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In our school textbook, it is written that sometime forest fires can occur due to rubbing of two horsetails. In the image that I searched online, the plant itself looks thin and rod like with leaflike structures arising from nodes. But many other plants have similar structure. So it might not be its morphology.

What special thing about horsetail starts the fire? Why don't other plants show same thing?

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I didn't believe this was a real thing, and I'm still very skeptical, but it is something that's claimed to happen!

Their stems & leaves have a siliceous epidermis, so if rubbing b/n 2 horsetails occurs, a forest fire may be produced

--The Bottled Ocean of Biology, by Nisarg Desai, 2017

Why don't other plants show same thing? Another plant, bamboo, is supposed to do the same thing:

How these fires originate is uncertain ... it has been suggested that a probable cause is the rubbing together of the silica-coated bamboo stems as they sway in the breeze.

--Burma, by R. Talbot Kelly (2004)

The same claim about bamboo is made in an Indian Government document on forest fire management.

What special thing about horsetail starts the fire? Apparently the claim is that it's the silica in the stems of the plant that is the important factor, and that horsetail and bamboo are both unusual in that regard. This does seem to be true:

... in general, ferns, gymnosperms and angiosperms accumulated less Si in their shoots than non-vascular plant species and horsetails.

--Phylogenetic Variation in the Silicon Composition of Plants

Bamboo isn't mentioned in that article, but the presence of silicon in bamboo is supported elsewhere:

One of the advantages of bamboo over wood is that bamboo contains some silicon in both inner surface (pith-ring) and outer surface (rind) of the bamboo culm.

--Calcium phosphate formation induced on silica in bamboo

As I say I remain very skeptical. There's no evidence shown that this has actually happened. Is it telling that all these claims come from the same general region (India and Burma)? My guess is that this is an old wives' tale that's been written down and uncritically accepted, but if someone has actually seen this happen I'd be happy to be proven wrong, because the idea of plants sparking out flames in the wind is a pretty cool one.

Edit to add a link claiming that bamboo-on-bamboo generates sparks due to the silicates:

The method consists of striking sparks out of the culm of Schizostachyum bamboo with flint, broken pottery, china or even another piece of bamboo. The sparks that occur by striking the Schizostachyum bamboo are presumably generated by the high silica content in this genus of bamboo.

--Bamboo Strike-A-Light, by Tom Lourens, Ash Kivilaakso and Ed Read (My emphasis)

Edit to add that the overall concept of plants starting their own fires has been seriously looked at; the author is skeptical:

Individual plant traits (such as leaf moisture content, retention of dead branches and foliage, oil rich foliage) are known to affect the flammability of plants but there is no evidence these characters evolved specifically to self-immolate, although some of these traits may have been secondarily modified to increase the propensity to burn. ... It is more parsimonious to conclude plants have evolved mechanisms to tolerate, but not promote, landscape fire.

--Have plants evolved to self-immolate?

Notably, in his list of factors potentially allowing self-immolation, he does not list silica, and horsetail isn't mentioned at all.

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  • $\begingroup$ What would the underlying chemistry/physics of this be – can silica generate sparks through friction? $\endgroup$ – Alan Boyd Aug 30 '17 at 19:03
  • $\begingroup$ Apparently it does! See edit $\endgroup$ – iayork Aug 30 '17 at 19:19
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It's a cool idea and it's visibly a myth. No, Apparently it's never been seriously attempted or proven.

No wood friction ignition techniques that I know of use the sides of stems to obtain the 233 degrees combustion temperature of paper, they use the cross section and atm pressure. Equisetum couldn't achieve that because it's not heavy enough to get that kind of pressure on the stems, and it's not tinder plant. It sounds like they were having 300 kph winds!

There is an Equisetum species known as the scouring rush:

The rough stems have been used to scour or clean pots, and used as sandpaper.

Boiled and dried Equisetum hyemale is used as traditional polishing material, similar to a fine grit sandpaper, in Japan.

The stems are used to make fires: The silicated stems were used by Native Americans (and still used by some people today) to start hand-drilled fires.

It's probably the basis of a myth that they can start fires. Many horsetails are marsh plants with very deep roots and water storing rhyzomes. They are well studied for growth after forest fires, but there are no clear references to friction of the plants causing spontaneous combustion of the plant, a dubious theory.

... Ignition temp of various woods 190°-260°

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Could be that horsetails, as primitive plants, in certain soils, accumulate and concentrate atoms (hyperaccumulators) and just like certain minerals that spark or certain compounds that flare when dry, are source of ignition ie Flash Powder

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    $\begingroup$ this would be better suited as a comment $\endgroup$ – The Last Word Jan 23 '18 at 16:53
  • $\begingroup$ Is this purely speculation, or is there any evidence for this? $\endgroup$ – iayork Jan 23 '18 at 17:09
  • $\begingroup$ ...accumulate atoms? $\endgroup$ – LinuxBlanket Jan 23 '18 at 21:26
  • $\begingroup$ hyperaccumulation genes (HA genes) are found in over 450 plant species, including Arabidopsis and Brassicaceae. $\endgroup$ – com.prehensible Jan 24 '18 at 3:52

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