Kantstraße 20, D-65719 Hofheim, Germany.
Institute of Organismic and Molecular Evolution, Johannes Gutenberg-University, Anselm-Franz-von-Bentzel-Weg 2, D-55099 Mainz, Germany.
Ann Bot. 2023 May 15;131(5):851-866. doi: 10.1093/aob/mcad049.
Water lilies are of particular interest with regard to the evolution of angiosperms. They live in an aquatic environment and have been regarded as links to the monocots by some authors. Vascular bundles are sometimes described as scattered or atactostelar as in monocots. However, this view needs to be clarified as the morphology and vascularization of Nymphaea rhizomes remain to be understood.
The rhizome of Nymphaea alba was re-investigated morphologically and histologically. Developmental studies were conducted using scanning electron microscopy. Comprehensive histological analyses, including hand and microtome sections and a variety of specific staining procedures, were conducted to re-evaluate the composition of longitudinal and transverse tissue.
The rhizome is covered by parenchymatous nodal cushions each bearing a leaf and several adventitious roots. Internodes are extremely short. The apex is flat and early overtopped by developing leaf primordia and cushions. The phyllotaxis is spiral and passes alternately through vegetative and reproductive phases. Flowers appear in the leaf spiral, and lack a subtending bract and a cushion below the peduncle. The reproductive phase includes two or three flowers which alternate with a single leaf. The rhizome is histologically subdivided into a central core, an aerenchymatic cortex, and a parenchymatic exocortex formed to a great extent by the nodal cushions. The core contains strands of vascular bundles united to a complex vascular plexus. Vascular elements continuously anastomose and change shape and direction. Provascular strands originating from leaf primordia merge with the outer core vascular tissue whereas the flower strands run into the centre of the core. Roots originating from the parenchymatous cushions show the characteristic actinostelic pattern, which changes into a collateral pattern inside the rhizome. Several root traces merge and form one strand leading to the central core. Early cell divisions below the apical meristem dislocate leaf, flower and root primordia and their provascular strands outwards. Consequently, fully developed vascular strands insert horizontally into the vascular plexus at advanced rhizome stages.
The absence of bracts and cushions below the flowers, the alternate leaf-flower sequence and the course of the peduncle strand suggest that the rhizome is sympodially instead of monopodially organized. The spiral phyllotaxis extends in this case over several shoot orders, masking the branching pattern. The vascular strands in the central plexus differ considerably from vascular bundles in monocots, confirming the unique vascularization in Nymphaea. Sclerenchymatic bundle sheaths are lacking, and vascular bundles continuously split and anastomose throughout the rhizome. Though vascular bundles in petioles and peduncles of N. alba show similarities with some Alismatales, the vascular system of N. alba in general has little in common with that of monocots.
睡莲花对于被子植物的进化特别重要。它们生活在水生环境中，一些作者认为它们是单子叶植物的纽带。维管束有时被描述为单子叶植物的散生或无中柱的。然而，这种观点需要澄清，因为睡莲根茎的形态和血管系统仍有待了解。
对白色睡莲的根茎进行了形态学和组织学的重新研究。使用扫描电子显微镜进行了发育研究。进行了综合的组织学分析，包括徒手和切片以及各种特定的染色程序，以重新评估纵向和横向组织的组成。
根茎被肉质的节间垫覆盖，每个节间垫上都有一片叶子和几个不定根。节间非常短。顶端是平的，早期被发育的叶原基和垫所覆盖。叶序是螺旋状的，交替经历营养和生殖阶段。花出现在叶螺旋中，没有托叶和花梗下面的垫。生殖阶段包括两到三朵花，它们与一朵单独的叶子交替出现。根茎在组织学上分为中央核心、通气皮层和大部分由节间垫形成的薄壁外皮层。核心包含维管束束，它们联合成一个复杂的血管丛。血管元素不断吻合，并改变形状和方向。来自叶原基的原导管束与外核心血管组织合并，而花束则进入核心中心。起源于薄壁垫的根显示出典型的中柱模式，在根茎内部变为侧生模式。几个根迹合并形成一条通向中央核心的束。顶端分生组织下的早期细胞分裂将叶、花和根原基及其原导管束向外移位。因此，在根茎的后期阶段，完全发育的维管束束水平插入血管丛中。
花下无托叶和垫，叶-花交替出现，花梗束的走向表明，根茎是合轴而不是单轴组织的。螺旋状的叶序在这种情况下延伸到几个枝序，掩盖了分枝模式。中央丛中的维管束与单子叶植物的维管束有很大的不同，这证实了睡莲属的独特血管系统。缺乏厚壁束鞘，维管束在整个根茎中不断分裂和吻合。尽管白色睡莲叶柄和花梗中的维管束与某些泽泻目有相似之处，但白色睡莲的总体血管系统与单子叶植物的血管系统几乎没有共同之处。
