What do you think of this fact?
Picture a tree that doesn’t wait for anyone to spread its seeds. It just blows its fruit up. That’s the sandbox tree, Hura crepitans, also known as the dynamite tree.
A tree you don’t want to hug
The sandbox tree grows in the American tropics, and in the forest it can reach 60 meters. Its trunk is covered in sharp thorns, and its milky sap is toxic. So far, so unfriendly.
The real show is the fruit. It’s about 6 centimeters across and built from 12 to 16 segments, a bit like a tiny pumpkin made of wedges. Once it is ripe, the capsule bursts apart, and the seeds shoot out in all directions.
Fast, far and with backspin
In 1977, researchers measured launch speeds of up to 70 meters per second, about 250 km/h, with an average of 43 meters per second. Seeds landed up to 45 meters away from the tree.
More than 40 years later, a team filmed exploding fruits in Costa Rica at 2,000 frames per second and spotted something the older measurements had missed: the seeds fly with backspin, not topspin, as had been assumed. They spin at around 80 rotations per second. Like a CD you throw with a flick, a seed with backspin stays stable in the air and keeps an orientation with low drag. According to the team’s model, a seed launched at 43 meters per second with backspin could fly about 41 meters, compared with about 17 meters for one that tumbles.
Their own video only showed about 13 meters per second. The researchers suspect the reason: their fruits had been kept in a freezer, which probably weakened the blast. For explosions in nature, they consider the older, faster numbers more accurate.
Bonus fact: The name “sandbox tree” comes from the days before modern pens. People cut unripe capsules in half and used them as little boxes for sand to dry wet ink.
So when this tree drops its fruit, it doesn’t drop anything. It fires.