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Persistent Juveniles Among the Cycads
M. A. Chrysler
Vol. 98, No. 4 (Jun., 1937), pp. 696-710
Published by: The University of Chicago Press
Stable URL: http://www.jstor.org/stable/2471656
Page Count: 15
You can always find the topics here!Topics: Tracheids, Xylem, Genera, Tree trunks, Juveniles, Plants, Recapitulation theory, Petioles, Plant roots, Parenchyma
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1. An explanation is offered for the apparently anomalous composition of the xylem of the stem in Zamia and Stangeria. 2. It is shown that although the stem wood of tuberous species of Zamia does not pass beyond the scalariform stage, certain trunk-producing species from Central America show abundance of tracheids with circular bordered pits. 3. The presence of circular pits in the tracheids of the leaf rachis is hence explained; otherwise the occurrence of pits would constitute an exception to the doctrine of conservative organs. 4. The tuberous species of Zamia are regarded as persistent juveniles with respect to their growth habit and their xylem; that is, they remain immature vegetatively although they reproduce freely. 5. A comparison is made between these juvenile cycads and such persistent juveniles as the Retinispora species and other conifers. 6. A similar explanation is considered to apply to Stangeria. 7. All organs of the cycad plant are surveyed in order to test the validity of the explanation, and the differences shown by the xylem in stem, cone axis, root, and leaf are interpreted in terms of relative completeness and rapidity of the recapitulation which the different organs exhibit in the various genera. The stem exhibits a complete although gradual recapitulation, incomplete, however, in the tuberous species of Zamia; the reproductive axis has a much more condensed series, generally incomplete; the root quickly passes through the early phylogenetic stages; finally the leaf shows in its centripetal xylem a much abbreviated recapitulation, which tends to be incomplete in the tuberous members of the group.
Botanical Gazette © 1937 The University of Chicago Press