Growth Architecture of Thucydia mahoningensis, a Model for Primitive Walchian Conifer Plants.

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  • A large number of vegetative and fertile branching systems of Thucydia mahoningensis provide data for interpreting the growth architecture and life-history pattern of a primitive Paleozoic conifer. Internal anatomy is similar to modern conifers, indicating an arborescent life-history pattern. Three categories of vegetative plagiotropic branching systems are recognized, and these resemble developmental variations among lateral branching systems in juvenile trees of the extant conifer Araucaria heterophylla (Salisb.) Franco. Thucydia mahoningensis is considered to be a small tree on the basis of the size and branching pattern of the stem and lateral branching systems and on the basis of internal anatomy. Growth architecture of T. mahoningensis conforms to Massart’s model, but fertile remains reveal interesting differences from living species. The T. mahoningensis tree has an orthotropic stem that bears two orders of plagiotropic vegetative shoots. Fertile shoots have a compound ovulate fertile zone and lack vegetative lateral branches. Ovulate branches bear penultimate-type vegetative leaves in the basal region, a compound ovulate fertile zone in the midregion, and ultimate-type vegetative leaves in the distal region. Axillary ovuliferous dwarf shoots of the compound fertile zones correspond to ultimate branches of the vegetative systems. Compound pollen cones are terminal on branches. These and supplementary data from previously described walchian species reveal that the architecture of the most primitive conifers resembles juvenile araucarian conifers and suggest that giant conifers evolved from diminutive trees by the combination of an extended vegetative growth phase and hypermorphoses in leaf development.

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    Article (Published)
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    Copyright 2003 by The University of Chicago
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    • Hernandez‐Castillo, G.R., Rothwell, G.W., Stockey, R.A., and Mapes, G. (2003). Growth Architecture of Thucydia mahoningensis, a Model for Primitive Walchian Conifer Plants. International Journal of Plant Sciences , Vol. 164, No. 3, 443-452