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A Reevaluation of the Zosterophyllophytina with Comments on the Origin of Lycopods
Karl J. Niklas and Harlan P. Banks
American Journal of Botany
Vol. 77, No. 2 (Feb., 1990), pp. 274-283
Published by: Botanical Society of America, Inc.
Stable URL: http://www.jstor.org/stable/2444648
Page Count: 10
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The primary paleobotanical literature pertaining to Zosterophyllophytina (a now extinct group of Devonian vascular plants) was reexamined for evidence concerning the activity of the apex of fertile shoots as it might be revealed by the presence or absence of terminally located sporangia (terminate or nonterminate axes, respectively). The symmetry of sporangial arrangement (radial or bilateral), the presence or absence of enations, circinate axial tips, and the shape of the vascular strand were also recorded. We found terminate axes usually are (but not invariably) associated with radial symmetry and nonterminate axes are typically bilateral in symmetry. Other morphological features are consistent with this observation, e.g., enations, circinate tips, and (when preserved) elliptic vascular strands are found in association with bilateral symmetry and nonterminate axes. We hypothesize that there are two distinct patterns of fertile axial growth within the Zosterophyllophytina. Nonetheless, all taxa currently assigned to the zosterophyllophytes share a reniform or globose sporangial shape and a distal line of dehiscence. Accordingly, we view Zosterophyllophytina as a monophyletic group of plants, whose members show two distinct patterns of growth in their fertile axes. We speculate that lycopods arose from an early zosterophyllophytelike group characterized by nonterminate, radially symmetrical fertile axes. We speculate that zosterophyllophytes with terminate fertile axes and those with nonterminate, bilaterally symmetrical fertile axes were phylogenetic deadends.
American Journal of Botany © 1990 Botanical Society of America, Inc.