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The main structural material in a plant is cellulose, a linear polymer made from the sugars that photosynthesis provides. Cellulose is stiff (about 130GPa), durable and is the major component of the walls of the cells of which all plants are made. The cells expand and grow, actuated by the liquid inside them that, because it contains chemicals such as sugars, pulls water into, and so pressurises, them.
The shape of the expanding cell is largely controlled by the orientation of cellulose in the cell walls controlling directional compliance. Growth and morphology are related but can be separate. Shape, established in the bud, is expressed by origami-like deployment that can inspire engineering design, for instance the Kresling pattern.
A simple static example of this construction is the dandelion flowering stem. Its cells work at 5 to 10 bar. The pre-strained cell walls give structural stiffness, so that with a minimal amount of solid material (7% dry weight) the dandelion can hold its head up. Smaller cells with thicker walls on the outer part of the stem set up a gradient of 25:1 of cellulose from outside to inside, maximising the second moment of area. Under wind loading the tensile pre-strain on the compressive side of the stem has to be paid off before it fails by local buckling.
Plants can move quickly using power amplification, storing energy slowly in a spring, triggering its release. The Venus fly trap is a well-known example – an opposing pair of bistable anticlastic plates. The trigger is a sensory bristle, although nobody knows how it works; nor do we know how the trap opens again after it’s digested its prey. Each trap is a resettable mechanism that can be used several times.
The cell wall contains other components such as pectin – a gel that can absorb water and swell (hence ‘swellulose’) and is essential for making jam and marmalade – that provides a matrix for the cellulose fibres. Since swelling is a property of the chemistry rather than life, it’s common for ‘dead’ parts of plants to swell and shrink as water becomes more, or less, available. For instance humidity can vary with weather, season and diurnal changes in temperature. This partnership of local chemistry and wider changes in water is very reliable, so plants can tune their lifestyle to varying external conditions.
Probably the best-known example is the pine cone, whose dead scales (bracts) open in the dry to release the seeds, and close in the damp to protect them from fungal attack. Actuation relies on a bilayer. The matrix swells uniformly but is constrained by the cellulose fibres that allow more expansion on the lower than the upper side of the bract. The pine-cone model has inspired actuation of textiles (originally for military clothing), architectural facades and kinetic art.
Many seed pods are bilayers with cellulose arranged at ±45deg. The pods dry, developing pre-strain, breaking open when brittle. The stored elastic energy feeds the fracture and shoots the seeds. Seeds of some grasses with helically arranged fibres in protrusions (awns) generate circular movement and drill themselves into the ground or even ‘walk’ away from the mother plant.
Seeds also fly. Maple and sycamore seeds are sophisticated autogyros, and the seeds of the dandelion are sophisticated balloonists. Air flowing between the bristles of the umbrella is constrained as a permanent vortex, increasing drag fourfold at no cost to the seed. Biology maximises structure and information, and minimises energy and materials.
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Content published by Professional Engineering does not necessarily represent the views of the Institution of Mechanical Engineers.