Rhizome architecture, development and vascularization in the water lily Nymphaea alba

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.