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The world of Baros (b'a:rəs) has low gravity (around 0.5g), thick, moist atmosphere (2-3 bar), and lots of volcanoes. Humans mostly inhabit sunny elevated areas, separated by relatively dark and CO2-rich lowlands. The humanity is at roughly medieval tech level.

Now one of the features I want there is a plant that grows on bare cliffs, using symbiotic bacteria and its own roots to cut its way through micro cracks. This plant is domesticated and selected for rapid growth and deeper rooting. Bacteria are producing oxalic acid which is already used by lichens to crush basalt. UPD: The wild variation relies on birds for nutrients and seed distribution, and grows berries to attract them. Domestic one is dependent on humans' aid.

The rock in question is similar to Earth's alkali basalts (Mg/Ca/Na/K silicates with Fe, Ti and Al oxides) and should be able to withstand temperature changes, water, knocking, and scratching for ages.

Say I want the roots to penetrate 5-10cm of rock in 2-3 months. Afterwards other means (driving in wedges and making them swallow? Or maybe just a sledgehammer?) would be used to dismantle weakened rock into convenient small blocks of stone. Nice terraces are left behind.

Is it possible at all? Should it be a shrub? A grass? (Trees seem to be ruled out, for one cannot plant trees in tight rows - or...?) Maybe such plants already exist on Earth? Last but not least, is such technique feasible at all for a medieval society?

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    $\begingroup$ The issue is: where will the plant feed from? It is very different from a weed to grow in a grove of the rock that has some soil on it, and whose roots cause some damage of the rock, than a plant growing out of bare rock. $\endgroup$ – SJuan76 Nov 12 '15 at 11:31
  • $\begingroup$ @SJuan76 Good point. The wild species was supposed to grow slowly and rely on bird droppings, spawning berries to attract birds. The domestic one is dependent on supply of fertilizers and water by humans. $\endgroup$ – Dallaylaen Nov 12 '15 at 11:56
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The main benefit of being able to grow on rock (of any kind) is that other plants can't, which gives the species that can an edge in getting living space and sunlight. The downside is that shortage of nutrients requires either some other form of getting them or very slow growth.

You presumably want the process to be as fast as possible, so the question essentially comes down to alternate ways of getting nutrients. I will suggest two as an example, but I am sure you will be able to think up more.

The plant can grow underwater with the water carrying the nutrients. You can find such plants on many coastal areas, so real world examples to base the plant on are available. I think real world plants prefer adhesives though. Deep roots gives no advantage over shallow roots and a good adhesive. This would require that people build temporary pools of water for the plant to grow on.

The plant could get extra nutrients by catching insects. Such plants could presumably be simply fed with cultivated insects. Plants like this also generally have shallow roots. If you do not get nutrients from your roots, there is no benefit in making them deeper than needed to anchor the plant. And solid rock makes for superior anchoring point.

So I think the optimal plant to use would be fast growing and spreading plant with shallow roots and very low and small visible part. Something along the lines of moss. I also think underwater solution would work better than feeding insects to plants few millimeters in size.

You would only be able to crush a very shallow layer of rock per cycle, but the cycles would potentially be very fast. And since a plant with roots penetrating deep into rock seems unlikely due to lack of compelling benefit, shallow roots version is probably the choice of necessity. And of course the shallow roots fast cycles version would also be much more precise. You would be able to create ornamentation and inscriptions with precise outlines and delicate features.

Also smaller the plant is less water you will need to provide. Small enough plant could be painted as a gel to surface in the morning and rubbed out of the surface along with some softened rock in the evening.

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  • $\begingroup$ Shorter cycles mean more manual labour, making the whole thing less competitive against a pickaxe. One thing that didn't occur to me, though, is that I can plant the weed many times in the same place, each time deeper. A cliff dweller certainly must be able to climb out of deep cracks towards the sunlight. $\endgroup$ – Dallaylaen Nov 12 '15 at 15:45
  • $\begingroup$ Making pools around the seeds on a steep slope looks like more extra work again, but this may be a good drive to invent plumbing. This is generally the kind of tech I want there - sophisticated and well-though yet clumsy and primitively looking. $\endgroup$ – Dallaylaen Nov 12 '15 at 15:52
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    $\begingroup$ @Dallaylaen The steep slope issue is why I started talking about paint/gel at the end. If you have something that is transparent enough for plants to grow on the surface and keeps the surface wet for several hours, you can replace pools with paint. // The amount of manual labour scale with cycles and depth removed, so it should balance out reasonably well. Essentially while work per depth would increase, the work per area would decrease to compensate. // Given they live on cliffs they'd almost certainly have sophisticated irrigation. Check Andean civilizations for ideas on that. $\endgroup$ – Ville Niemi Nov 12 '15 at 16:51
  • $\begingroup$ Yes, I really like the gel idea. $\endgroup$ – Dallaylaen Nov 12 '15 at 17:19
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High CO2 concentrations is good for agriculture and also anaerobic bacteria. Maybe it's most feasible to make bacterias use that CO2 to produce carbonic acid (if you find a large source of hydrogen, like water) to attack the rock.

Plants will need water anyway. Maybe you can provide carbonic acid resistant bioengineered crops using that bacteria in symbiosis to bricks/soften that rock.

Also remember some plants grow better in excess of sunlight others not. Maybe the thick atmosphere clouds visible light but can let UV pass. In that case you plants can also be engineered to get more of this wavelength. If genetics is that advanced on your world you even can mess with chlorophyll making a variant. It can have a interesting effect on leaf colors making it range from dark blue/purple to white.

Edit

OP make a good point with oxalic acid. In that case to rapid deteriorate rock a plant/bacteria symbiosis to acid attack the rock and soften it sounds good for me. If sunlight is not a problem it can even help with some cultures like potatoes, is left to OP to decide how rock-eating potato crisps will taste.

More Edit

Light atmospheric scattering and absorption is complicated because depends on the many atmosphere layers, each one interacting in a different way. Without the ozone layer UV can be a problem. Also you star can have a very different light spectrum.

Also if your planet got lots of deep valleys with gaps where there are lacks of oxygen it can get cyanobacterias for example. If you want to goo deep (no pun intended) you can read this article about photosynthesis & solar radiation

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  • $\begingroup$ Atmosphere is not that thick, there is excess sunlight in the highlands where the weed grows, and CO2 is still high, just not dangerously high. Oxalic acid is stronger than carbonic, but not very hard to synthesize (it's present in many of earth's plants). That's why I took it. $\endgroup$ – Dallaylaen Nov 12 '15 at 12:16
  • $\begingroup$ I believe UV would be caught by this atmosphere first, however, far red or infrared-based photosynthesis may be viable in the deeper regions of this world. Never thought of this, thank you! I wonder if Earth's rainforests or oceans have something like that. They do $\endgroup$ – Dallaylaen Nov 12 '15 at 15:11

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