Category: Ecology

Bryophytes : Seeking The Root(less) Origins of Mud

Bryophytes : Seeking The Root(less) Origins of Mud

Reading Time: 9 minutes

For 90% of its existence, planet Earth (mostly) lacked the one thing that cradled civilizations and made millions for detergent companies : mud. Then, sometime around 458 million years ago, it suddenly began seeing a lot more of Mr Mud. Why the sudden rise? How the sudden rise? What in the sodden rice is mud, anyway? And where exactly do those rootless plants called  bryophytes fit in? The Nerd Druid investigates the root(less) cause behind this muddy mystery.


India and farmers

It is March, spring in the Northern Hemisphere. In East India, along the hot dusty flyovers of Kolkata, crimson Rudropolash flowers bloom, giving the speedway a red bannister. Thousands of kilometers away, tens of thousands of impoverished farmers march 180 km barefoot from Nashik to Mumbai to make the government hear their plight. Their red headdress contrast and clash with the saffron of the Indian summer like a tide of Rudropolash, reminding ideologies and ideologues that governments are, ultimately, of, for, and by the people. It is time for the Indian spring [Note:IndianSpring].

Image of impoverished Indian farmers marching 180 km from Nashik to Mumbai to voice their grievances. Their red turbans, headdresses, and flags give the illusion of red Rudropolash flower in full bloom.
Indian Spring

This is not a political blog. However, science and politics sometimes seem rather (unhappily) closely entwined. In this particular article, we shall have no opportunity to explore said entwinement.

A very brief history of farming

Instead, we come to the basic reason why farmers are. Given arable land, farmers grow food. Without farming and agriculture, civilisation as we know it (and as we clearly are very not aware of) would not have existed. Primitive hunter-gatherers, perhaps exhausted with having to chase after or pluck food all day, decided, about ten thousand years ago, that enough was enough. Here be river. Here be fertile land. Here be seed. Where be hammock?

And that is how modern civilisation arose. Through extremely sturdy king-sized hammocks.

Of course, just like you can’t make fire without fuel, or babies without incredibly strong storks, you need a proper substrate for good crops to grow. Luckily, riverbeds and its frequently flooded adjoining areas provided such substrates. The Indo-Gangetic plains in India, Pakistan and Bangladesh, and the North China plains are one of the largest such alluvial plains, as also one of the most fertile in the world.

Map of the Indo-Gangetic Plains. A large percentage of the world's alluvium deposits are found here.
The Indo-Gangetic Plains

Alluvium

Alluvium (from the Latin alluvius, from alluere, “to wash against”) is loose soil or sediments that have been eroded and deposited by water on land, and which has not yet compacted into rock. Alluvium comprises fine particles of silt and clay, as also larger particles of sand and gravel. Agriculturally, alluvium is gold. The high mineral content, optimal water-retention capacity, and optimal breathing room for plant roots make it ideal for a successful harvest. Unsurprising then, that a third of the world’s population now lives in the two largest alluvial plains I spoke of a minute ago.

Geologically, alluvium is quite young, with most deposits in the world having formed in the Quaternary Period of the Cenozoic Era.

Wait. The what period of the what era?

Earth’s geological eras

Well, Earth’s geological timeline is divided into several time-sections, for lack of a better word. The largest sections are called Eons. There are four of them, though for all intents and purposes, especially for the intent and purpose of this article, we’ll talk about two : the Precambrian [Note:Precambrian], and the Phanerozoic. The boundary between the two, situated 542 millions years in the past, is approximately when complex life (such as trilobites or corals) began appearing on Earth. The name phanerozoic, from the Ancient Greek words φανερός (phanerós) meaning visible and ζωή (zōḗ) meaning life, is quite apt.

A cartoon schematic of Earth’s geological timeline, designed by Ray Troll. It shows the geological ages stacked up like a pyramid.
Earth’s geological timeline, in cartoon version. Designed by Ray Troll.

Actually, I lied. I’m not going to talk about the Precambrian, not in this article anyway.

Back to the classification systems. Eons are divided into Eras; the Phanerozoic Eon has three Eras — the Paleozoic (paleo = old, ancient), the Mesozoic (meso = middle), and the Cenozoic (ceno = recent, new). We live in the Cenozoic Era, in the Quaternary Period, in the Holocene Epoch. Epochs can be further subdivided into Ages, though the Holocene hasn’t been long enough to admit such a division.

Eons > Eras > Periods > Epochs > Ages

There. That should clear things up.

The Paleozoic Era

We are, today, more interested in the Paleozoic Era. Beginning 542 mya (mya = million years ago) with the appearance of complex life, the Paleozoic Era ends 251 mya and gives way to the Triassic Period of the Mesozoic Era.

Hold on. Triassic? Does that mean the Mesozoic Era…?

Yep. The Mesozoic Era has three Periods : Triassic (251 – 200 mya), Jurassic (200 – 145.5 mya), and the Cretaceous (145.5 – 65.5 mya). The Mesozoic is the “Age of the Dinosaurs”!

Which (a) makes no sense, it should have been the “Era of the Dinosaurs” and (b) makes no sense, because we ain’t talking about dinos today. Curse you Michael Crichton [Note:Crichton].

Rewind. Back to the Paleozoic.

Schematic of Earth’s geological timeline, in all its detailed glory. This is the International Chronostratigraphic chart, updated February 2017.
Earth’s geological timeline, in all its detailed glory/

Incorporating 291 million years of Earth history, the Paleozoic Era is divided into six Periods : Cambrian, Ordovician, Silurian, Devonian, Carboniferous, and Permian. The Cambrian saw the greatest number of species evolve in a single Period, in an evolutionary event called the Cambrian Explosion. The animals, or rather, the fauna of this Period was dominated by the hard-shelled trilobites. Fish, coral and cephalopods (octopuses and squids, among others) evolved in the Ordovician, though fauna were dominated by trilobites, snails and shellfish. The Cambrian Explosion was balanced out by the Ordovician-Silurian extinction event, where almost half of all marine life disappeared. Things warmed up in the Silurian Period, which saw an explosion in the evolution of fish (both jawed and jawless), as well as the rise of vascular plants on land.

Vascular plants

Most plants and trees that we see today have a highly evolved, though simply explained, mechanism of life. Roots burrow deep into the soil, providing support and extracting water and minerals. These raw materials are then transported throughout the plant body via the rigid xylem tissues. Leaves “cook” these raw materials using sunlight to photosynthesize “plant food”, which is then transported via the less rigid phloem tissues to other parts of the plant body. Plants that have the xylem-phloem (XP) transport structure are vascular plants. The earliest such plants, Cooksonia being a wonderful representative, were the earliest precursors to such XP plants. Nowadays, ferns, conifers, and angiosperms (flowering plants) are all vascular, and all have XP.

Photographs showing xylem elements in the shoot of a fig tree (Ficus alba): crushed in hydrochloric acid, between slides and cover slips
Photographs showing xylem elements in the shoot of a fig tree (Ficus alba): crushed in hydrochloric acid, between slides and cover slips

Which shouldn’t be surprising. All those millions of years gives one lots of experience [Note:XP].

Roots, of course, do more than support and feed the plant. In rocky terrain, they sneak into crevices and break the rock apart into stones and pebbles, enabling wind and water and sun to break them further into finer particles of sand, silt, and clay. Thus, soil! In plain land, or mountain slopes, they hold onto the topsoil, preventing rainwater and wind from blowing it away. Roots are thus both friend and foe to Sir Erosion, and are quite critical for Sirs Environment and Ecology.

Classifying soil and rocks

Since we are on a classification spree today, would we like to try and classify rocks, pebbles, and soil based on how large or tiny the particles are? Bet we would. The Wentworth scale will help immensely.

Image of the Wentworth scale of classifying soil particles
The Wentworth scale of classifying soil particles

Anything above 256 mm (diameter) are boulders. 64 – 256 mm are cobbles, 2 – 64 mm are gravel (4 – 64 mm are pebbles), 62.5 µm – 2 mm is sand, 3.9 – 62.5 µm is silt, and 0.98 – 3.9 µm is clay [Note:Wentworth]. Silt and clay together make up what is usually called mud.

That takes care of one question. Two more to go.

The rise of mud is a bit of a chicken-and-egg problem. Rooted plants break apart rocks which are then eroded into silt and clay; thus, mud. Therefore, rooted plants must have come first, right? However, rooted plants also need mud for the roots to carry out their primary functions. You can’t leach water and minerals from hard rock, can you, Mr Joe Root [Note:JoeRoot]?

Houston, we seem to have a problem.

Bryophytes

Which is where bryophytes come in.

The term bryophyte comes from Greek βρύον (bryon), meaning tree-moss or oyster-green and φυτόν (phyton), meaning plant. Bryophytes are land plants and include various species of liverworts, hornworts, and mosses. Bryophytes arose in the Ordovician Period, sometime around 470 – 460 mya. Around 458 mya, they began to proliferate.

Image of a bryophyte. This is Marchantia, an example of a liverwort.
Bryophyte. This is Marchantia, an example of a liverwort.

Mud

But was there mud at that time? Yes, mud was present at that time, mostly in ancient river deposits. This mud originated from the action of non-biotic erosional agents (wind, rain, water, sun) as well as biotic ones (microbes and fungi). However, the percentage of mud (silt+clay, remember?) with respect to the coarser and larger sand and gravel was woefully small.

How small?

William McMahon and Neil Davies, both geologists from the University of Cambridge, decided to find out. They trawled through nearly 1200 published papers for data on mud rock in 704 ancient river deposits. As if that wasn’t enough, they themselves collected new data from 125 ancient river outcrops. Then they calculated, based on relative thickness of mud layers and sand+gravel layers, the percentage of mud (PoM), 458 mya. Their median result?

1%

Woeful doesn’t even begin to describe it.

However, McMahon and Davies weren’t done yet. They went ahead and calculated the median PoM of times after 458 mya. And they found that, over the next 100 million years or so, the PoM kept on increasing until, at about the end of the Devonian Period (359 mya), the PoM had reached 26%!

This was, of course, good news for rooted vasculars, which arrived in mid-Siluria around 430 mya. Given mud to grow on, rooted vasculars would be further able to make more mud. And the chain was established.

Except, what happened between 458 – 430 mya? How did the PoM increase from 1% to about 10% in the intervening 28 millions years? Do we have someone to blame?

Graph showing the percentage of mud and the proliferation of microbes and plants in the Paleozoic Era. Bryophytes start thriving around mid-Ordovician, about 470 mya. The percentage of mud begins to rise from late Ordovician and early Silurian, about 460-440 mya.
Percentage of mud and the proliferation of microbes and plants in the Paleozoic Era

Bryophytes and the origin of mud

I can see the tiny bryophytic child jumping about at the end of the classroom, raising her hand and trying desperately to say,

“It was me! It was me! I made mud.”

To which the geology teacher would inevitably ask, “How?”

Well, to start with, even tiny scrappy little green mats are, in large enough numbers, enough to stop wind and water wash away existing mud into the river and then the sea, ensuring enough mud stays on riverbanks. By stabilising riverbanks such, they might have even altered the paths of rivers and streams, thus changing the landscape towards a state conducive to more plant growth. Finally, organic acids secreted by the bryophytes during photosynthesis would have altered soil chemistry and led to the actual creation of mud.

So, basically, to recap, bryophytes (liverworts, hornworts, and mosses)

  1. Made mud
  2. Made sure the mud stayed
  3. Which led to the rise of the rooted plants
  4. Which led to more mud
  5. And more rooted plants
  6. And thus, agriculture!

Moss are the Boss!

Image of the evolution of plants. Bryophytes such as mosses, liverworts, and hornworts feature prominently.
The evolution of plants

 

Footnotes

Note:IndianSpring : Or not. As of today, March 13 2018, the Maharashtra government, which was inclined to acquiesce to the farmers’ demands, has indeed acceded to said demands, and have arranged for special trains to put the farmers back in their places.

Note:Precambrian : Well, technically, the Precambrian is a super-Eon, further divisible into three Eons : Hadean, Archaen, and Proterozoic. However, as papa Feynman used to say, what’s the point of learnin’ names if ya don’t know what they do?

Note:Crichton : Actually, no, bless you. You, sir, single-handedly made people fall in love with the prehistoric (more precisely, the mesozoic). Dinosaurs became cool, thanks to you. Scary, too. As The Nerd Druidess loves singing (in perfect rhyme and rhythm)

Oh Michael Crichton

Your dinosaurs

They so wonderfully frighten

Note:XP : XP, in roleplaying game (rpg) lingo, means Experience Points. Given that Dungeons and Dragons was one of the first popular rpgs, it is entirely possible Gary Gygax came up with the name. I wonder how many XP a Nerd Druid class might accumulate.

Note:Wentworth :  Humans speak in decimal. That is, (most of) our activities are carried out in powers of 10. We have ten fingers (101). A hundred meter (102) race is the most exciting, now that Bolt has retired. A thousand kilos (103) makes a ton(ne), though cricket commentators seem to think a century is a ton. Blame the Brits for that. Agriculture arose ten thousand years ago (104), a hundred thousand (105) is called a lakh in India, while a million (106) is probably the most used word in this article.

Computers, however, speak in binary (or octal, or hexadecimal, but never mind). Powers of 2. The method of classification of grain sizes, that is, the Wentworth Scale, also uses powers of 2. Rewriting the paragraph in the main text in terms of powers of 2 and millimeters, we have

Anything above 28 mm (diameter) are boulders. 26 – 28 mm are cobbles, 21 – 26 mm are gravel (22 – 28 mm are pebbles), 2-4 – 21 mm is sand, 2-8 – 2-4 mm is silt, and 2-10 – 2-8 mm is clay.

Binary rules!

Note:JoeRoot : Absolutely no intention of evoking the fine English batter here. Although, truth be told, Root is definitely no average Joe.

Sources

  1. McMahon, William J. and Davies, Neil S. : Evolution of alluvial mudrock forced by early land plants, Science (2018) [doi : 10.1126/science.aan4660]
  2. Fischer, Woodward W. : Early plants and the rise of mud, Science Commentary (2018) [doi : 10.1126/science.aas9886]
  3. Yirka, Bob : Ancient rootless plants linked to increase in production of mud rock, Phys.org (2018)
  4. Gramling, Caroline : Early land plants led to the rise of mud, ScienceNews (2018)