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Generally it is thought that silicon is taken by plants up as soluble
monosilicic acid, [1] a neutral species. This quite unusual as most
elements are taken up as cations or anions. The other element thought
to be taken up as a neutral species is boron. Si is thought to be
transported through to plant mostly as monomeric silica, but I think in
some species the xylem sap may get so concentrated that polymers
form. The soluble silica is then concentrated in certain cells- often,
but not exclusively at the end of the xylem stream. Above a certain
concentration it comes out of solution, and forms solid deposits of
amorphous silica (phytoliths). These are not really a "store" as the
process seems irreversible. The phytoliths take the shape of the cells
etc., and have uses in archaeology and palaeoecology as markers.
Functions? Defence against herbivores and pathogens. Helps keep
plants upright (particularly grasses). In soluble form it seems to reduce
Al and heavy metal toxicity.
Hope that helps!
Martin

[hanson]
Thanks Martin. I too would love to think that [1] monosilicic acid,
Si(OH)4 or H4SiO4 [2] is the transporting agent for phyto-Si and
I have neither any problems with the rest of your assertions.
Tell me more about the chem and physics of [2] from rocks like
(polyvalent Me-silicates) granite or feldspar [3] into the phytoliths
deposits, for it doesn't seem to be a very well known thing.

IIRC, mono [2] is stable in water and in rather high concentrations
under near freezing conditions, but polymerizes (polycondensates
with water loss) quickly to become immediately insoluble as meta-
silicate, especially with electrolytes (salt) or higher temps present.
Also [2] is disassociates so weakly that even the H2CO3 from
dissolved CO2 in water will liberate it out of the rocks [3].

So the explanation for the origin of the necessary [2] for the phyto-Si
in nature is an "acceptable" one to me. But now the info available to
me for [2]'s next step into the bio domains of the plant gets sparser.
What other events/processes do play a role? Does the root system
alone do the job or are there symbiotic interplays required?

Does the possible presence of Fluoride and Mg / Zn factor in
to form these highly soluble Fluosilicates, SiF6--. compounds?...
Or do heavy metals like tungsten, W, facilitate the H4SiO4 [2]
transport by forming the highly soluble Silico Tungsten acid,
H4[SiO4(W3O9)4], .... and/or Mo & V, who in similar fashion
together with phosphates do form soluble [2] complexes?

..... or on the organic front does [2] form those soluble 5-ligand
coordinates (instead of the usual 4) in the presence of your
Xylem-sap, it presumably being mostly a 2-6 C (ring or chain)
keto/aldo carbohydrate agglomerate. Is there a known possibility
that these sugars can substitute, be or use Si(OH)4 as an ligand
and transport this insoluble item [2] in a "clatherated" soluble
form thru the system until conditions arise inside the plant where
[2] gets "expelled"/precipitated or exchanged and forms your
phytoliths?

Tell me more, Martin, dudes and dudettes. Gimme some urls.
This [2] chem and physics is fascinating, from its mechanism of
eroding mountain ranges, influencing climate change, being a
necessary building block in the global food chain all the way to
GE-plant modification improvement, and the preparation of hi-tech
electronic nano sized electronic components. Thanks.
hanson