Hierarchical Organization of Peel Confers Impact Resistance

close up photo of seven pomelo citrus fruits, one with exposed orange flesh

Biological Strategy

Pomelo

AskNature Team

Image: JPCNX / Shutterstock / Some rights reserved

Manage Impact

An impact is a high force or mechanical shock that happens over a short period of time, such as a hammer hitting a nail rather than a hand pushing slowly against a wall. Because of their speed and force, impacts don’t allow materials to slowly adjust to the force, which can lead to cracks, ruptures, and complete breakage. Therefore, living systems have strategies that can absorb, dissipate, or otherwise survive that force without the need to add large amounts of material. For example, the Toco toucan’s large beak is very lightweight, yet can withstand impacts because it’s made of a composite material with rigid foam inside and layers of a hard, fibrous material outside.

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Prevent Fracture/Rupture

High force impact or stress can cause materials that comprise living systems to separate into two or more pieces (called fracturing) or to break or burst suddenly (called rupturing). For example, a scallop prevents structural failure from fracture because its shell is comprised of two materials of varying stiffness. When a crack moves from the scallop’s stiff material to the less stiff one, the latter reduces the force at the tip of the crack, thereby stopping it from spreading farther.

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Flowering Plants

Clade Angiosperms (“receptacle seed”): Dandelions, oaks, grasses, cacti, apples

With 416 families containing some 300,000 known species, angiosperms are the most diverse group of plants, and they can be found around the globe in a wide variety of habitats. They are characterized by seeds that grow enclosed in ovaries, which are enclosed in flowers. The floral organs then develop into fruits of myriad kinds and dimensions, from simple seed casings on maples to elaborate fleshy growths like papayas. The oldest flower known from fossils, Montsechia vidalii, appeared during the Jurassic Period 130 million years ago. They are the primary food source for herbivorous animals, which in turn makes them the indirect food source for carnivores as well.

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The pomelo fruit has excellent damping properties due to the hierarchical organization of its composite peel

Hierarchical structure of the pummelo.

Image: High Contrast / wikimedia commons / CC BY - Creative Commons Attribution alone

One half of a cut pummelo.

Image: Allen Timothy Chang / wikimedia commons / GFDL - Gnu Free Document License

Pummelos of various sizes, taken Jan. 30, 2005.

“Natural materials often exhibit excellent mechanical properties. An example of outstanding impact resistance is the pummelo fruit (Citrus maxima) which can drop from heights of 10 m and more without showing significant outer damage. Our data suggest that this impact resistance is due to the hierarchical organization of the fruit peel, called pericarp.” (Fischer 2010: B658)

“Citrus maxima is the largest fruit among the genus Citrus with a fruit weight up to 6 kg and a maximal height of the fruit bearing trees of 15 m. In combination these two factors, fruit weight and the height of the fruit bearing branches, cause a high potential energy of the hanging fruit. After the fruit is shed its potential energy is converted into kinetic energy which reaches its maximum just before impact with the ground. If the high kinetic energy was to cause the pummelos to split open when impacting with the ground, the fruits would perish within a short time due to the tropical climate in Southeast Asia, the region of origin of the genus Citrus.” (Fischer 2010: B659)

“Semi-quantitative analyses of thin sections of pummelo peel revealed a gradual transition in density between exocarp and mesocarp. Thus, structurally, the dense exocarp cannot be separated clearly from the spongy mesocarp. We hypothesize that due to this lack of an abrupt change in tissue composition and therefore in structural and mechanical properties the risk of delamination of the tissues during impact is reduced. The impact force acting on the pummelo depends on the velocity of the fruit before impact and its weight, but also on the consistency of the ground. Under natural conditions, part of the total energy is dissipated by the relatively pliable ground, as typically existing in the regions where pummelos grow naturally. In the tests presented mechanical loads acting on the fruits were increased by dropping the fruits onto a hard ground. Thus all kinetic energy must have been dissipated by the fruits themselves. The mesocarp with its air-filled intercellular spaces represents a compressible foam. As the Young’s modulus of this spongy part of the peel is rather low, we conclude that its ability to dissipate large amounts of energy must result from the structural composition of the peel.” (Fischer 2010: B662)

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