Thursday, April 13, 2017

Answer: You mean.. they explode?


It's true: some plants DO explode, including ones that look quiet and serene in botanical gardens

Waimea Arboretum

As I did this research, the biggest surprise to me was that there are a LOT of plants that explode.  Luckily, most of them are reasonably small and gentle.  But there are some that can be deadly.  

The comments this week were especially fantastic.  This week I'm going to lightly edit Regular Reader Mike's answer because it was great.  Nice job, Mike!  (I've added my comments in a different font so you can see them.)  

Remember that the Challenges this week are:  

1.  Are there, in fact, dangerous trees that can somehow eject sharp bits of themselves, potentially hurting a human being?  (If so... HOW would it do this?  So far as I know, few plants - venus fly traps aside - are capable of much movement.)   
2.  More generally, are there other plants that can hurl seeds?  You can imagine this might be a handy evolutionary mechanism to have--but again, how would that seed-hurling mechanism actually work?  Do they have little plant muscles??   

AlmadenMike writes: 

A few years ago, I let a small section of backyard grass go untended before a remodel that would replace it with a patio. When it went to seed, I experienced a spectacular ankle-high show of seed-spouting fireworks with every step. Merely brushing against my pants was enough to set off the blasts. I thought I’d start with this search to try to identify that grass and go from there:

     [grass projectile seeds]

But the fourth hit was an article in a gardening blog that described the entire phenomenon: Examples of plants that disperse seeds by shooting them.  

The article starts: 
“Some plants disperse their seeds forcefully by ejecting them. Sometimes the tension is so great, seeds may be ejected up to 200 feet away from the mother plant. This method of seed dispersal is called ballistichory, a label that hints at the projectile-like emergence of seeds from their pods or capsules. This type of seed dispersal occurs because the fibers in the dried fruit pull against each other to create tension, and when the tension is great enough, the fruit splits open and the walls of the fruit spring back, flinging the seeds out with force.”

It listed four plant families that had significant ballistichoric seed ejections: Pea family (Fabaceae), Euphorbia (Euphorbiaceae), Acanthus (Acanthaceae) and Mallow (Malvaceae). "In the pea family are “Orchid trees” (Bauhinia spp.), (which) bear large pods that can fling seeds nearly 50 feet."

I didn’t see any plant list for the Waimea Arboretum or Botanic Garden ... and no searches for those places and “dangerous tree” got any hits. But I thought your “dangerous tree” might have been a Bauhinia

Searching for:

    [bauhinia ballistichory] 

led to this informative article (second hit) about two more capable exploding seed capsule trees.  It's called, "Going Ballistic."  From the article:  
“One interesting example from the tropics is the genus Pentaclethra, a relative of Bauhinia. An experienced botanist reported that Pentaclethra was the only plant he knew whose pods opened so explosively and forcefully that they could break the sturdy oaken presses that botanists use to press and dry their plant specimens. For all its press-shattering strength, Pentaclethra throws its seeds only about 33 feet from the parent tree, which, ironically, is not as far as the gentle Bauhinia. Just for the record, the world champion ballistics title belongs to an African tree in the Legume family, Tetraberlinia moreliana, which throws its seeds almost 200 feet!”
The latter may have come from the 1997 paper: “Explosive Seed Dispersal of the Rainforest Tree Tetraberlinia moreliana (Leguminosae - Caesalpinioideae) in Gabon” by Xander M. Van Der Burgt, (Journal of Tropical Ecology - Vol. 13, No. 1 (Jan., 1997), pp. 145-151).  (You can find this paper by searching for Tetraberlinia moreliana seeds ] in Google Scholar--it's the first hit.  In this paper I read that: 
“The dehiscence [i.e., the rupture of the seed pod] is caused by tension that builds up between the two valves [sides of the seed pods] during drying. Mature pods explode with a crack during periods of sunshine.”

One tree he monitored produced an estimated 15,000-20,000 seeds, with 1.5-2% of those seeds projected over 50 meters away. The longest distance was 60 meters (nearly 197 feet). (With discussions of launch angle and wind-speed dispersions, this paper read almost like an analysis of a golfer testing drivers.) The author also speculates that such explosive dispersal is handy for "...high forest trees, from which large seeds lacking dispersal
devices are thrown many metres away."  

[ Dan:  I couldn't help but include a diagram fro Van Burgt's paper. It neatly summarizes why plants would develop such explosive seed dispersal mechanisms...] 

Van Der Burgt’s paper also mentions that the exit velocity for the test tree was 37.1 meters/second … which was less than nature’s fastest.  That high speed mark belongs to Hura creptitans, aka Sandbox tree or Dynamite tree, which launched its seeds at 70m/s (157 miles/hour). 

This tree, Hura creptitans, may well be your “dangerous tree.” (FWIW, The sap is also poisonous.)

Googling simply: 

     [“Sandbox tree” danger] 

turned up a host of articles and videos. Among the best is this article from a gardening blog, Sandbox Tree Facts.   
“Considered one of the most dangerous plants in the world, the sandbox tree isn’t suitable for home landscapes, or any landscape actually. … Sandbox tree fruit looks like little pumpkins, but once they dry into seed capsules, they become ticking time bombs. When fully mature, they explode with a loud bang and fling their hard, flattened seeds at speeds of up to 150 miles per hour and distances of over 60 feet. The shrapnel can seriously injure any person or animal in its path.”
I found two videos (although none show the H. crepitans seeds exploding on their own without a tap to get them going).  

This is a nice video of Hura crepitans (Sandbox tree), showing its formidable spines:   (

and a Smithsonian Channel video of several exploding-seed plants including the remarkable exploding cucumber, worth watching on its own (as it's an example of explosive seed dispersal that uses fluid pressure, not dehiscense to eject its seeds). Unfortunately, there's no Sandbox tree in this video.  It's still worth watching: 

The “Going Ballistic” article also succinctly described the ballistichory mechanism: “All of these plants rely on the drying of the fruit wall to generate tension. The wall of a ballistic fruit has at least two layers in which the woody fibers are oriented in different directions, usually at right angles. As the woody fibers dry, the layers pull against each other. When at last the fruit fractures along a predetermined seam or weak spot, the walls curl, throwing the seeds a considerable distance. … It is the drying of the fruit that supplies the tension.”

So my answers to your questions would be:

1) Several trees do eject their seeds with an explosive speed of 30-70 meters per second that can send them nearly 200 feet away. One could imagine that it might be hazardous to stand very close to one of the seed pods at the time of its ballistichoric dehiscense. Especially those of the Sandbox/Dynamite Tree, Hura creptitans.   
2) Yes, quite a few plants eject their seeds with explosive force. The mechanism (described in more detail above) involves increasing tension within a seed pod as it dries, and explosive ejection of the seeds when the pod eventually ruptures.

Finally, after finding all this, I did not look further to identify the grass that I’d had in my backyard.  


AlmadenMike has it right. The magical words to know here are dehiscense and ballistichoric. Once you start running into technical terms in the middle of your search, its time to start paying attention--especially once you look them up and learn that these concepts are central to what you seek.  

Similar topic search:  Mike started his search by looking for explosive dispersal of seeds, and then learned the word "ballistichory," which then led him to the Bauhinia plant, which led him to the Sandbox Tree, Hura creptitans.  

I followed a similar path, but started with the query: 

     [ explosive seeds Hawaii ] 

which got me to Hura creptitans fairly quickly.  

Once I knew the Latin name, a YouTube search found me a video of the fruit exploding (after being hit with a piece of wood). 

Look twenty seconds into this video for the impressive explosion, which would account for the sounds I heard in the arboretum.  

And, just to verify that this was the tree that I'd walked near, I did this search: 

     [ Hura creptitans Waimea ] 

I will admit that I ended up having to go to Google Images as a way to ensure that there IS a Hura creptitans at the Waimea arboretum.  (Like AlmadenMike, I wasn't able to find a plant list for the arboretum.  I'm sure they have one, but not online.)  

In any case, my Google Image search led me to a Pictaram image collection of Hura creptitans (who knew such a thing would exist?), and in that collection, there are a couple images of the Sandbox tree with the hashtags #arboretum and #WaimeaValley.  That convinces me that there really is at least one Sandbox tree in the Waimea Arboretum.  

P/C Ronald Casallas

AlmadenMike started his search by looking for a rapidly ejecting grass seeds.  Luckily, I remembered seeing such a thing in my own yard, and was able to find out what causes it.  (And, double lucky, AlmadenMike also live in the Silicon Valley.)  

I tracked my "exploding grass seeds" to the Yellow Woodsorrel (Oxalis stricta).  Wikipedia tells us that: "The mature seed capsules open explosively when disturbed and can disperse seeds up to 4 meters (about 13 feet) away." There's a lovely video of these seeds ejecting when touched by a human... 

I will tell you from personal experience that when the conditions are right, touching just one seed pod can cause it to fling seeds every which way, causing a kind of mass seed ejection chain reaction, when thousands of seeds from neighboring seed pods are sent flying after being hit by an errant seed.  

Search Lessons 

1.  Search for a parallel that you know about.  I really liked AlmadenMike's strategy of searching for something he already knew about (the exploding grass seeds), and then learning what terms he should use in later searches. From an article about exploding grass seeds he picked up the word "ballistichory," and was able to use that term to focus in on ballistichoric tree pods and seeds as well.  
2. Learn to notice special technical terms.  Before starting this search, I'd never heard of dehscience, OR ballistchory.  While they probably won't be part of my everyday speech, they're exactly the right words to use in this bit of research.  By using specialty terms, your chance of finding exactly the right concept is improved.  (But be sure to get the right definitions... you don't want to use the wrong technical term!)  
3.  Sometimes Images are where you'll find the connection.  Honestly, I was expecting the Arboretum to have a plant list, or at least an online page highlighting some of their more spectacular plants.  But I wasn't able to find that.  However, by using Image search I was able to discover at least one person who knew that there is a Sandbox Tree at the Waimea Arboretum.  



As I was writing this blog post, I got an email from Regular Reader Ramón who took the initiative to write to the Director of the Waimea Arboretum to ask about explosive plants.  

He wrote: 
"[I'm searching in the Waimea Arboretum for] .. a tree that has ballistic seed dispersal and we trying to find more information about it. [Dr. Russell] will give an answer next week. But, the intention is to find the information before searching online. 
Do you have more information about what tree is this in the valley? Maybe is not a tree but a plant."

And the Director wrote back! 

"What Dr. Russell mentioned could be these trees: The rubber tree Hevea brasilliensis or the Sandbox tree – Hura crepitans.  There may be others but these are what came to my mind.  Thanks, Josie."

This makes it pretty clear that it could be one of these two plants.  Both have explosive seed dispersal mechanisms.  

This leads to a 4th Search Lesson: 
4.  Ask questions of people who know.  Ramón's method of simply writing to someone who knows can be a real life-saver when dealing with complex research questions.  Google is great, but so are people--especially experts who are right there and can check the ground truth for you.    

After all this, my bet is on the Sandbox tree as the source of the overhead crack, whiz, and zing.  


I forgot to mention that I also remember standing near a large, very dangerous-looking spiny tree when the seed pod flew overhead.  It's possible that I was looking at the Sandbox tree while the rubber plant seed exploded behind me, but it seems unlikely.  

Of course, maybe I'll have to return to the North Shore of Oahu to find out... 

Search on! 

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