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A pteridophyte is a vascular plant (with xylem and phloem) that reproduces by means of . Because pteridophytes produce neither nor , they are sometimes referred to as "", meaning that their means of reproduction is hidden. They are also the ancestors of the plants we see today.
, (often treated as ferns), and Lycopodiophyta (, , and ) are all pteridophytes. However, they do not form a monophyletic group because ferns (and horsetails) are more closely related to than to lycophytes. "Pteridophyta" is thus no longer a widely accepted taxon, but the term pteridophyte remains in common parlance, as do pteridology and pteridologist as a science and its practitioner, for example by the International Association of Pteridologists and the Pteridophyte Phylogeny Group.
where the monilophytes comprise about 9,000 species, including horsetails (Equisetaceae), whisk ferns (Psilotaceae), and all eusporangiate and all leptosporangiate ferns. Historically both lycophytes and monilophytes were grouped together as pteridophytes (ferns and fern allies) on the basis of being spore-bearing ("seed-free"). In Smith's molecular phylogenetic study the ferns are characterised by lateral root origin in the endodermis, usually mesarch protoxylem in shoots, a pseudoendospore, plasmodial tapetum, and sperm cells with 30-1000 flagella. The term "moniliform" as in Moniliformopses and monilophytes means "bead-shaped" and was introduced by Kenrick and Crane (1997) as a scientific replacement for "fern" (including Equisetaceae) and became established by Pryer et al. (2004). Christenhusz and Chase (2014) in their review of classification schemes provide a critique of this usage, which they discouraged as irrational. In fact the alternative name Filicopsida was already in use. By comparison "lycopod" or lycophyte (club moss) means wolf-plant. The term "fern ally" included under Pteridophyta generally refers to vascular spore-bearing plants that are not ferns, including lycopods, horsetails, whisk ferns and water ferns (Marsileaceae, Salviniaceae and Ceratopteris). This is not a natural grouping but rather a convenient term for non-fern, and is also discouraged, as is eusporangiate for non-leptosporangiate ferns.
However both Infradivision and Moniliformopses are also invalid names under the International Code of Botanical Nomenclature. Ferns, despite forming a monophyletic clade, are formally only considered as four classes (Psilotopsida; Equisetopsida; Marattiopsida; Polypodiopsida), 11 orders and 37 families, without assigning a higher taxonomic rank.
Furthermore, within the Polypodiopsida, the largest grouping, a number of informal clades were recognised, including leptosporangiates, core leptosporangiates, polypods (Polypodiales), and eupolypods (including Eupolypods I and Eupolypods II).
In 2014 Christenhusz and Chase, summarising the known knowledge at that time, treated this group as two separate unrelated taxa in a consensus classification;
The two major groups previously included in Pteridophyta are phylogenetically related as follows:
In addition to these living groups, several groups of pteridophytes are now extinct and known only from . These groups include the Rhyniopsida, Zosterophyllopsida, Trimerophytopsida, the Lepidodendrales and the Progymnospermopsida.
Modern studies of the land plants agree that seed plants emerged from trimerophytes. Therefore, pteridophytes do not form a clade but constitute a paraphyletic grade.
These terms are not the same as monoecious and dioecious, which refer to whether a seed plant's sporophyte bears both male and female gametophytes, i.e., produces both pollen and seeds, or just one.
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