The collateral vascular bundle lacking a cambium between xylem and phloem is called closed type, eg, monocot stem. The collateral (phloem outside the xylem) vascular bundle having a cambium between xylem and phloem is said to be open type, eg, dicot stem. A vascular bundle having both xylem and phloem together is called conjoint. The stele with central pith is called siphonostele or medullated stele. Pith or medulla is mostly made up of parenchymatous cells.