Cannons and Catapults - Nature's Projectiles
Catapults and cannons aren’t just military devices invented by humans; plants and fungi have been using them for millions of years to help them survive and prosper. This essay explores these tiny machines. Expect to be amazed!
While inspecting a mound of horse dung(1) for mushrooms, I felt the tiniest drop of wetness hit my lower lip. “Lip” and “dung” are not words that go comfortably in the same sentence, however, in this case the tiny drop strengthened my suspicion that the “fuzz” I saw were the fruiting bodies of the coprophilous(2) fungus Pilobolus.
Pilobolus, also called the shotgun fungus, is a genus of tiny fungi that might ordinarily pass unobserved in the human world, but for its ability to blast its spores(3) at record-breaking accelerations. In fact, Guinness World Records states that the Pilobolus sporangium – the spore package – accelerates to 12 mph in 2 millionths of a second or at about 20,000g, nearly 400 times the maximum acceleration survived by a human.
Many Pilobolus launching their sporangia in slow motion
The fungus’s elegant cannon is a miniscule, fluid-filled, balloon-like vesicle perched on a stalk with a spore-filled sporangium topping the vesicle off like a bowler hat without the brim. All told, the stalk and vesicle are only 2-10mm tall, about the size of a skinny rice grain. The fluid in the vesicle has a high sugar content which draws water into the vesicle through osmosis, increasing the pressure inside until the vesicle ruptures. The subsequent jet of fluid from the collapsing vesicle propels the spore package toward its target. Despite its diminutive size Pilobolus can send a sporangium up to 2 meters away.
Close-up video of a single vesicle propelling a sporangium
So, why does Pilobolus go to all this trouble to spread its spores? In order to answer this question, we need to know a bit about Pilobolus’s life cycle. First, the sporangium isn’t simply launched randomly without a goal. Pilobolus has a light-detecting mechanism that orients its cannon toward the sun, and an internal clock that fires the cannon around dawn. Together, these capabilities help Pilobolus aim its cannon at a low angle—the morning sun is low on the horizon—and therefore increase the distance the sporangium will fly to its target.(4)
What is this target, and why is it special? The ideal target is grass or other vegetation that an herbivore like a cow or horse might eat. Once consumed along with the grass,(4) spores pass unscathed through the herbivore’s digestive tract and are deposited, once again, in a mound of dung, completing the cycle. But if they start and end in the same place why not just cut out the middle part: the cannon? Without the launch step, the spores would repeat the Pilobolus life cycle on the same food source until it was depleted. In other words thanks to the cannons the pioneering sporangia are able to explore strange new worlds and boldly go where no spore has gone before!(6)
Plants aren’t fungi, however, they have independently evolved inventive ways to disperse their pollen, spores, or seeds.(7) This is called convergent evolution. A favorite of mine is the bracken fern, Pteridium aquilinum, a distinctive fern common in Maine where I live. Ferns release spores from sporangia clumped in structures called sori—think nested Russian dolls with sori being the largest, outer dolls, sporangia being the middle size dolls, and spores being the smallest, inner dolls. Unlike many ferns whose sori are prominently located in the middle of the ferns’ leaflets, bracken sori are nestled under the curled leaflet edges. Yet, despite their concealment, bracken ferns are anything but shy when it comes to spore dispersal, hurling their genetic material with microscopic catapults. The catapults are so small that about 150 can fit on the head of a pin, significantly more than the number of angels.
However, unlike the Pilobolus’s vesicles which rupture when their internal water pressure becomes high, Pteridium’s catapults activate when the internal water pressure becomes low. Essentially, cells in the bracken ferns’ sporangia—the catapults—contract as they dehydrate to the point that microscopic, internal bubbles form.(8) The formation of these bubbles is the release mechanism that causes the catapults to fire their payload of spores.
In the accompanying video the slow, initial movement of the sporangium, lasting tens of seconds, is sped-up to show it opening due to dehydration and contracture in the catapult mechanism. The rapid phase,(9) complete within less than 0.001 seconds and coincident with the formation of the microscopic bubbles, snaps back the sporangium and ejects the fern’s spores.
Bracken fern catapult
Although bracken fern’s life cycle is complicated, it doesn’t require an intermediary like Pilobolus’s herbivore to eat spore-laden vegetation, nor does it take advantage of a pollinator like a bee, so why does it eject its spores with a catapult? In short, bracken ferns need to overcome challenges associated with dispersing the diminutive spores—about 20-40 microns(10) in diameter—which will not travel far from the parent leaflet unless they are ejected with a high initial velocity, like that provided by a catapult. If the spores simply stay with the parent plant, the fern may simply regrow in the same location and potentially compete with the parent for resources.
Not Just Any Pollinator Will Do
What if you combine a catapult with a pollinator? That is exactly what the mountain laurel, Kalmia latifolia seems to do in order to improve its reproductive chances. The flower’s male, pollen-producing structures—the anthers— are on the tips of long, thin filaments, which are bent outwards under tension with their tips tightly held in pockets in the laurel’s petals. You can see this in the beginning of the video below.
The pollen needs to get from the male anthers to the female stigma located in the center of the flower.(11) The pocket releases the anther when a pollinator of the right size,(12) ideally near the female stigma, manipulates the filament, typically by brushing against it while searching for nectar. The right size is bumblebee-sized and shaped— if you’re too small you’re not as effective at releasing the anther, and if you’re butterfly-shaped your long, thin proboscis won’t release the anther either.
Mountain Laurel catapult
When the anther is freed from its pocket, the tense filament springs toward the flower’s center and its pollen is hurled onto the pollinator and/or the stigma.(13) The pollen can be thrown up to several inches away. Once dusted with pollen, the pollinator may carry it to the stigma of the same or different laurel flower. In fact the pollen-carrying bumblebee is better at transferring pollen to a stigma than is the catapult alone.
Keep an Eye Out for the Small Things
Before nature literally hit me in the face, I hadn’t thought much about these beautifully diverse solutions with which plants and fungi face the challenges of reproducing and propagating themselves. But nature can thrill you with the smallest things, like a dung-loving Pilobolus sporangium on your lip.
1 Scat from some animals, particularly raccoons and some carnivores, can carry health risks, so I strongly recommend against any casual inspection of scat you may find. In other words, don’t do what I did.
2 Coprophilous means“dung-loving”, which Pilobolus are.
3 Think of Pilobolus’s spores as its microscopic seeds. A tiny package of spores is what hit my lip.
4 Roughly, a 45 degree angle will maximize the distance a projectile like the sporangium will fly.
5 The sporangium is sticky and will adhere to the vegetation or any object like my unfortunate camera lens.
6 Apologies to Star Trek fans.
7 Even animals can use slingshot-like devices like the tongues of certain amphibians and reptiles. As an aside, porcupines cannot throw their quills, however, they can swat very quickly with their quill-loaded tails.
8 Technically this is called “cavitation” and occurs in low-pressure regions within a liquid.
9 The rapid phase in the video is greatly slowed down to demonstrate the release.
10 An average human hair is 50-80 microns in diameter (1 micron is about 0.00004 inches)
11 The pollen can also be transferred to stigmas in other flowers.
12 Or my thumb.
13 An arrow in the video first points to the anther that will be released and second to where flying pollen grains will be visible.