The xylem is normally at the inside, the phloem at the outside. In dicots are both components separated by the fascicular cambium . It is also spoken of an open vascular bundle in contrast to the 'closed' bundles of monocots that lack the cambium. Xylem and phloem are surrounded by a bundle sheath of parenchyma that is often starch-containing. Although the vascular bundles seem to be scattered in cross-section, do they occur at the periphery in larger numbers.
Central bundles are usually bigger than peripheral ones.
Many monocots have hollow stems. It seems as if their vascular bundles were arranged in a ring, because space is rare. This arrangement is of secondary nature, since it developed as an adjustment to aquatic life. A comparable structure can be found in some submerse dicots. It decreases the strength of the stem, but increases its flexibility.