Herbaceous Dicot Vine: Cucurbita - Long Section: Cucurbita Stem Magnification: 40x Bicollateral Vascular Bundles Have Two Zones Of Phloem And Two Zones Of Cambium On Either Side Of The Central Xylem With Outer Facing Tissues More Heavily Developed. The Sequence Of Tissues In The Bicollateral Vascular Bundle Is Outer Phloem, Outer Cambium, Xylem, Inner Cambium And Inner Phloem With Outer More Heavily Developed. Walls Of Xylem Vessels Have Lignin Free Pits And Lignin Thickenings That Form Rings, Spirals, Networks And Solid Blocks. Phloem Consists Of Large Clear Sieve Tubes With Pitted Cell Walls And Sieve Plates And Small Narrow Green Stained Companion Cells. Many Sieve Plates Have Deposits Of A Mucoid P-protein That Forms As Part Of The Trauma Response In Injured Phloem. Between The Xylem And Phloem Are A Few Regularly Arranged Layers Of Elongated Cambium Cells. Photo