in plants, a vascular bundle with adjoining mechanical tissue that arises as a result of the differentiation of procambial cells. In dicotyledons, mechanical tissue is usually represented by lignified fibers that are derived from the primary phloem, as seen in the stems of sunflowers, clovers, and buttercups. In monocotyledons, sclerenchymatous fibers often surround the entire bundle, forming the mestome sheath, or facing of the bundle; this type of bundle occurs in the stems of corn and the leaves of cane and rough bluejoint reed grass.