Witch hazel plants propel seeds at bullet speeds

A recent study from Duke University has revealed the remarkable physics behind the ability of witch hazel plants to launch seeds with incredible velocity.

The team discovered that members of the witch hazel family can propel heavier seeds just as swiftly as lighter ones, thanks to the spring-loaded mechanism of their fruits. 

Focus of the study

The study, led by graduate student Justin Jorge and senior author Sheila Patek, explored the world of plant biomechanics through high-speed video camera recordings and meticulous measurements.

The witch hazel, commonly found in forests, is often admired for its sweet-smelling shrub with crinkly ribbon-like petals. 

However, according to Jorge, it is more akin to a howitzer due to the impressive firepower of its fruits. 

Rapid seed dispersion 

When witch hazels are ready to disperse their seeds, their woody seed capsules split open. Pressure builds up, and eventually the seeds shoot out like bullets fired from a rifle, hitting 30 feet per second in about half a millisecond.

This rapid seed dispersion happens so swiftly that it is almost impossible to capture with a regular camera. “If you blink you’ll miss it,” said Jorge. 

How the study was conducted 

The research team utilized a high-speed video camera capable of recording at 100,000 frames per second to document the phenomenon.

The team collected fruits from three different witch hazel species found in Duke Gardens or Duke Forest. 

The collected seeds varied in weight, with the lightest ones weighing only 15 milligrams – lighter than a grain of rice, while others were ten times more massive. 

What the researchers discovered 

Surprisingly, the witch hazels were able to fling the heavier seeds just as fast as the lighter ones. 

“We found that the launch speeds were all roughly the same,” said Jorge. “Given the order of magnitude difference in seed masses, I was not expecting that at all.”

Their findings, published in the Journal of the Royal Society Interface, delved into the mechanics of how the plants achieve this feat. 

The researchers discovered that the secret to the seed’s velocity lies in the spring-loaded launch of the fruit capsule.

Seed propulsion 

The three species investigated in the study employed the same mechanism for seed propulsion. 

Before the seeds are ejected, the fruit capsule dries out and deforms, akin to a piece of wood warping. The compression of the woody fruit capsule’s walls is what eventually sends the seed flying.

“It’s similar to how you can shoot out a watermelon seed by squeezing it between your fingers,” explained Jorge.

Spring-like seed capsule 

Typically, to propel a heavier object at the same speed as a lighter one, more force is required. The witch hazel achieves this by using springs. 

The researchers estimated the elastic potential energy stored in the spring-like seed capsule by measuring the force needed to wedge the seed back into place. 

They found that witch hazel species with heavier seeds have larger capsules that can store more elastic energy.

Jorge plans to investigate further by examining the forces acting on witch hazel seeds as they fly through the air and assessing how far they can travel. 

Study implications 

This study not only sheds light on the remarkable adaptations of the witch hazel, but also contributes to the broader understanding of seed dispersion mechanisms in the plant kingdom.

The researchers say that some of the lessons learned from nature could lead to better designs for robots. 

“People ask me all the time, ‘why are you looking at seed-shooting plants?’” Jorge said. “It’s the weirdness of their springs.”

“When we think of springy things, we typically think of rubber bands, coils, or archery bows,” Jorge said. “But in biology, we have all these weird, complex shapes.”

“Perhaps there are some benefits to these shapes that can be used to improve the design of synthetic springs, such as those used in small jumping robots, but first we need to understand how these biological springs work.”

Video Credit: Patek lab, Duke University

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