles open as soon as the plant or a branch dies, suggesting that the structure changes its equilibrium by a simple drying process. Since both Banksias and Hakeas display a large range of follicle geometries and, as a consequence, various motion patterns they have the potential to serve as bio-inspiration for the development and manufacturing of robust active materials and structures.

By extending the range of species and serotinous traits, we will be able to mechanistically understand seedpod properties and how they develop with growth, maturity and opening. It will substantially contribute to a deeper understanding of the interactions between environment/ecology and material properties. The consideration of various geometries will also allow to extend the range of design space for new material developments.

Left: Banksia photograph. Right: Micro-computed tomography (┬ÁCT) scans of a Banksia prionotes follicle (seed capsule) in the closed (grey), heat-triggered open (yellow) and water-fuelled fully open state.

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