Extraxylary fibers as a continuous peripheral ring not related to phloem

These so-called pericyclic fibers constitute a ring-like layer near the periphery of the stem (seen at the top) in the cross-section of a stem internode of squash (Cucurbita sp.), and provide support for the stem. Rings of fibers are often found along the outer periphery of vascular cylinders, but do not originate in the phloem. In such cases, they are designated as perivascular fibers. Often, the two terms (pericyclic and perivascular) are used interchangeably.

Due to their location, these fibers can be designated as extraxylary. Outside of the ring of pericyclic fibers are two-to-three layered cortical parenchyma and collenchyma in a thicker layer that is not stained as dark as the fiber ring. The vascular bundle is embedded in ground tissue.

C = collenchyma; F = fibers; P = phloem