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Group consisting of two extant genera, Psilotum and Tmesipteris, in one family, Psilotaceae. These plants are primitive in structure: Psilotum lacks both roots and leaves and is structurally similar to the fossil genus Rhynia.. Recent molecular systematic studies suggest that the family is actually related to primitive ferns. Tmesipteris has a more complex morpholgy in that it has structures on the aerial shoot that are foliar. Bith Psilotum and Tmesipteris have compound sporangia called synangia. In the case of Psilotum these are three-parted.
While both genera have aerial branches arising from stems embedded in its substrate, they both lack roots. The rhizomes are infected with mycorrhizae. Branching occurs dichotomously by the division of the apical meristem and is most obvious in the areal branches of Psilotum. While Psilotum lacks true leaves, it possesses leaf-like extentions of the stem called enations. Because these lack vasculature, they are not considered leaves. However, in Psilotum complanatum, a vascular trace occurs below the enations. The foliar structures of Tmesipteris are vascularized.
The gametophytes of both genera are non-photosynthetic and live in association with a fungus. In the case of Psilotum, the gametophyte of certain strains produce vascular tissue.
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