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AOBPreview originally published online on April 27, 2007
Annals of Botany 2007 99(6):1121-1130; doi:10.1093/aob/mcm065
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© The Author 2007. Published by Oxford University Press on behalf of the Annals of Botany Company. All rights reserved. For Permissions, please email: journals.permissions@oxfordjournals.org

INVITED REVIEW

Developmental Morphology of the Shoot in Weddellina squamulosa and Implications for Shoot Evolution in the Podostemaceae

Satoshi Koi* and Masahiro Kato

Department of Botany, National Science Museum, 4-1-1 Amakubo, Tsukuba, Ibaraki 305-0005, Japan

* For correspondence. E-mail skoi{at}kahaku.go.jp

Received: 25 November 2006    Returned for revision: 20 December 2006    Accepted: 16 February 2007    Published electronically: 27 April 2007

Background and Aims: In angiosperms, the shoot apical meristem produces a shoot system composed of stems, leaves and axillary buds. Podostemoideae, one of three subfamilies of the river-weed family Podostemaceae, have a unique ‘shoot’ that lacks a shoot apical meristem and is composed only of leaves. Tristichoideae have been interpreted to have a shoot apical meristem, although its branching pattern is uncertain. The shoot developmental pattern in Weddellinoideae has not been investigated with a focus on the meristem. Weddellinoideae are in a phylogenetically key position to reveal the process of shoot evolution in Podostemaceae.

Methods: The shoot development of Weddellina squamulosa, the sole species of Weddellinoideae, was investigated using scanning electron microscopy and semi-thin serial sections.

Key Results: The shoot of W. squamulosa has a tunica–corpus-organized apical meristem. It is determinate and successively initiates a new branch extra-axillarily at the base of an immediately older branch, resulting in a sympodial, approximately plane branching pattern. Large scaly leaves initiate acropetally on the flanks of the apical meristem, as is usual in angiosperms, whereas small scaly leaves scattered on the stem initiate basipetally in association with the elongation of internodes.

Conclusions: Weddellinoideae, like Tristichoideae, have a shoot apical meristem, leading to the hypothesis that the meristem was lost in Podostemoideae. The patterns of leaf formation in Podostemoideae and shoot branching in Weddellinoideae are similar in that these organs arise at the bases of older organs. This similarity leads to another hypothesis that the ‘branch’ in Weddellinoideae (and possibly Tristichoideae) and the ‘leaf’ in Podostemoideae are comparable, and that the shoot apical meristem disappeared in the early evolution of Podostemaceae.

Key words: Anatomy, development, evolution, Podostemaceae, shoot, shoot apical meristem, Weddellina squamulosa


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