Plant Biomechanics

Abstract

The mechanical design of plants is based on the tensile properties of cellulose molecules which combine to form stiff nanofibrils from which the wall surrounding all plant cells is mainly constructed. Simple plants such as liverworts are made from few cell types and rely for their stiffness on the osmotically maintained pressure (turgor) of water inside the cell reacting against the stiff cellulose. As plants evolved and grew towards the light they developed larger fibres which transmit the forces more effectively and convert the plant into a complex pre‐stressed structure, but still largely dependent on water pressure for its rigidity. Increased structural complexity leads to increased fracture resistance and durability. Lignin glues the nanofibrils together so that the fibres, and then the cell walls in general, can take compressive forces, and tall herbs and trees evolve. Some plants (lianas, creeping herbs, Spanish moss, etc.) can grow large relying on simple tension under gravitation.

Key Concepts:

  • Cellulose is the commonest, and most technically most useful and important, biological fibre‐forming molecule.

  • The cell wall is a composite material of fibres in a matrix.

  • Since the plant cell wall is a relatively open fibrous structure, it is very difficult to ascribe unique mechanical properties to it. This variability gives it adaptability.

  • Intrinsically plants move slowly using turgor, but they can move quickly by storing elastic energy over time and releasing it suddenly. This is called power amplification.

  • The more cells, the more tissue types are possible and the plants can be larger and more adaptive.

  • Cellulose is distributed nonuniformly in a way that maximises its effectiveness.

  • Lignified tissues are mostly dead and so are simple prototypes for biomimetics.

Keywords: plant; cell wall; cellulose; turgor; stiffness; fracture

Figure 1.

Production of cellulose fibril from an enzyme (cellulose synthetase) ‘floating’ in the cell membrane and guided by one or more microtubules beneath the cell membrane. Adapted from Burgert and Fratzl with permission from Oxford University Press.

Figure 2.

The cell wall is made of cellulose fibrils joined together by more flexible and smaller strands of cellulose (hemicellulose). The spaces between the fibrils are filled with pectin and water. The middle lamella (top) forms the boundary between the cell walls and the cell membrane (bottom) surrounds the cell.

Figure 3.

Three cylindrical cells with different orientations of cellulose fibrils in the cell wall (top row) and the change in shape when the cell is pressurised. The stiff fibrils stop the cell from stretching, but they can separate laterally. Thus the cell can increase in diameter (a) or length (b) or in both directions (c).

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Further Reading

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Vincent, Julian FV(May 2011) Plant Biomechanics. In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1002/9780470015902.a0002062.pub2]