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  • Chances are, if you've turned on a lightbulb or a computer,

  • you've harnessed the power of some very ancient, and very strangetrees.

  • The vast majority of the electricity produced in human history has come from coal.

  • And like all fossil fuels, coal is made from the remains of extinct organisms that have

  • been exposed to millions of years of heat and pressure.

  • But in the case of coal, these organisms consisted largely of some downright bizarre plants that

  • once covered the Earth, from Colorado to China.

  • Meet the Lepidodendrales, also known as scale trees.

  • Named for their scaly looking bark, scale trees were tall plants topped with crowns

  • of branches that sported long, narrow leaves.

  • They thrived during the Carboniferous Period, from 299 million to 359 million years ago.

  • And they're partly why we call the Carboniferous ... the Carboniferous!

  • This period is named after the carbon-rich layers of sediment that formed from countless

  • millions of scale trees, ferns, and other early plants that died, piled up, and fossilized.

  • You know these layers as coal.

  • During the Carboniferous, much of the Earth was covered in lush forests and swamps, and

  • this is where the scale trees flourished, sometimes growing as tall as a 15-story building.

  • Their remains have been found throughout the Northern Hemisphere, but they're especially

  • common in North America, where fossilized roots, trunks, and leaves have been found

  • from almost every stage of the plant's life cycle.

  • And because we know so much about the anatomy of these trees, we know that they were very

  • different from the trees that you and I know today.

  • For one thing, they had no need for seeds!

  • This is because scale trees were a type of plant called lycopods, which originated way

  • back in the Silurian Period -- before seeds were even a thing!

  • Some lycopods are still around today, like the low-lying, water-loving club mosses and

  • quillworts.

  • But instead of having seeds, these plants reproduce by releasing tiny, single-celled

  • spores.

  • Scale trees produced cone-like pods that dropped their spores into the water below, to be whisked

  • away to the nearest patch of soil.

  • But lycopods were also the earliest vascular plants, which means they were the first plants

  • to contain xylem, a special tissue that moves water and nutrients from the roots to

  • the leaves.

  • This tissue is what allows modern trees to grow tall and straight, because it contains

  • lignin, a rigid organic polymer that gives the plant structural support.

  • But scale trees only produced a small amount of lignin.

  • And they also had much shallower anchoring roots than today's trees.

  • So how did they manage to grow so large without simply falling over?

  • Their secret was in their bark.

  • The outer layer of the scale tree, the periderm, was extremely thick and actually made up most

  • of the trunk.

  • This allowed scale trees to reach great heights -- some growing as large as 50 meters high

  • and more than 2 metres across!

  • But they were still a lot more delicate than the hardwoods you find in today's forests.

  • A single storm could probably knock down a whole swath of them.

  • And they didn't always look quite so majestic.

  • As juveniles, scale trees looked more like giant spikes sprouting out of the ground.

  • At this stage, they had no branches to speak of, so their leaves just grew right out of

  • the bark, in a spiral pattern.

  • As the trees matured and shed their leaves, diamond-shaped marks were left in the bark,

  • called leaf scars, giving the scale trees their name.

  • So, scale trees were weird by the standards of your modern oak or pine.

  • But when you're talking about evolutionary history, you can't really argue with results.

  • Scale trees were built to grow fast and die young, allowing them to spread across North

  • America, Europe and Asia, leaving huge swaths of organic matter in their wake.

  • At their peak, they made up nearly half the biomass in Europe and North America, and provided

  • food and shelter for the giant insects and early reptiles that called these forests home.

  • But the scale trees' success may also have led to their downfall.

  • Their vast forests pumped out huge amounts of oxygen, which was great for us animals.

  • But at the same time, the scale trees were also sucking up tons of CO2 from the atmosphere.

  • And back in the Carboniferous, many of the decomposing microbes that could break down

  • the hardy lignin -- and release CO2 as a result -- had yet to evolve.

  • So, over time, scads of scale trees collapsed and piled up, without decomposing, keeping

  • much of the CO2 that they had taken up, locked away in their tissues.

  • In addition to really putting the carbon in the Carboniferous, this storage of CO2 also

  • created a sort of reverse-greenhouse effect.

  • Falling CO2 levels, combined with a sudden burst in volcanic activity, plunged the Earth

  • into an early ice age.

  • The resulting extinction became known as the Carboniferous Rainforest Collapse, and the

  • scale trees were among its casualties.

  • A few species did manage to hang on, in places like Asia and South America, into the Permian

  • Period, but by 272 million years ago, the last of the scale trees were gone.

  • Nonetheless, these bizarre plants left behind a big legacy.

  • After living and dying in prolific numbers for millions of years, scale trees and other

  • early plants formed a thick layer of carbon-rich peat across much of our swampy world.

  • The peat was eventually buried, compacted, and exposed to extreme heat, turning into

  • coal.

  • And eventually, some hairless ape dug it up and burned it, releasing the energy that the

  • scale trees absorbed from the Carboniferous sun over 300 million years ago.

  • Think about that the next time you turn on a light, or charge your phone, orwatch

  • a video about natural history.

  • As plants go, scale trees were powerful.

  • What do you want to know about the story of life on Earth?

  • Let us know in the comments.

  • And don't forget to go to youtube.com/eons and subscribe!

  • Now do yourself a favor and check out some of our sister channels from PBS Digital Studios.

  • Your brain will thank you!

Chances are, if you've turned on a lightbulb or a computer,

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B1 US scale co2 coal bark north america peat

History's Most Powerful Plants

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    Liang Chen posted on 2019/04/09
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