Culm anatomy of B. balcooa, showing “double broken-waist type” vascular bundles intermingled with those of “broken-waist type.” The fiber sheath is present only at the protoxylem side (A). F, discrete fiber bundle; M, metaxylem; Ph, phloem and Pr, protoxylem of a “double broken-waist type” vascular bundle are marked (B). (A color version of this figure is available at afs-journal.org.)