Stems Vary Stiffness

grasses with ridges stand upright through the photograph with more grasses in the background

Biological Strategy

Rough horsetail

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Prevent Buckling

When a living system undergoes compression to the extent that it causes structural damage, it results in buckling. For example, if a person pushes down on the top or the side of a paper cup, the cup’s wall will eventually give way, or buckle. Although a living system could add material to strengthen a structure, this requires expending precious energy. Instead, it must use energy and materials conservatively to avoid buckling, strengthening structures through careful placement of materials to resist, absorb, or deflect compressive forces. For example, instead of one long, tubular stem, some plants like bamboo have stronger nodes scattered along their stems. When compressed, these nodes keep the round stems from taking on an oval shape that weakens the structure and could result in buckling.

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Modify Pressure

For many living systems, modifying pressure provides extra strength. For others, it provides ways to move air. To use pressure effectively requires a reliable source of pressure, as well as mechanisms to create and release the pressure as needed. Often, modifications in pressure within a living system are created by water, although air can also be a source. An example of a water-facilitated pressure system is wilted leaves that use hydrostatic pressure to stiffen. They do so by bringing solutes (such as salts) into their cells, which causes them to draw in water.

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Ferns

Class Polypodiopsida (“many footed appearance”): Ferns, horsetails, whisk ferns

Ferns are a class of plants that reproduce without flowers or seeds, but by spores. With over 10,500 known species, ferns are the second most abundant group of vascular plants after angiosperms (flowering plants). Though they can be found in a variety of habitats, ferns prefer moist, humid environments. Ferns first evolved in the Carboniferous Period about 360 million years ago. Today, ferns are grown for food, medicine, ornamental landscaping, and hold importance in culture and folklore. Several species can also remove pollutants from the atmosphere.

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Stems of scouring horsetail vary their stiffness by having rings of supportive tissues that react to changes in turgor.

“Plants with hollow axes, e.g. various horsetails and grasses, serve as
generators of biological concepts for technical structures with
variable stiffness. Their structure is characterised by a thin outer
ring of strengthening tissue stabilised by a lining of parenchyma cells
(Fig. 1A-C). The hollow stems are divided into shorter segments
(internodes) by transverse walls and stem thickenings at the so called
nodes. The nodes significantly reduce the danger of local buckling in
these light-weight structures. The stability of these stems depends
significantly on the internal pressure (turgor) of the parenchymatous
cells. If the turgor pressure is reduced, e.g. by water deficiency,
stiffness and stability of the stems decrease. In some species–such as
the Brazilian Giant Horsetail (Equisetum giganteum)…the resistance to
ovalisation is extremely turgor-dependent. In other horsetail
species–such as the Dutch Rush (E. hyemale)–the outer ring of
strengthening tissue is connected via wedge-shaped elements with an
inner ring of strengthening tissue forming a mechanically resistant
sandwich structure (Fig. 1D, E). These stems are also stabilised by the
pressurised lining of parenchymatous cells but depend much less on the
turgor pressure of the parenchyma cells. The mechanical stability
resisting stem ovalisation is diminished by only about 20% due to
reduction of the turgor pressure.

“Potential technical
implementations are manifold, inspired by plants with mechanical
properties of the stem varying with the internal pressure of the
pressurised cellular lining. These include light-weight structures with
chambered pressure-stabilised pneumatic structures that feature a
segmental variation of stiffness and the ability to adapt their
stiffness or form to changing outer conditions, facilitated either
adaptively or via integrated active control. Envisaged technical
applications for these types of biomimetic technical smart materials
include: (1) shells of airplane wings and other aircraft (adaptation to
changing aerodynamics); (2) shells of buildings of innovative
construction; (3) car parts, e.g. aerodynamically adjustable spoilers.”(Speck et al. 2004:199-200)

Last Updated August 18, 2016

References

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