If you've ever stopped after a hike to pull burrs off your socks or tossed aside a pit after eating a piece of fruit, you've actually done plants a huge favour by helping spread their seeds. 
Even though you might not spend too much time thinking about seeds, those hardy little plant embryos are kind of amazing! They've evolved to be an almost perfect way to get the next generation going, because they pack in everything a baby plant needs to get started. 
A seed usually has a little package of nutrients called the endosperm, which feeds the baby plant while it's establishing itself.  

All of this is sealed inside a durable shell that lets the seed stay viable for a long time until conditions are just right to germinate or sprout.  Some seeds also have adaptations to help them find a good place to grow.  Ideally, somewhere far away from their parent plant so they aren't directly competing for light or nutrients.  That's called dispersal, and that's where you came in when those burrs hitched a ride on your clothes.  A seed grabbing on to a passing mammal is one thing, but some plants try to give their seeds the best possible start in life in ways that are a little more extreme.  

So here are five plants that have some of the weirdest seeds and dispersal strategies on the planet.  

Birds and mammals are some of the most common seed dispersers, but other animals can also get involved sometimes.  In South America, the tucum plant has to rely on a particularly unusual partner: a fruit-eating fish.  The tree grows in the largest freshwater wetland in the world, a vast eco-system in Brazil called the (?~1:40).  Every year, seasonal rainstorms flood the region and that's when the tucum palm drops its fruit, drawing in huge relative of the piranha called the pacu, aka that super creepy fish with human-looking teeth.  Pacu have a taste for the palm fruit and local fisherman even use it as bait, but for a long time, it wasn't clear whether all that fruit eating was actually an important way of spreading seeds.

In the 2000s, a team of ecologists finally traveled to the area to find out.  Over several seasons, they checked the guts of 70 fish and found that nearly 3/4 had at least seed in their digestive tract.  One fish had more than 140 and many of the seeds were intact, meaning they'd still be able to sprout after the fish pooped them out and the floodwaters eventually receded.  The scientists also poked around in the feces of other animals to see if they might be spreading seeds too, but for the most part, either the palm fruit wasn't a very popular meal or the seeds ended up being destroyed.  So pacu seemed to be the main way tucum palm seeds get around.  

Pacu are being overfished, though, which is a problem.  The biggest fish eat the most seeds and are therefore the best dispersers, but they're also the ones most sought after by fishermen.  So if something doesn't change, that could spell trouble for the tucum palms.  

Over in the sunny Mediterranean, there's a plant that disperses its seeds by squirting them out in a stream of goo.  It's called, appropriately, the squirting cucumber.  When a squirting cucumber's fruits are ripe, each one pops off the end of its stem and shoots a stream of slime-coated seeds from its base, spewing the seeds over six meters away from the parent plant.  It's an example of an unusual adaptation called rapid plant movement, he same basic mechanism that lets Venus Fly Traps snatch their prey.

Plants don't have muscles like animals do, but they do have rigid cell walls, which means they can sustain high water pressure or turgor pressure within their cells and that pressure is what drives their movement.  The squirting cucumber's explosion is a one-time event powered by the release of pressure that buils up inside the fruit as it ripens and fills with liquid.  Once the fruit is fully ripe, even the slightest nudge will break it from the stem, unleashing its contents in a gooey high-powered stream.  The squirting cucumber is actually related to the cucumbers we eat and it's popular amongst gardeners as a novelty, but don't try to eat one.  

Those cukes are poisonous.  The squirting cucumber's abilities are impressive, but one South American tree has seeds so explosive that its common name includes the dynamite tree and monkey's pistol.  As its pumpkin-shaped seed cases dry out, the individual compartments holding the seeds start to shrivel.  Eventually, they shrink so much that they separate from the seed case's central axis and the whole thing bursts with a loud gunshot-like bang.  The seeds are each about an inch across and this mini-explosion propels them as far as 45 meters at speeds of 70 meters per second.  That's enough to injure anyone unlucky enough to be in their path.  Most trees don't use this method because  thanks to air resistance, it takes a lot of force to get the seeds very far from the parent tree, but the dynamite tree is unusually well set up to propel seeds.

In the 1970s, scientists used high-speed photography to measure the velocity and angle of the seeds as they burst from the drying capsules in a lab.  They found that the shape and angle of the seed capsules gives the seeds maximum range.  The flat disc-like seeds shoot out of the capsules edge on, minimizing drag, and at close to the optimal angle that would let them fly as far as possible.  Despite their tendency to hurl dangerous projectiles, dynamite trees are grown for shade in residential neighborhoods in some tropical parts of the world so um, maybe try not to stand too close?

The smallest seeds in the world are made by orchids.  Each one is about the size of a speck of dust and the ultimate record holder for the tiniest seed is a (?~5:34) species of orchid with seeds just 5/100 of a millimeter across.  One reason they're so incredibly small is because orchids are all about quantity.  A single seed capsule from one tropical species contains almost four million seeds.  Orchid seeds also need to be tiny and light because they rely on wind to get where they need to go and lighter seeds have an easier time riding air currents.  

The seed coat is only one cell thick and it traps air inside to form a tiny balloon around the embryo, which lets the seed travel even farther through the air.  Individual seeds from one species have traveled over a thousand kilometers and thanks to that mini balloon, the seeds will also float if they land in water.  Now, if you've ever played with dandelions, you know that adaptations for wind dispersal are pretty common amongst plants, but orchids have taken this tiny but numerous strategy to the max, because they have very specific needs.  

To germinate, orchid seeds have to find a mycorrhizal fungus, a fungus specially adapted to partner with a plant to help nourish them.  Orchids are also epiphytes, or plants that grow on other plants, in this case, usually a tree.  They get their nutrients from the air and rain rather than from the soil.  The chances that any individual seed will land on a welcoming host plant, meet up with the right fungus, and be able to sprout are very small, so orchids produce a lot of these super-tiny seeds to increase the odds of at least a few seeds making it.  Normally, making a ton of seeds is very costly for plants, but orchid seeds don't get any prepackaged food, which both helps keep them tiny and helps their parent plant make literally millions of them. 

On the opposite side of the seed size extreme is the coco de mer, or coconut of the sea, with the largest and heaviest seeds in the plant kingdom.  the coco de mer is a species of palm found only in the Seychelles, an archipelago off the east coast of Africa.  It's related to the coconut tree you're probably more familiar with, but its fruits are much bigger, up to a half a meter long and weighing as much as 40kgs.  Definitely not something you want falling on your head.  Each fruit usually contains a single huge seed and it takes 6-10 years to ripen.  Then, once they drop from the tree, the seeds take another two years to germinate.  The whole process requires a ton of energy, so the trees dedicate a huge amount of resources towards producing their massive seeds.  The trees leaves only contain about a third as much nitrogen and phosphorus as other plants in the Seychelles, which lets them reroute more of those nutrients toward seed production.  

Their huge leaves also act as rain funnels, providing a steady source of water and flushing nutrient-rich organic material to the base of the plant where it acts like a fertilizer to further fuel the production of the huge seeds.  But how do they get these gigantic seeds in the first place?

Researchers think the dense, shady conditions and a lack of animal seed dispersers in the Seychelles' forests helped drive their evolution.  On mainland Africa, elephants spread the seeds of the coco de mer's relatives, but there aren't any elephants in the Seychelles, so there's no pressure to stay a certain edible size.  The seeds just fall at the base of the parent plant and have to establish themselves there.  Competition for light is intense in the humid, dense, tropical forests, so giving young plants a head start with extra nutrients stored in their huge seeds means they can keep going longer while they try to grow tall enough to access the sunlight, but peoples' fascination with the bizarre seeds has led to overharvesting for souveniers and curiosities.  There are only about 9,000 coco de mer trees left today and the species in endangered.  

So the next time you're trying to revive your wilting vegetable garden or cursing the new crop of weeds sprouting in your yard, just be thankful that at least you aren't being sprayed with goo or hit by high speed tiny discs or dealing with 40 kg fruits that take ten years to ripen.

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