>"Primitive vascular types consist of wood, a central pith and, in
>addition, usually two thin layers of different cells which are
>external to the wood." p. 22
I don't know what you mean by "primitive vascular types" which Carboniferous
coal floras were certainly not, but many Carboniferous plants had minimal
wood compared with modern trees (generally only primary xylem), but had very
extensive and complex cortices, which in lycopods comprised much of the
trunk diameter. The tree ferns were tree ferns (what can I say) having
again, lots of "cortical" tissues. The calamites had hollow stems, as the
modern equisetum does. We know the detailed anatomy of these forms because
of the exquisite preservation found in "coal balls".
Art
http://chadwicka.swau.edu