Pressurized Struts Dissipate Impact

yellow fruit hanging from tree with green leaves

Biological Strategy

Pomelo

Eleanor Banwell

Image: Luhaifeng279 / Pixabay / Free non-commercial use

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.

Search this Function

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.

Search this Function

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.

Search this Living System

Skin of pomelos absorb impact by rupturing fluid-filled struts

The pomelo, or Citrus maxima, is the largest citrus fruit in the world. The fruits usually weigh around 2 kg and occasionally as much as 6 kg, while the trees reach 15 meters in height. Such large fruit falling from such a height are at risk of being damaged by the impact, but to maximize their chances of reproducing, the trees need the fallen fruit to stay good for as long as possible. This gives time to the animals that eat pomelos to find them, carry them away, and spread the eaten seeds. The trees are native to hot and humid regions and any splits in the skin caused by the fall will give access to microorganisms that cause fruit to spoil. To protect from impact, pomelos have a compressible skin 2-3 cms thick. Most citrus fruit have been altered by selective breeding, but the pomelo is one of the three “true” citrus and so its thick skin is due to the need to protect the fruit and not due to human intervention.

The skin of the pomelo has multiple layers. The outermost layer, called the exocarp, is densely packed with cells. The next layer, which forms the bulk of the peel, is called the mesocarp. Inside the mesocarp is the endocarp, which contains all the seeds and pulp segments of the fruit. The mesocarp, or pith, is the layer primarily responsible for protecting the fruit from impact, and it is an open porous foam. That is, the mesocarp is made of many air cells that are interconnected with each other. When the fruit falls, these air pockets collapse like a cushion, absorbing the energy of impact, protecting the endocarp and exocarp from damage. Unlike a simple foam cushion, the struts that make up the physical structure of the foam mesocarp are fluid-filled and under pressure. Rupturing these struts absorbs even more energy and ensures the pomelo is exceptionally good at surviving a fall.

Layered materials can separate under stress, or become delaminated. When this occurs they lose their physical properties. The layers of the pomelo are not clearly defined. Instead, the exocarp gradually transitions into mesocarp that at first has small air-filled gaps that become gradually larger closer to the center of the fruit. By gradually transitioning from a dense, tough, material to a soft foamy one, the fruit cannot delaminate and its protective nature is preserved.