a bundle of conducting tissues of xylem and phloem in the stems and leaves of plants. In stems the exterior side of the collateral bundle consists of phloem, and the interior of xylem. Collateral bundles may be closed or open. Closed collateral bundles lack secondary growth, because they contain no cambium. Closed collateral bundles are present in almost all monocotyledonous plants. In open collateral bundles, which are characteristic of the stems of dicotyledonous, gymnospermous, and some pteridoid plants, there is a layer of cambium cells between the phloem and the xylem. As a result of cell division in this layer, new elements of the phloem and xylem are formed (secondary thickening of the stem).