AOBPreview originally published online on April 18, 2005
Annals of Botany 2005 96(1):51-58; doi:10.1093/aob/mci148
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Leaf Development in the Absence of a Shoot Apical Meristem in Zeylanidium subulatum (Podostemaceae)
1 Department of Chemical and Biological Sciences, Faculty of Science, Japan Women's University, 8-1, Mejirodai 2-chome, Tokyo 112-8681, Japan and 2 Department of Biological Sciences, Graduate School of Science, University of Tokyo, 3-1, Hongo 7-chome, Tokyo 113-0033, Japan
* For correspondence. E-mail ryoko{at}fc.jwu.ac.jp
Received: 24 November 2004 Returned for revision: 12 January 2005 Accepted: 2 March 2005 Published electronically: 18 April 2005
• Background and Aims The Podostemaceae are a family of unusual aquatic angiosperms that live in rapids and waterfalls. To adapt to such extreme habitats, the family shows unusual morphologies. This study investigated the developmental anatomy of the shoot of Zeylanidium subulatum borne on the prostrate root attached to submerged rock surfaces.
• Methods Shoots of Z. subulatum were observed under the microscope using resin-sections.
• Key Results The shoot has no shoot apical meristem (SAM) and, without it, forms leaves distichously dorsiventrally facing the immediately older leaf. A new leaf forms on the adaxial side of a pre-existing leaf and also on the abaxial side of a leaf on flowering shoots. In both cases, the young leaf is endogenous below the older leaf and maintains histological continuity with it. Shortly after internal initiation, the leaf primordia become separate from each other due to cleavage between adjacent leaves of opposite ranks. The cleavage is caused by intercellular separation as well as by degeneration of vacuolated cells. Loss of the SAM is probably linked with the speculated shift of the site of leaf formation to the root.
• Conclusions The ‘shoot’ of Z. subulatum is characterized by the absence of a SAM, endogenous leaf formation in the absence of a SAM, cleavage between leaf primordia, and adventitious leaf formations. These innovations occur in some Podostemaceae that have become increasingly adapted to extreme aquatic habitats.
Key words: Adventitious leaf, developmental anatomy, dorsiventrality, endogenous leaf, Podostemaceae, shoot apical meristem (SAM), Zeylanidium subulatum
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