Fig. 11.5-4. Transverse section of stem of rush (Juncus). This is an amphivasal vascular bundle: one in which the phloem is surrounded by xylem. The Xs mark just three of the many xylem elements, which are large enough here that we have no trouble distinguishing them from the fibers of the bundle sheath (in many species, it can be difficult to be certain if a cell is a wide sheath fiber or a narrow tracheary element). In the phloem, two of the many sieve tube members have been marked with “s.”
Although collateral bundles are most common in seed plants, a number of monocot species have amphivasal bundles instead, or have amphicribral ones (in which phloem surrounds xylem).