AOBPreview published online on October 24, 2002
Annals of Botany, doi:10.1093/aob/mcf259
© 2002 by Annals of Botany Company
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Submitted on June 25, 2002
Affiliation of the authors:
1 Department of Chemical and Biological Sciences, Faculty of Science, Japan Women's University, 2-8-1 Mejirodai, Tokyo 112-8681, Japan;
2 Department of Biological Sciences, Graduate School of Science, University of Tokyo, 7-3-1 Hongo, Tokyo 113-0033, Japan
* To whom correspondence should be addressed.
Podostemaceae have markedly specialized and diverse roots that are adapted to extreme habitats, such as seasonally submerged or exposed rocks in waterfalls and rapids. This paper describes the developmental anatomy of roots of four species of Zeylanidium, with emphasis on the unusual association between root branching and root-borne adventitious shoots. In Z. subulatum and Z. lichenoides with subcylindrical or ribbon-like roots, the apical meristem distal (exterior) to a shoot that is initiated within the meristem area reduces and loses meristematic activity. This results in a splitting into two meristems that separate the parental root and lateral root (anisotomous dichotomy). In Z. olivaceum with lobed foliose roots, shoots are initiated in the innermost zone of the marginal meristem, and similar, but delayed, meristem reduction usually occurs, producing a parenchyma exterior to shoots located between root lobes. In some extreme cases, due to meristem recovery, root lobing does not occur, so the margin is entire. In Z. maheshwarii with foliose roots, shoots are initiated proximal to the marginal meristem and there is no shoot-root lobe association. Results suggest that during evolution from subcylindrical or ribbon-like roots to foliose roots, reduction of meristem exterior to a shoot was delayed and then arrested as a result of inward shifting of the sites of shoot initiation. The evolutionary reappearance of a protective tissue or root cap in Z. olivaceum and Z. maheshwarii in the Zeylanidium clade is implied, taking into account the reported molecular phylogeny and root-cap development in Hydrobryum.
Revised on August 21, 2002
Accepted on September 16, 2002
Developmental Anatomy and Branching of Roots of Four Zeylanidium Species (Podostemaceae), with Implications for Evolution of Foliose Roots
Y. HIYAMA1,
Key words: Branching, developmental anatomy, exogenous vs. endogenous origin of root, meristem, Podostemaceae, root, root cap, shoot, Zeylanidium.
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