Polysporangiophytes, also called polysporangiates or formally Polysporangiophyta, are plants in which the spore-bearing generation (sporophyte) has branching stems (axes) that bear sporangia.

In 1859 he published a reconstruction of a Devonian plant, collected as a fossil from the Gaspé region of Canada, which he named Psilophyton princeps.

Dawson's discoveries initially had little scientific impact; Taylor et al. speculate that this was because his reconstruction looked very unusual and the fossil was older than was expected.

The fossils were better-preserved than Dawson's, and showed clearly that these early land plants did indeed consist of generally naked vertical stems arising from similar horizontal structures.

[3] Since these discoveries, similar megafossils have been discovered in rocks of Silurian to mid-Devonian age throughout the world, including Arctic Canada, the eastern US, Wales, the Rhineland of Germany, Kazakhstan, Xinjiang and Yunnan in China, and Australia.

[citation needed] Prior to that, most of the early polysporangiophytes had been placed in a single order, Psilophytales, in the class Psilophyta, established in 1917 by Kidston and Lang.

Trimerophytes comprised plants with large clusters of downwards curving terminal sporangia that split along their length to release their spores and had centrarch xylem strands (e.g., Psilophyton).

[14] A major cladistic study of land plants was published in 1997 by Kenrick and Crane; this both established the concept of the polysporangiophytes and presented a view of their phylogeny.

[9] Since 1997 there have been continual advances in understanding plant evolution, using RNA and DNA genome sequences and chemical analyses of fossils (e.g., Taylor et al. 2006[24]), resulting in revisions to this phylogeny.

[25] † Horneophytopsida (Caia, Horneophyton, Tortilicaulis) † Aglaophyton † Rhyniaceae (Huvenia, Rhynia, Stockmansella) † basal groups (Aberlemnia caledonica [=Cooksonia caledonica], Cooksonia pertoni) Cooksonia cambrensis, Renalia, Sartilmania, Uskiella, Yunia † Hicklingia Adoketophyton, Discalis, Distichophytum (=Rebuchia), Gumuia, Huia, Zosterophyllum myretonianum, Z. llanoveranum, Z. fertile Zosterophyllum divaricatum, Tarella, Oricilla, Gosslingia, Hsua, Thrinkophyton, Protobarinophyton, Barinophyton obscurum, B. citrulliforme, Sawdonia, Deheubarthia, Konioria, Anisophyton, Serrulacaulis, Crenaticaulis Nothia, Zosterophyllum deciduum extant and extinct members † Eophyllophyton † basal groups (Psilophyton crenulatum, Ps.

dawsonii) moniliforms (ferns; extant and extinct members) † basal groups (Pertica, Tetraxylopteris) spermatophytes (seed plants; extant and extinct members) More recently, Gerrienne and Gonez have suggested a slightly different characterization of the early diverging polysporangiophytes:[26] †'Protracheophytes' †Paratracheophytes Eutracheophytes The paraphyletic protracheophytes, such as Aglaophyton, have water-conducting vessels like those of mosses, i.e., without cells containing thickened cell walls.

In particular, they did not have tracheids: elongated cells that help transport water and mineral salts, and that develop a thick lignified wall at maturity that provides mechanical strength.

[28] The remainder of the polysporangiophytes divide into two lineages, a deep phylogenetic split that occurred in the early to mid Devonian, around 400 million years ago.

[31] However, it has been suggested that the poorly preserved Eohostimella, found in deposits of Early Silurian age (Llandovery, around 440 to 430 million years ago), may be a rhyniophyte.

[6] Boyce has shown that the sporophytes of some Cooksonia species and allies ('cooksonioids') had stems that were too narrow to have supported sufficient photosynthetic activity for them to be independent of their gametophytes – inconsistent with their position in the cladogram.