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Carbon dominates the makeup of life on Earth.

But is there any way that plants could be made out of metals, or somehow integrate metals with their constitution?

For example, we use calcium in our bones, which is an alkaline metal.

Its there any way that a plant might use metals (alkali or otherwise) to strengthen its structure so as to resist strong winds and protect itself from predators?

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  • $\begingroup$ Related: worldbuilding.stackexchange.com/q/10884/7351 $\endgroup$
    – apaul
    Apr 26 '15 at 16:02
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    $\begingroup$ Talking about calcium as "an alkaline metal" is entirely beside the point. No organism uses metallic calcium. As an example of how wrong this is, consider a skyscraper made, not from metallic iron, but from iron oxide - that is, rust. $\endgroup$ Apr 26 '15 at 16:46
  • $\begingroup$ Your point is ? $\endgroup$
    – Jorge Aldo
    Apr 26 '15 at 16:48
  • $\begingroup$ Many elements are classified as metals. Look up a recient public lecture on youtube from SLAC concerning the use of metal atoms in organic materials esp. catalysts. They go over how the properties of what makes it metalic is complementary to what is typically organic building blocks like protein. $\endgroup$
    – JDługosz
    Apr 26 '15 at 17:50
  • $\begingroup$ I'm not asking about any use of metals, but the specific use of metals as strenghtening elements $\endgroup$
    – Jorge Aldo
    Apr 26 '15 at 19:52
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Lets look at some real-world examples of plants which already absorb and use metals, namely hyperaccumulators.

Hyperaccumulating plants already use metals within their tissues.

Hyeraccumulators are plants which can withstand extremely high concentrations of metals otherwise toxic to non-hyperaccumulating plants.

They extract metals from the soil and store them within leaves, shoots and their roots. Other plants can also extract metals from soils, but hyperaccumulators can do this at a much, much faster rate and are also able to store incredible quantities of these toxic metals within their tissues.

Because of this extraction of metals, hyperaccumulators are commonly used in phytomining, where we use such plants to take minerals out of the soil for us.

phytomining process

Of course, hyperaccumulators absorb many metals, not just the ones valuable to humans.

Because of the toxicity of the metals which are absorbed by hyperaccumulating plants, scientists speculate that the primary purpose of hyperaccumulation, at least, the primary defensive purpose, is to prevent them from being eaten. The concentration of toxic metals within these plants is so high that animals which eat them will die, and so never be found eating them again.

Toxic metals used mainly to deter animals.

So, here on Earth, hyperaccumulators only pad their tissues with toxic metals to decrease the likelihood of their being eaten, but you've stated specific interest in using metals to improve structure, rigidity and strength.

However, if a plant were able to pull metals from the ground like a hyperaccumulator, there's no reason those metals couldn't then be used to strengthen the plant.

First, let's have a look at which metals hyperaccumulators are known to handle:

Name           Symbol        UTS tensile strength
Aluminium      Al                700
Silver         Ag                170
Arsenic        As                  3
Beryllium      Be                448
Chromium       Cr                689
Copper         Cu                220
Manganese      Mn                  -
Mercury        Hg                  -
Molybdenum     Mo                690           Disclaimer:
Lead           Pb                 17           I am pretty certain that the
Palladium      Pd                325           listed tensile strengths are
Platinum       Pt                240           inaccurate and inconsistent.
Selenium       Se                500           They should be used just as a
Zinc           Zn                 28           rough idea of the actual
                                               strengths.

From this list, two metals stand out; Aluminium and Chromium.

Al/13/27 Cr/24/52

Both incredibly strong metals; this is the same Aluminium used in skyscrapers and jet engines, and the same chromium as used in Chrome plating.

If your plants were to hyperaccumulate large enough quantities of these metals, they could capitalise on their strength in many ways. Some examples I thought of were:

  • Reinforcing cell-walls by chrome plating. Currently, plant cell walls are made almost entirely from cellulose, which is (compared to chromium) very weak. By reinforcing this cellulose with chromium, individual cells of your plant would become nigh-indestructable.
  • Building skeletal systems. Along with the Phloem and Xylem of current plants, metal plants might have a third system of vessels, filled with Aluminium, keeping the plant structurally sound. Good luck snapping a twig laced with one of the toughest metals of which we know.
  • Plating the entire plant. If the plant's epidermis were to secrete chromium instead of wax, it could build up a thick layer of chrome plating, which cannot be scratched, corroded or otherwise damaged.

Metal plant uses chrome-plating to protect itself.

Of course, now that you have the ability to absorb minerals and metals from the ground, you can use them however you want!

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    $\begingroup$ Plating, short for "electroplating," is an energy-intensive, electrochemical process. A plant couldn't perform it on itself at the cellular or structural level, and it couldn't survive the process if it was performed by some motile creature, either. Aluminum doesn't exist in nature as an elemental metal; you need electricity and lots of it just to extract it from ore. How could a plant secrete a solid metal, anyway? And how could a metal-covered plant absorb sunlight to sustain itself? $\endgroup$ Jun 3 '15 at 16:22
  • $\begingroup$ The plant would just excrete, through pores in the epidermis, the same metals they absorb, in the same solutions they are found, to keep them liquid. Eventually, the liquid will dry off, leaving just the metal. $\endgroup$ Jul 8 '15 at 20:41
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    $\begingroup$ A metal-covered plant would obviously be unable to absorb sunlight, so don't cover it entirely in metal. Leave leaves exposed, they would need to protrude outside of the metal coating. An iron tree trunk alone is still a lot of protection. $\endgroup$ Jul 8 '15 at 20:42
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As you stated previously we use calcium in our bones but, given several bone sustenance changes, it would be trivial to use said calcium to create skeleton based flora. Unfortunately...

Where do we get it?

On average soil is around a tiny 1 percent calcium. If your goal is to reinforce the plant, then this might work if you add small amounts of hydroxyapatite(bone) to the cell walls of the plant but not if want a complete skeleton. For that i suggest.

Carnivorous Plants

The human body, and most other large animals, contain a great amount of calcium just waiting to be used by your potential plant. Although large carnivorous plants are more fantastical they would allow for the vast need of calcium to be met.

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  • $\begingroup$ Well. But there are many herbivores (animals, not plants) that form exosceletons of some kind, be it calcium-based (calcium carbonates) or silica based (silicon dioxide). So it could be conceivable to have a plant satisfy its need on metal or silicon compounds without turning carnivorous. (Question remains, would a plant's structural integrity improve upon incorporating such means.) $\endgroup$
    – Ghanima
    Apr 26 '15 at 15:40
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Not carbon-based plants as we know them.

The advantages like structural strength that you associate with metals are generally a result of what humans have done to the metal—accumulation, processing, and forging/casting or plating. Those processes require a lot of energy, a lot of heat, and a lot of motility, none of which plants are going to be able to pull off or survive on their own.

Metals as they exist naturally are not generally useful for their metallic properties. They are almost always bound in some sort of ore, highly impure, rare even within the mass of the ore, and they usually require considerable and energy-intensive physical and chemical processing before you get a usable quantity of elemental metal.

At that point, even more processing must be done. Two of the basic properties of metals are ductility and malleability, which are the ability to deform under tensile or compressive stress. Thus metals aren't known for their ability to withstand stress, but rather for their ability to deform under it without fracturing.

The metals known or used for their strength or rigidity owe that strength to a crystal structure imparted by the process of forging or casting. Those processes require levels of heat (the metal has to melt) which are impossible for any carbon-based lifeform to create on its own without technology or to withstand at close range.

Electroplating, a process used to coat things in useful metals—although usually for corrosion-resistance or decoration (which are the reasons you know what chrome is) and not for strength—is both energy-intensive and an electrochemical, technological process. Plants aren't going to be able to use or survive plating. Even if they could, metals are opaque to sunlight, so any plant coated in them won't be able to survive.

Besides, plants already have the use of another light-weight, rigid material for structural strength: cellulose. Humans were using wood for its strength and rigidity long before we discovered how to work metals. And, while cellulose doesn't have the rigidity of structural steel, it also doesn't have to be heated to 1,539 °C in order to have that strength, which is a big advantage when you're constructing a plant.

tl;dr: You don't get structural steel without a lot of heat and technology

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