Submerged Dicot Stem: Xylem Elements in Nymphaea

cross section: Nymphaea stem

common name: Water Lily

magnification: 40x

Berkshire Community College Bioscience Image Library

The cells of the epidermis are single-layered, thin-walled and mucilaginous. There is no apparent cuticle.

The outer cortex consists of a narrow zone of 5 – 6 layers of parenchyma cells. The bulk of the cortex is occupied by network of thin walled aerenchyma cells. Chloroplasts may form throughout the cortex in stems exposed to sufficient light. The numerous air spaces of the aerenchymatous cortex store respiratory gases and provide bouncy. The only mechanical support within the cortex is provided by branched or stellate sclereids.

There is no well-defined stele, rather several smaller poly steles composed of two or more confluent vascular bundles loosely linked in a ring in the mid cortex. Single vascular bundles are distributed to the inside and outside of the zone of polysteles. All vascular bundles are closed and collateral with proto xylem in polysteles facing towards each other, followed by zones of metaxylem, and most externally, caps of phloem. A well-defined starch containing endodermis with casparian strip is evident in the poly stele systems.

Vascular tissues are often poorly differentiated into xylem and phloem. Xylem is entirely composed of spiral and annular tracheids of unusual length. In many vascular bundles a xylem lacuna can be seen. Phloem bundles are primarily phloem parenchyma and sieve tubes.