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Inspired by the many great questions asked for this fortnight's tag challenge, I got to thinking, could a plant develop/evolve to eventually consume an entire planet, and keep on living? What I mean by this is that a single organism, or perhaps a colony, both covered the surface of the planet and also converted the vast majority of the planet's mass into plant matter, most of which would either be alive or recently dead but ready to be consumed and converted back into living matter. Essentially, there would be no part of the planet not used by the plant, and yet with these resources the plant could survive indefinitely.

Also, just imagine exploring such a planet. You'd be like "Where's the ground on this planet?" and your scientist would incredulously reply "There is no ground. This tree is the planet."

I know many plants on Earth are pretty good at thriving in the craziest environments and overcoming all sorts of obstacles, but this seems a few levels more difficult. What sorts of hurdles would the plant have to evolve over to make this happen, and what things might make it impossible? Is there perhaps a theoretical planet that would make this process more plausible?

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The Hurdles

  • Resource Transport All parts of the plant need resources, so you xylem/phloem is going to need to transport things over large (vertical) distances.
  • Heat The mantle of a planet can get quite toasty. Enough, in fact, to burn any wood or organic material. The mantle is considered to be a huge area of molten rock.
  • Climate A planet, even an apparently lifeless one like Mars, has differing climates based on how much sun it gets. There are other factors, like water availability and soil quality.

Resource Transport

Trees can only grow so tall before the cost of hauling those resources becomes too much. See this article on the limits of tree height. They estimated that the maximum height of a plant is 400 m. Given this, you don't want your planet-sized plant to grow very high.

Resource transport may still be a problem, though, as you get down further into the planet. Your roots will need to have enough energy to transport the nutrients they gather to the rest of the plant. You can see an article talking about maximum root depth from Duke, and a list from wikipedia on maximum root depth. (The record appears to be 68 meters!)

As described in the wiki article, roots generally do not go very deep simply because the soil becomes inhospitable to the tree the further down you go. So, you planet-plant will need some tough roots that enjoy making airways but also breaking up heavily compacted soil. Bacteria and fungi could help your roots grow, as they do in real life, but they only help so much.

Heat

The world gets hot the further you go down. The geothermal gradient of Earth makes it abundantly clear that your tree will need to have a higher melting point than that of rock to get down really far. Additionally, it would need to withstand enormous amounts of pressure, simply because of all that atmosphere/rock bearing down upon it. (The mantle is, after all, is where diamonds can be made.)

The solution will be for your plant to cool the ground as it digs through it. This can be tough, especially if your planet is earth like and has radioactive elements providing heat. This would also take a very long time; radioactive decay can take a while to stop. Even if your roots could cool the planet as it digs through it, it would only compound the resource transport issue.

Climate

Last of all, your tree needs to be able to withstand any climate. It needs to out-perform things at the equator of your planet as well as anything at the poles. (The poles of earth, at least, seem to be forsaken by plant life.)

So your plant will need solutions for surviving both hot and cold, extreme wet and extreme dry, extreme nutrient deficit and nutrient plenty. Every plant has adaptations to its environment, but each adaptation could be a liability in another climate.

Some Models

There are some big trees out there. Here are some models of live trees which are rather successful at covering the surface area, especially compared to "normal" trees.

  • Pando: an aspen colony which is thought to be made from the same tree. Technically, every tree there is the same tree because of interconnecting root systems. Pando covers 106 acres (428 966 $m^2$ ) of land.
  • Maior cajuerio do Mundo: a cashew tree which, for some reason, carried a mutation that let its branches touch the ground, put down roots, and then move on. This tree covers around 1.8 acres (7 300 $m^2$) of land.
  • The Great Banyan Tree: like the previous tree, this ficus tree has branches touch the ground, put down roots, and then make more branches. This one holds the title for widest tree in the world. It covers 4 acres (14 500 $m^2$) of land.

An Ideal Planet

An ideal planet for such a plant would:

  • Possess an unusually homogeneous global climate (because of water or air convection? Multiple brown dwarves heating it?)
  • Possess an abundance of nutrients for this plant everywhere
  • Possess a single, connected landmass. No islands or other inaccessible places.
  • Somehow cool as the plant digs deeper without the plant above dying
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  • $\begingroup$ "Possess an unusually homogeneous global climate" could be solved by its host being a rogue planet with dense clouds. With no star to circle, the global climate would be highly homogeneous, and the dense clouds should keep it warm enough for life (also help water the plant; you don't need to pump water up if you can grab it from the clouds!). $\endgroup$ – Tim S. Feb 26 '15 at 17:16
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    $\begingroup$ Don't forget about Earth's largest organism. Of course, a fungus is not a plant, but still, that sucker is huge. $\endgroup$ – Samuel Feb 26 '15 at 20:31
  • $\begingroup$ @Samuel Good point, but the OP asks for a plant... :( Maybe I should put it in for an honorable mention. $\endgroup$ – PipperChip Feb 27 '15 at 0:17
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    $\begingroup$ Regarding climate, the Earth hasn't always had polar ice - we're currently in an ice age - and there seems to have been significant forests on and around the South Pole. It would seem all you need is enough greenhouse gases, and perhaps some decent oceanic currents (which might be a problem to arrange if the plant covers the water). $\endgroup$ – gustafc Feb 27 '15 at 14:04
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While I expect that a plant colony could in theory cover a planet or moon, and even be very deep, maybe a mile for a smaller planet, I don't think it would be possible to make the entire planet biomass. Here's my thinking why.

First plants need energy to grow, a mile deep forest is going to be pretty dark near even half way down. so no light, at least not from above.

Second, plants of course need nutrients, and so roots or what ever method they use to collect them need something to collect them from. Rocks, soil, other dead things.

Third, even a planet as small as the moon, it's mass is still large enough to crush things in it's center, the moon is round because of it's own gravity. At some point the gravity of the plant planet would be crushing everything in the center. and nothing would be able to live. So At the least there would be a core supporting all the plant life. Life would likely not ever get more than a few miles deep, just the weight of the plants would be crushing. The diameter of the moon is 3474km, if you even made a 3 mile layer of plant life, you would still have 3469km of non-plant life in between.

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    $\begingroup$ Just a thought, what if the center of the planet became a waste dump for the plant, where all the dead bits fall in, get churned up and heated by the molten core, then get eaten again by some other part of the plant? $\endgroup$ – DaaaahWhoosh Feb 26 '15 at 16:05
  • $\begingroup$ @DaaaahWhoosh well you'd still need some kind of convection to move the material in and out. $\endgroup$ – bowlturner Feb 26 '15 at 16:35
  • $\begingroup$ Right, convection from the core. Generated and sustained by gravity. Perhaps regulated by the plant in some symbiotic way. $\endgroup$ – DaaaahWhoosh Feb 26 '15 at 16:41
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    $\begingroup$ Something like this: what-if.xkcd.com/4 $\endgroup$ – SSumner Feb 26 '15 at 19:31
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1. Depends on the planet - such planet should have similar climate on poles and equator. Water resources should be under ground, but not very deep. Also there should be plenty of water and water should be accessible anywhere. Most of such planet should be ground (to provide place for such plant to grow).

I believe any other type of planet would make such big "plant" not plausible

2. Depends on evolution My gut feeling tells me, that there should be nothing "eating" such plant. So we should wave out all bigger animals and keep maybe insects and some germs. That's it.

Other suggestion is evolution done in a way as in great barrier reef - everything is living in co-existence

The plant will share roots - So it will not cover all the land visually, but under ground

You cannot change planet into plant As bowlturner pointed out in this answer, you cannot expect roots going very deep. Also do not forget, that

Planet has to have hot core in order to support life and atmosphere. So going deep will burn your plant

Overall: You can have plantet which looks like it is actually one big plant. But going closer will show you it is planet which has ground covered by one big plant and the rest is ground, microbes and some small animals.

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Here's an idea. Instead of a plant, why not a fungus?

Some are superficially similar to plants, to the point where early taxonomists misclassified them as such.

There's no theoretical size limit on a fungus. Indeed some people would say that, depending on your definition of an individual organism, the world's largest living things are fungi (I see one commenter has already provided a link for this).

Most grow by feeding on decomposing matter, which is going to become a problem when they've consumed all the planet's resources. However, there is limited evidence to suggest that some have limited capacity to convert radiation into energy, like plants do with sunlight. This would have the added bonus of allowing your planet-fungus to grow in deep space, out of range of sunlight.

Fungi are also better at growing in extreme conditions than plants, and are better at lying dormant during periods of poor nutrition. So your theoretical plant planet could be alive, even when it had no source of nutrition at all.

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When you say "plant", I think it depends on what you mean. In an Earth context, a plant seems to be an organism evolved to live as part of the environment here, which is a symbiosis of many life forms in the conditions that have existed here. Some plants (e.g. Kudzu) may tend to spread very aggressively and smother other plants to an unusual degree, but every organism on Earth ultimately requires conditions which come from many other species, and relatively stable climate conditions. Every species on Earth which is "too successful" at dominating its environment, tends to eradicate the conditions which allow it to live, resulting in a serious die-off and possible extinction. The planet as a whole however can be said to be alive as a whole, and has much more potential for long-term survival.

In order for one species to be able to take over and dominate an entire planet, it would need to be able to somehow generate its own requisites for continued existence. The easiest way to accomplish that, is not to eradicate the other species, but to encourage and aid other species, which in turn have their own survival requirements and needed conditions to keep in balance. When different species interact to support each other, it's called a symbiotic relationship. Even predator/prey relationships tend to be ultimately symbiotic, as they help control the imbalance of different species' population levels, avoiding toxic conditions, starvation, die-offs and extinctions. This tends to be best supported by a naturally-evolved ecology with plenty of biodiversity, perhaps best exemplified by the Amazon rainforest before foolish humans started destroying it for low-grade cattle pasture.

A planet-consuming "plant" would also need something to cause it to organize itself to eradicate other species and generate its own conditions in some other way, which on Earth or another natural planet, I don't see a likely reason for without something like deranged human thinking at play.

You also describe it as one planet-sized plant, which is a challenge in itself. One species with many members is one thing, but requiring it to be a single member ... well it depends on what you mean. I think it could work as long as much smaller pieces could function independently. To achieve the self-symbiosis, I imagine it would need to have many different forms or components to create its own life cycles.

At which point, I think you're falling back towards something more like life on Earth anyway. Until relatively recently over the span of human existence, most humans have considered themselves to be a part of our living world as a a whole, and not as a separate thing struggling against nature. The wisest answer may be that we are already on a planet-size life-form or even plant, at a macro level. We're all part of the organism, and the idea that we're not, and our nastier industrial activities, are a disease which we need to outgrow to avoid our own die-off.

Back to a more literal vision of your question, another aspect is that if you start with an Earth-like planet and an Earth-plant-like "plant" (say, genetically modified by the evil Doctor Monsanto to intentionally devour the entire planet), there would be challenges to the literal physical task in that only the topsoil and oceans of the planet are very useful nutritionally. If evil Doctor Monsanto intends his plant to devour the very planet beneath the outer layers, he needs to find something that will do something to inorganic rock, and then to magma, molten iron, etc...

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