Vascular bundle having pholem at the centre encircled by xylem is known as
When phloem is surrounded by xylem on all sides, such a concentric vascular bundle is called amphivasal or leptocentric. Such vascular bundles are found in Dracaena, Yucca, Aloe, etc. Amphicribal bundle is the exact opposite of an amphivasal bundle. In an amphicribal bundle, the xylem lies at the centre and is surrounded by the phloem on all sides.
When a vascular bundle has xylem at the centre and phloem on both the inner and the outer sides with a strip of cambium separating each from the xylem, the bundle is called bicollateral.
In a conjoint collateral bundle, the xylem is on the inner side and the phloem is on the outer side separated from the xylem by cambium.