Ontogeny of progymnosperms. I. Shoots of Upper Devonian Aneurophytales

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Configurations of the primary xylem strands of the progymnosperms Tetraxylopteris schmidtii, Triloboxylon ashlandicum, and Proteokalon petryi (Aneurophytales) have been determined for most levels of their shoot systems. In general there is a decrease in diameter of xylem and number of protoxylem strands from proximal to distal branchings. Variation, however, occurs within each order of branching. Primary xylem of vegetative branches is smallest at the proximal and distal ends and largest in the midregions. Size and form of primary xylem of living plant axes seem to relate to size (volume) and vigor (potential for growth) of their apical meristems. Thus the variation of primary xylem found in vegetative axes of these Aneurophytales may reflect epidogenetic, menetogenetic, and apoxogenetic episodes of apical growth of these organs. Fertile organs and vegetative ultimate appendages differ by having primary xylem strands that are largest proximally and that decline with each more distal division. This reflects the apoxogenetic growth of their apices and suggests that these organs are appendicular. Vegetative branches of all three plants seem to be at first indeterminate but eventually become determinate. Distal branches appear to have less potential for growth than proximal ones. Ultimate appendages and fertile organs differ by being fully determinate. The type of organ that a primordium becomes is rigidly fixed in Aneurophytales and, judging from the diameter of its trace, may relate to the size of the primordium itself rather than to the size of the parental axis. One may postulate that above a size threshold a primordium develops into a branch with indeterminate potential (expanding apical volume). Below that threshold a primordium is determinate (contracting apical volume) and develops into either a fertile organ or an ultimate appendage, depending on its position within the branching system. Determinate primordia on the last order of vegetative branches develop into the dichotomous ultimate appendages. Determinate primordia on other orders of vegetative branches develop into dichotomous fertile organs.