In article Y9rRf.41547$CI6.33560@trnddc07, Hanson wrote:
Thanks Jo. AFAYK, is [1], such a high Si uptake, common to
all such primitive plants (Fern, Lichen etc)?

Don't *know* the answer to that one, but seeing lichens and
ferns happily growing on clean limestones with 2/10th of damn-all
percent silica in their composition, I don't think that all of them
*require* appreciable environmental silicon.

In marine plankton, the radiolarians, all do have skeletons made
of beautiful microscopic SiO2 structures.

Radiolarians ... yes, many produce siliceous skeletons. Some
produce strontium sulphate skeletons. some produce no skeletons. But
radiolaria are only a relatively small part of the general planktonic
fauna and flora.

But what I have not seen
a good/elegnat explanation yet for in what form this Si4+ or
H4SiO4 is transported into and through the plant.

Well, having had to work with silicate-based drilling muds (pH
12.3 and higher - keep those face shields on! And they're an absolute
bitch to wash samples in.) I'd suspect that solution-load silicate is
moved as a range of complexes with organic compounds for 2 reasons : as
you point out, in simple inorganic systems the pH required to keep
silicate in solution is high (very alkaline); and the rate of
weathering of rocks (and the *style* of weathering) increases rapidly
as the plant cover increases, and by implication as the quantities of
organic acids and complexing agents increases. But I don't have the
chemistry to put any detail on that.
Seawater silicate concentrations are (generally) appreciably
lower than temperate runoff (river water, essentially), suggesting that
there are some organisms or processes in the ocean that are very
efficient at removing silicate from the oceans.
Come to think of it - ISTR that the effluvia of mid-ocean
hydrothermal systems also have a significant silicate input to the
oceans, which something is removing. Now where did I read that ... it's
relatively recent.

--
Aidan Karley FGS
Aberdeen, Scotland,
Location: 57°10'11" N, 02°08'43" W (sub-tropical Aberdeen), 0.021233