Division Rhyniophyta - Upper Silurian to Middle Devonian

Among the oldest convincing fossils of early vascular land plants are those of the Division Rhyniophyta. The members of this division consist of: (1) simple axes with cuticle, stomates, epidermis, cortex and a terete (circular) centrarch xylem (xylem development occurs from the center of the stem to the periphery), without well-developed appendages or roots; (2) simple, isotomous branching systems to anisotomous, dichotomous, pseudomonopodial systems with up to four or five orders of branching, whole plants rarely exceeding ten centimeters; (3) sporangia globose to almost reniform (kidney-shaped) with longitudinal dehiscence; (4) spores with sporopolleinin and distinct trilete marks on their proximal side from their placement in the tetrad. These contain the most morphologically primitive plants as well as the oldest, suggesting that morphological complexity diversified from the simplest parenchymatous forms during the middle to upper Silurian periods. Likely the Rhyniophytes disappeared when their derivatives modified to more complex forms and diverged from this primitive assemblage.