The bamboos are evergreen perennial flowering plants in the subfamily Bambusoideae of the grass family Poaceae. In bamboo, as in other grasses, the internodal regions of the stem are usually hollow and the vascular bundles in the cross section are scattered throughout the stem instead of in a cylindrical arrangement. The dicotyledonous woody xylem is also absent. The absence of secondary growth wood causes the stems of monocots, including the palms and large bamboos, to be columnar rather than tapering.