Vascular Tissue

Below is a listing of the grades of vascular tissues found in extinct and extant plants

Paravascular plants and bryophytes

• The eophytes are a recently-described group of Paleozoic plants that possessed sugar-conducting cells (leptoids), but not water-conducting cells

• Some modern bryophytes have conducting cells that lack secondary thickenings. These cells are referred to as hydroids. These cells can transport water, similar to xylem, but tend to provide little strengthening.

H-type conducting cells

• • Plants such as Horneophyton have thick-walled H-type conducting cells, which would have moved water, minerals, and sugars from the rhizome to the aerial axes

  • These cells were involved in active solute transport versus passive water movement in tracheids (Kenrick & Long, 2025)

  • Along with eophytes, this evidence in Horneophyton may indicate that phloem-like cells evolved first, and xylem appeared secondarily to aid in passive water-flow

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)

Basal eutracheophytes

• e.g., cooksonioids and zosterophyllphytes

C-type tracheids 

• • Plants such as Cooksonia had tracheids with conventional annular and spiral tracheids, except that the lateral walls are imperforate (lack holes) and thick compared to the primary wall in protoxylem. Named after the plant Cooksonia †.

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 the ancestral clubmoss Asteroxylon †.

Eutracheophytes: 

• e.g., euphyllophytes and derived lycophytes

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 †.