Developmental Anatomy and Branching of Roots of Four Zeylanidium Species (Podostemaceae), with Implications for Evolution of Foliose Roots
R. IMAICHI*,1 and
1 Department of Chemical and Biological Sciences,
Faculty of Science, Japan Women’s University, 2-8-1 Mejirodai, Tokyo
112-8681, Japan and 2 Department of Biological Sciences, Graduate School of Science, University of Tokyo, 7-3-1 Hongo, Tokyo 113-0033, Japan
Annals of Botany 90: 735-744, 2002.
Podostemaceae have markedly specialized and diverse roots thatare adapted to extreme habitats, such as seasonally submergedor exposed rocks in waterfalls and rapids. This paper describesthe developmental anatomy of roots of four species of Zeylanidium,with emphasis on the unusual association between root branchingand root-borne adventitious shoots. In Z. subulatum and Z. lichenoideswith subcylindrical or ribbon-like roots, the apical meristemdistal (exterior) to a shoot that is initiated within the meristemarea reduces and loses meristematic activity. This results ina splitting into two meristems that separate the parental rootand lateral root (anisotomous dichotomy). In Z. olivaceum withlobed foliose roots, shoots are initiated in the innermost zoneof the marginal meristem, and similar, but delayed, meristemreduction usually occurs, producing a parenchyma exterior toshoots located between root lobes. In some extreme cases, dueto meristem recovery, root lobing does not occur, so the marginis entire. In Z. maheshwarii with foliose roots, shoots areinitiated proximal to the marginal meristem and there is noshoot–root lobe association. Results suggest that duringevolution from subcylindrical or ribbon-like roots to folioseroots, reduction of meristem exterior to a shoot was delayedand then arrested as a result of inward shifting of the sitesof shoot initiation. The evolutionary reappearance of a protectivetissue or root cap in Z. olivaceum and Z. maheshwarii in theZeylanidium clade is implied, taking into account the reportedmolecular phylogeny and root-cap development in Hydrobryum.
Key words: Branching, developmental anatomy, exogenous vs. endogenous origin of root, meristem, Podostemaceae, root, root cap, shoot, Zeylanidium.