plant organ systems 2.3.1

• the xylem and phloem are arranged in vascular bundles in the stem
• in mature trees, the xylem are located in the woody part of the trunk, and the phloem are found in a thin ring just under the bark
• in the roots, the xylem are in a cross shape in the centre of the root
• in the roots the phloem are between the arms of the xylem
• in the stem the xylem are on the inside edge of the vascular bundle
• in the stem the phloem are on the outside edge of the vascular bundle
• in the leaves the xylem are on the upper side of the leaf vein
• in the leaves the phloem are on the lower side of the leaf vein
• insects like aphids use their mouthparts to feed on the dissolved sugars found in the phloem, this shows that phloem are located just under the surface of the stem, this is because phloem are found in the outer edge of a vascular bundle
• food dye is transported up the xylem and stains them, therefore making them visible, this shows that xylem are found in vascular bundles, as shown in the stained circular areas this is because having vascular bundles rather then a complete ring of xylem gives the stem more flexibility
• removing a complete ring of bark (called tree ringing) will kill a mature tree, this shows that phloem are located just under the surface of bark, wheras xylem are found in the woody part of the trunk, this is because cutting the phloem stops sugars from reaching the routes so they cant respire and the whole tree eventually dies
• young trees are vulnerable to damage from animals because they may scratch the bark and remove it, causing the tree to die
• translocation is the movement of sugars and amino acids from the leaves to the rest of the plant
• plants have two transport systems, xylem transports water and mineral ions from the roots to the stems and leaves. the phloem transports dissolved sugar from the leaves to the rest of the plant