Nymphoides stem

This cross section, cut through the basal portion of the petiole, shows the central vascular system in some detail. Note that there are two mirrored vascular bundles (only part of the lower bundle is shown in this micrograph). The Phloem (P) and xylem are primary, that is, it consists of metaxylem (MX) and protoxylem (PX). The bundle is ensheathed by a starch sheath - this is the layer of cells containing small starch granules immediately  exarch (outside of) the phloem.

Notice the complete absence of any mechanical supporting tissue.