Koi Satoshi, Kato Masahiro
Department of Botany, National Science Museum, 4-1-1 Amakubo, Tsukuba, Ibaraki 305-0005, Japan.
Ann Bot. 2007 Jun;99(6):1121-30. doi: 10.1093/aob/mcm065. Epub 2007 Apr 27.
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.
The shoot development of Weddellina squamulosa, the sole species of Weddellinoideae, was investigated using scanning electron microscopy and semi-thin serial sections.
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.
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.
在被子植物中，茎尖分生组织产生由茎、叶和腋芽组成的地上部系统。川苔草科三个亚科之一的川苔草亚科，具有独特的“地上部”，其缺乏茎尖分生组织，仅由叶组成。三肋川苔草亚科被认为具有茎尖分生组织，尽管其分枝模式尚不确定。对川苔草亚科地上部发育模式的研究尚未聚焦于分生组织。川苔草亚科在系统发育上处于揭示川苔草科地上部进化过程的关键位置。
利用扫描电子显微镜和半薄连续切片，对川苔草亚科唯一的物种鳞状川苔草的地上部发育进行了研究。
鳞状川苔草的地上部具有由原套-原体组织的顶端分生组织。它是有限的，并且在紧邻的较老枝条基部腋外连续地起始一个新枝，从而形成合轴的、近似平面的分枝模式。大型鳞片叶如在被子植物中常见的那样，在顶端分生组织的侧面由顶向基起始，而散布在茎上的小型鳞片叶则与节间伸长相关联地由基向顶起始。
川苔草亚科与三肋川苔草亚科一样，具有茎尖分生组织，由此提出川苔草亚科中分生组织消失的假说。川苔草亚科中叶片形成模式和川苔草亚科中地上部分枝模式相似，即这些器官在较老器官的基部产生。这种相似性导致了另一个假说，即川苔草亚科（可能还有三肋川苔草亚科）中的“枝”与川苔草亚科中的“叶”具有可比性，并且茎尖分生组织在川苔草科的早期进化中消失了。
