Vascular Tissue
In modern plants, we tend to differentiate between vascular and non-vascular plants. In the fossil record, we observe several prevascular and early vascular plants with tracheids that differ from modern plants. As would be expected, there is a grade of plants and vascular cells that evolved during the Silurian Period.
Below is a listing of the grade of vascular tissues found in extinct and extant plants
Pre-vascular plants and bryophytes
The eophytes are a recently-described groups of Paleozoic plants that possessed sugar-conducting cells, but not water-conducting cells
Plants such as the horneophytes and some modern bryophytes have conducting cells that lack secondary thickenings. Sometimes these cells are referred to as hydroids. These cells can transport water, similar to xylem, but tend to provide little strengthening.
Basal tracheophytes
Some of the earliest vascular plants, such as rhyniophytes, had thickened conducting cells referred to as "S-type" tracheids.
S-Type tracheids
These cells had annual or helical thickenings consisting of a spongy material. Prominent helical (possibly annular) thickenings show a spongy texture which is also present in the underlying and intervening lateral walls of the elements. Both thickenings and intervening lateral walls are covered by a very thin layer with numerous closely spaced holes. Named after the plant Sennicaulis (Kenrick et al. 1991)
C-type tracheids
G-Type tracheids
Secondary thickenings are annular, helical to occasionally approaching reticulate, and are connected by a sheet of resilient material that is fused to the presumed primary wall. The sheet is perforated by holes of varying size and usually rounded shape. Named after cells found in Gosslingia, and found in many zosterophylls as well as ancestral clubmoss Asteroxylon
P-Type tracheids
Metaxylem tracheids are long (< 3 mm) and markedly faceted (5–7 sides) compared with G-type. Pitting is essentially scalariform, and in superbly preserved specimens, there are pit-closing membranes present. Named after cells found in the Psilophyton.
