On July 10th, 2017 we learned about

Plants and fungi that spray, splatter and sling their seeds and spores

If the apple doesn’t fall too far from the tree, they both have a problem. The seeds in the apple may take root next to its parent where it will be forced to compete for nutrients and sunlight, possibly stunting its growth and wasting the investment the parent plant made in the seeds. Fortunately for apples, the seeds are packaged in yummy, sugar-filled fruit that animals eat, taking the seeds for a ride along the way. As those seeds are pooped or discarded elsewhere, the seeds have a chance to grow in new territory away from their parents. Not all plants make such attractive fruit though, and so many have had to find other ways to give their offspring a push to newer pastures. In some cases, that even means evolving mechanisms to squirt, eject or catapult seeds and spores to ensure a bit of distance between each generation.

Shooting spores

Starting small, many fungi have ways to launch their spores into the air when it’s time to reproduce. The Pilobolus mold, for instance, uses sap to build up pressure in a stalk called the sporangiophore. Once the pressure is too great to contain, the end of the sporangiophore bursts open, launching a payload of pinhead-sized spore capsules. Those tiny capsules are ejected at up to 55 miles per hour, sometimes traveling as far as six feet. For molds that grow less than half an inch high, that’s plenty of distance to ensure the spores end up on the grass they need to continue their life-cycle.

Slinging sori

On a larger scale, some plants throw their spores rather than fire them out of a fluid-powered cannon. The delicate ferns you find in shady forests have a two-stage life-cycle, and to get spores in a safe location to grow into gametophytes, the spores need to move away from the parent plant. To do this, clumps of spore pellets, called sori, grow on the underside of the fern’s leaves. Once the sori dry out, the a catapult mechanism flings them into the air where they can be carried on the wind, animal fur, or in local waterways.

Ferns don’t exactly look like catapults, but they can launch their spore in a process that takes less than a half-second to complete. A coiled group of cells called an annulus grows around spore capsules, bent in an arch to build a bit of mechanical tension. Once dried sufficiently, the annulus snaps forward to lob the spore capsule. To keep it from bending too far and flinging the spore back at the leaf, a tiny amount of water squeezes through pores in the annulus, blocking that forward movement and releasing the spore at an optimal trajectory. This tiny delivery structure can send spores flying at around 22 miles per hour once released.

Popping pods

Launching spores are one thing, but firing full-sized seeds into the air requires some heavier artillery. Various plants grow seed containers that dry out unevenly, squeezing the seeds from one side. For example, when gorse seed pods are sufficiently dried out, they fire seeds out at around 18 miles per hour. Gorse seeds usually only travel a few feet, but pinching seeds for propulsion is used by the Bauhinia tree to send seeds as far as 49 feet.

Self-firing fruit

The biggest payload to be propelled off a plant may be Ecballium elaterium, better known as the Squirting cucumber. Like the cucumber you put on your salad, this plant’s seeds grow in a protective, oblong fruit, although there’s a lot less flesh to actually eat. Instead the two-inch fruit fills mostly with fluid, with enough room for a 20 or so seeds to go flying away from the parent plant.

When filled with fluid, there’s enough pressure in a single cucumber to give it a bit of a hair trigger, ready for wind or a passing animal to kick things off. When “activated,” the cucumber will detach from the stalk it grows on, ejecting water and seeds into the air out of newly formed opening where the stem attached. Like a rocket booster, the cucumber shell will be pushed towards the ground while the seeds will fly as far as 20 feet away. It seems like this should make for the most exciting, kid-pleasing vegetable ever, but aside from being mostly water, Squirting cucumbers contain a lot of cucurbitacins, pest-deterring chemicals which are toxic if ingested. It seems that Squirting cucumbers are better to watch than to eat.

Source: An explosive start for plants: Plants get up to some ingenious tricks and aerial acrobatics to ensure their survival by Paul Simons, New Scientist

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