Leaves Have Elasticity, Shape Memory

Biological Strategy

Resurrection fern

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Modify Size/Shape/Mass/Volume

Many living systems alter their physical properties, such as size, shape, mass, or volume. These modifications occur in response to the living system’s needs and/or changing environmental conditions. For example, they may do this to move more efficiently, escape predators, recover from damage, or for many other reasons. These modifications require appropriate response rates and levels. Modifying any of these properties requires materials to enable such changes, cues to make the changes, and mechanisms to control them. An example is the porcupine fish, which protects itself from predators by taking sips of water or air to inflate its body and to erect spines embedded in its skin.

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Store Liquids

Many living systems must store liquids, such as water or nectar, so that it is available over long periods of time, including when moisture levels are low. Because of their properties, liquids tend to disperse unless they are confined in some way. Each liquid has its own unique properties. For example, water is polar, exhibiting a strong negative charge on one side of the molecule and a strong positive charge on the other. Living systems have strategies to confine fluids by taking advantage of these properties. A good example of taking advantage of water’s polarity is using materials that repel water. In doing so, a living system can keep water on one side of a barrier, such as a membrane.

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Capture, Absorb, or Filter Liquids

The most common liquid used by living systems is water, which they require to survive. But there are many other liquids that provide nourishment, play a role in defense mechanisms, or serve other purposes. Water varies in its availability; it is sometimes plentiful and sometimes very scarce or only available as fog or mist. To minimize the energy required to capture, absorb, or filter liquids, living systems have strategies that take advantage of the unique properties of the given liquid. For example, water moves from a gaseous to liquid state when it encounters a surface colder than the air. Plants in forests that experience fog and clouds more than rain have strategies that condense liquid water from moist air.

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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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Leaves of the resurrection fern regain shape after dehydration due to hierarchical structure of palisade and spongy layers.

Image: s /

Image: Christine (and John) Fournier /

“The resurrection fern Polypodium polypodioides has a remarkable elastic reponse, where the fast water uptake of the fern upon rehydration is accompanied by a significant reduction in its Young’s modulus. In this letter, we discuss the fern’s elastic response and suggest that by mimicking its structure, one should be able to design materials exhibiting interesting elastic behavior.

“For many years, plants have been a rich information source for designing and optimizing materials and biomimetic systems. For example, Burdock plants had a direct impact on the invention of a novel hooking system, while the lotus leaf has inspired the creation of very hydrophobic surfaces. The elastic response of plants when exposed to external stimuli water, light, etc. is also rather interesting,and emerging biomimetic materials may just take advantage of this. Plant leaves are often stiff while fully hydrated but loose turgor and become soft under dry conditions. If they dry completely up, only the cellular matrix remains, and the leaf appears to be hard and brittle. However, some plants have a conceptually simpler elastic response, where the plant leaf is stiff in the dry state and soft in the wet state. Here, we study the resurrection fern Polypodium polypodioides, which has an amazing ability to take up water while at the same time altering its elasticity from a soft hydrated state to a stiff dehydrated state in order to cope with drought. Moreover, as reported in Ref. 6, the fern can reproducibly switch between a curled-up dry state and a fully extended and soft wet state and is, therefore, a natural shape-memory material.” (Helseth 2008:1)

“The structure of the resurrection fern is hierarchical (see Fig. 5 in Ref. 6), where the smallest elastic units are the plant cells arranged into palisade and spongy layers. Water flows into the layered structures due to capillary pressure, allowing the cells to absorb water as well. An artificial structure aiming at reproducing the elastic reponse of the fern must display a hierarchical structure which mimics that of the plant.” (Helseth 2008:3)

Last Updated October 24, 2016

References

Journal article

Elastic response of the resurrection fern Polypodium polypodioides during rehydration

Appl. Phys. Lett. | 30/01/2008 | L. E. Helseth

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Reference

Young’s modulus (E) is defined as the ratio of the stress applied to the material along the longitudinal axis of the specimen tested and the deformation or strain, measured on that same axis. Young’s Modulus is also known as tensile modulus, elastic modulus or modulus of elasticity.

When a stretching force (tensile force) is applied to an object, it extends, and its behavior can be obtained using stress-strain curve in the elastic deformation region (Known Hooke’s Law). The extension that a force produces is not only dependent on the material but also on other factors like dimensions of the object (e.g. length, thickness etc.)

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Young's Modulus

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