Blades Take on 3D Ruffled Shape

close up photo of green plant underwater

Biological Strategy

Bull kelp

Dimitri Smirnoff

Image: August Rode / Flickr / Some rights reserved

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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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Optimize Shape/Materials

Resources are limited and the simple act of retaining them requires resources, especially energy. Living systems must constantly balance the value of resources obtained with the costs of resources expended; failure to do so can result in death or prevent reproduction. Living systems therefore optimize, rather than maximize, resource use. Optimizing shape ultimately optimizes materials and energy. An example of such optimization can be seen in the dolphin’s body shape. It’s streamlined to reduce drag in the water due to an optimal ratio of length to diameter, as well as features on its surface that lie flat, reducing turbulence.

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Phylum Plantae (“plants”): Angiosperms, gymnosperms, green algae, and more

Plants have evolved by using special structures within their cells to harness energy directly from sunlight. There are currently over 350,000 known species of plants which include angiosperms (flowering trees and plants), gymnosperms (conifers, Gingkos, and others), ferns, hornworts, liverworts, mosses, and green algae. While most get energy through the process of photosynthesis, some are partially carnivores, feeding on the bodies of insects, and others are plant parasites, feeding entirely off of other plants. Plants reproduce through fruits, seeds, spores, and even asexually. They evolved around 500 million years ago and can now be found on every continent worldwide.

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Blades of bull kelp take on 3D ruffled shape because of different growth rates along the edge and midline

Bull kelp (Nereocystis luetkeana) is a marine macroalga that resembles a vine-like plant with a long, thin stipe (stem-like structure) up to 30 meters long, anchored to the sea floor by a holdfast (root-like structure). A gas-filled float at the stipe’s other end holds numerous 4-meter-long photosynthetic blades close to the water’s surface. If bull kelp grows at a site exposed to strong wave action, blades are strap-like and flat. If grown at a sheltered site, blades are wide and ruffled. These differences in blade shape are at least partly due to a trade off between reducing drag and increasing solar exposure for (check out the related strategy).

How do bull kelp blades become ruffled? The cause appears to be different growth rates in different parts of the blade. Tissues of an organism can grow at different rates, even within the same appendage such as a kelp blade. For a flat strap-like blade, the tissues at the blade’s edge and the blade’s middle grow at the same rate. But for a ruffled blade, the tissue at the edge lengthens faster than the tissue at the midline. Extra material builds up on the blade’s edge, which then buckles and takes on a ruffled appearance. Because the blade material is flexible, these ruffles can be pushed into different configurations with shorter or longer wavelengths. This growth pattern illustrates a way that simple changes in growth rate can result in more complex 3D shape changes.

Highly Stretchable Stipe Resists Breaking

Bull kelp

Highly stretchable stipe of bull kelp resists breaking because of restraining cellulose fibers

Biological Strategy

Blades Balance Drag Reduction and Solar Exposure

Bull kelp

Blades of bull kelp balance drag reduction and sunlight exposure in different flow environments via changes in width and flatness.

close up of dark olive sea kelp in water

Biological Strategy

Pulled Support Stalks Experience Lower Stress Forces

Bull kelp

Long and skinny support stalks (stipe) of seaweed experience lower stress forces when pulled rather than bent.

Image: Dan Hershman / CC BY - Creative Commons Attribution alone

Last Updated January 12, 2017

References

“If rates of longitudinal growth at the edges of a blade exceed the rate of longitudinal growth along the midline of the blade, ruffles along the edges of the blade are produced by elastic buckling. In contrast, flat blades are produced when rates of longitudinal growth are similar across the width of a blade. Because ruffles are the result of elastic buckling, a compliant undulate N[ereocystis] luetkeana blade can easily be pushed into different configurations (e.g., the wavelengths of the ruffles along the edges of the blade can change, and the whole blade can twist into left- and right-handed helicoidal shapes)…” (Koehl et al. 2008:834)

Journal article

How kelp produce blade shapes suited to different flow regimes: A new wrinkle

Integrative and Comparative Biology | 01/01/2008 | Koehl MAR; Silk WK; Liang H; Mahadevan L.

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