Generally it is thought that silicon is taken by plants up as soluble
monosilicic acid, 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