Cambridge O Level Biology · Syllabus 5090 · Transport in Flowering Plants
Vascular Bundle
What is Vascular Bundle?
A strand of transport tissue in a plant containing both xylem and phloem. In a transverse section of a young non-woody dicotyledonous stem the vascular bundles are arranged in a ring near the outside, with the xylem toward the centre of the stem and the phloem toward the outside of each bundle, and the cortex lies outside the ring. In a young dicotyledonous root the vascular tissue lies in the centre, with the xylem often forming a star shape and the phloem lying between the arms of the star, surrounded by the cortex.
This definition is part of the Transport in Flowering Plants chapter in Cambridge O Level Biology.
Vascular Bundle in context
A flowering plant runs two separate transport systems in the same vascular bundles. Xylem carries water and dissolved mineral ions in one direction only, from the roots up to the leaves, through dead hollow vessels with lignified walls. Phloem is living tissue that carries sucrose and amino acids from a source that makes or releases them to a sink that uses or stores them, so its direction depends on where the source and the sink happen to be. The engine of the xylem stream is not a pump: water evaporates from moist mesophyll surfaces inside the leaf and diffuses out through the stomata, and the tension this creates — the transpiration pull — draws a continuous column of water up the plant, held together by the attraction between water molecules.
Questions students ask about Vascular Bundle
Where are xylem and phloem in a stem and in a root?
In a non-woody dicotyledonous stem the vascular bundles form a ring near the outside, with the xylem on the inner side of each bundle and the phloem on the outer side. In a young dicotyledonous root the xylem is central and often star-shaped, with the phloem lying between the arms of the star; the cortex surrounds the vascular region in both organs.