Editing Psilotum (section)

==Relation to ferns==
''Psilotum'' superficially resembles certain extinct early vascular plants, such as the [[rhyniophyte]]s and the trimerophyte genus ''Psilophyton''. The unusual features of ''Psilotum'' that suggest an affinity with early vascular plants include dichotomously branching sporophytes, aerial stems arising from horizontal rhizomes, a simple vascular cylinder, homosporous and terminal [[eusporangiate fern|eusporangia]] and a lack of roots.<ref name=GiffordFoster>{{cite book |last1=Gifford |first1=Ernest |title=Morphology and Evolution of Vascular Plants, Third Edition|year=1989|publisher=W.H. Freeman and Company|location=New York|isbn=0716719460|author2=Adriance Foster}}</ref><ref name="Friedman-et-al-2004" /> Unfortunately, no fossils of psilophytes are known to exist. A careful study of the morphology and anatomy suggests that whisk ferns are not closely related to rhyniophytes, and that the ancestral features present in living psilophytes represent a reduction from a more typical modern fern plant. Significant differences between ''Psilotum'' and the rhyniophytes and trimerophytes are that the development of its vascular strand is [[xylem#Development|exarch]], while it is [[xylem#Development|centrarch]] in rhyniophytes and trimerophytes.<ref name=Bell>{{cite book|last1=Bell|first1=P.R.|last2=Hemsley|first2=P.R.|year=1992|title=Green plants, their origin and diversity|edition=second|publisher=Cambridge University Press|location=Cambridge, U.K.|isbn=0521646731}}</ref> The sporangia of ''Psilotum'' are [[locule|trilocular]] synangia resulting from the fusion of three adjacent sporangia,<ref name=Bell/> and these are borne laterally on the axes.  In the rhyniophytes and trimerophytes the sporangia were single and in a terminal position on branches.<ref name=Stewart>{{cite book|last1=Stewart|first1=W.N.|last2=Rothwell|first2=G.W.|year=1993|edition=second|title=Palaeobotany and the evolution of plants|publisher=Cambridge university press|location=Cambridge, U.K.|isbn=0521382947}}</ref>

Molecular evidence strongly confirms that ''Psilotum'' is a fern (in the broad sense that includes horsetails) and that psilophytes are sister to ophioglossoid ferns.<ref name=QiuandPalmer1999>Qiu, Y-L and Palmer, J (1999) "Phylogeny of early land plants: insights from genes and genomes." Trends in Plant Science 4(1), 26-30</ref>