The sandbox tree (Hura crepitans)

 Tree which shoots its seeds at 150 MPH, What kind of tree is it?

The scientific name of this species is Hura Crepitans. Crepitans is a Latin term which refers both to something that breaks, and to the noise this makes. The reason this word refers to this tree is because of its fruits, which when ripe literally explode, projecting the seeds at great speed up to 15-20 metres away (some say up to 50 metres). That’s why this tree is also called the Dynamite Tree. This rainforest tree solved one of evolution’s hardest problems. Here’s how it evolved to use tension and geometry to fire its seeds with startling force. In the rainforests of Central and South America, there stands a tree which, without warning, detonates its fruit with a sharp crack. As a result, seeds are sent flying at speeds which can exceed 150 miles/hour (240 km's per hour). The explosion is loud enough to startle animals and force nearby humans to take a step back. Contrary to what some might believe, this isn’t a defensive tactic; it’s simple reproduction.

Hura crepitans, known by the common names sandbox tree, possumwood, monkey no-climb, assacu (from Tupi asaku) and jabillo, is a species of evergreen tree in the family Euphorbiaceae, native to tropical regions of North and South America including the Amazon rainforest. It is also present in parts of Tanzania, where it is considered an invasive species. Because its fruit explodes when ripe, it has also received the colloquial nickname "dynamite tree". The sandbox tree (Hura crepitans) belongs to a small but remarkable group of plants which rely on explosive seed dispersal. Here’s how this species, using nothing more than plant tissue, water tension and mechanical instability, has solved a fundamental evolutionary problem. The sandbox tree can grow to 60 metres (200 ft) in height, and up to 13.2 metres (43 ft) in girth at 1.8 m (6 ft) above the ground;[8] its large ovate leaves grow to 60 cm (2 ft) wide. The trees are monoecious, with red, un-petaled flowers. Male flowers grow on long spikes, while female flowers grow alone in leaf axils. The trunk is covered in long, sharp spikes which secrete poisonous sap. The fruit are large, pumpkin-shaped capsules, 3–5 cm (1–2 in) long, 5–8 cm (2–3 in) diameter, with 16 carpels arranged radially. Its seeds are flattened and about 2 cm (3⁄4 in) diameter. High-speed video analysis of its exploding fruit revealed that sandbox seeds fly with backspin as opposed to topspin, which had been previously assumed. Backspin helps seeds remain oriented to minimize their drag during flight.

Plants face one major dilemma that the vast majority of animals don’t: once rooted, they’re stuck in that position for good. Their offspring, however, benefit enormously from distance. This means seeds which land too close to the parent will have to compete with them for light, water and nutrients. It also renders them more vulnerable to predators and pathogens which prey on their species. For this reason, seed dispersal isn’t optional. It’s one of the strongest selective pressures in plant evolution. Many plants evolve in ways which allow them to outsource the task to animals, by packaging their seeds in sweet fruit or covering them with sticky coatings. Others rely on wind or water. The sandbox tree is unique for reasons beyond its explosive reproductive strategy. Its trunk is also covered in thousands of sharp, conical spines, and its sap is highly toxic. Historically, the tree earned its common name because its dried fruit was generally used as a container for ink blotting sand. Unsurprisingly, however, the fruit’s biomechanics are what have drawn the most scientific attention. However, in dense tropical forests, wind is weak and animal dispersers are unreliable. Because of this, some plants needed to find a more direct solution. For the sandbox tree, this meant turning its fruit into a spring-loaded weapon. The sandbox tree produces large capsules shaped similarly to a pumpkin, which are divided into wedge-like segments. As the fruit matures and dries, internal stresses build within the walls of the capsule. When those stresses eventually exceed the strength of the tissue holding the segments together, the fruit shatters violently. The seeds are launched radially at an astonishing mean speed of 43 meters/second (96 miles/hour), and sometimes as fast as 70 meters/second (156 miles/hour), often traveling tens of meters from the parent tree. From a biological standpoint, the sandbox tree challenges the intuition which plants are purely passive organisms. They may grow slowly and simply, but this doesn’t mean they aren’t complex organisms. The fruit of Hura crepitans is a composite structure optimized efficiently by evolution, especially in terms of:-

Fruit geometry

Fiber orientation

Moisture sensitivity

Cell wall thickness

     

Unlike animals, plants cannot actively make sudden movements with muscles. Instead, they can only rely on slow energy storage, followed by rapid release. In the sandbox tree, this energy comes from differential drying. In simpler terms, the fruit’s various layers of tissue lose water as it matures, each often at different rates. But because the plant cell walls are rigid and non-uniform, this uneven shrinkage creates internal tension. As a result, the fruit essentially becomes a biological pressure vessel. A early study on other explosively dehiscent fruits, specifically, the fruit of the Himalayan balsam (Impatiens glandulifera), shows that the cellulose fibers within these fruits’ walls are oriented in specific directions. In other words, as the tissues dry up, the fibers resist contraction along certain axes. This, in turn, causes the fruit to warp and store elastic energy. When the seams between the segments eventually fail under the pressure, the stored energy is released in milliseconds. The result is a burst fast enough to rival some engineered mechanical systems. From a physics standpoint, this is extraordinary. Given that the seeds themselves are relatively small, they undergo extreme acceleration over very short distances. What’s even more impressive is that the plant accomplishes this remarkable feat without the help of nerves, sensors or even any kind of active control over the behavior. The explosion occurs purely due to the intersection of material properties and geometry.

Its wood is light enough which people used it to make canoes. Fishermen have been said to use the milky, caustic sap from this tree to poison fish. The Caribs made arrow poison from its sap. The wood is used for furniture under the name "hura". In a time when most writing pens left wet ink on the page, the trees' unripe seed capsules were sawn in half to make decorative boxes (also called pounce pots) to hold the "sand" used to dry the ink, hence the name "sandbox tree". The sandbox tree is an example of a species which perfectly exploits elastic instability: when a system is slowly pushed into a stressed state, which leads it to release its energy catastrophically as soon as a certain threshold is crossed. This kind of explosive seed dispersal is incredibly risky; seeds are flung blindly, and there’s no guarantee that they’ll land in a favorable environment. But when it works, it works very well. And from an evolutionary perspective, the strategy only needs to succeed occasionally to be favored by natural selection. When successful, the child sandbox tree will have little to no competition with its siblings, and it also has a lesser chance of density-dependent mortality. Moreover, any predators which specialize in feeding near adult trees are also less likely to encounter the dispersed seeds.

Small genetic changes which alter tissue stiffness or drying rate can dramatically affect the seed’s dispersal distance. And over time, evolution fine-tunes these different parameters just as precisely as it does animal anatomy. The idea that a tree can hurl its seeds faster than a car on the highway feels almost absurd. Yet it’s a real, measurable and repeatable behavior you could observe for yourself. And in a forest that’s filled with countless small adaptations, the sandbox tree is one of nature’s loudest, fastest reminders which plants are anything but inert. This tree prefers wet soil and partial shade or partial to full sun. It is often cultivated for shade. Sandbox trees are tropical trees and prefer warmer, more humid environments. Extracts from this species have also been documented in herbal remedies. The seeds contain an oil which is toxic if ingested but can be made into biodiesel and soap; the starchy leftovers after extracting the oil from the seeds can be made into animal feed after cooking.