IGCSE Biology / Transport in plants
Where the tissues sit in the plant
Edexcel International GCSE Biology (4BI1), chapter 11: Transport in plants, specification point 2.53, 2.54. Free lesson.
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Xylem and phloem run through roots, stems and leaves, but sit in different positions in each. Their arrangement helps the plant resist forces and puts the tissues close to the cells they serve.
Where xylem and phloem sit
The position of xylem changes between a root, stem and leaf. In each, phloem is on the outer side of the xylem and close to the cells that need sugars or produce them. In a root, xylem is central and star-shaped. Phloem lies between the arms of the star. In a stem, vascular bundles form a ring near the outside. Within each bundle, xylem is on the inside and phloem is on the outside. In a leaf, the vein has xylem on its upper side and phloem on its lower side.
Why root xylem is central
A root is anchored in the soil and can be pulled lengthways as the stem sways. This force is called tension. The central core of lignified xylem helps the root resist this pulling force. The xylem forms a star in the centre of the root. The central position helps the root withstand tension along its length.
Why stem bundles form a ring
Wind can bend a stem sideways. Material further from the centre resists bending better than material close to the centre, so the bundles are arranged in a ring near the stem's outside. Each bundle has xylem on the inside and phloem on the outside. A ring near the outside helps the stem resist bending.
Why the leaf vein has xylem above phloem
Xylem is on the upper side of a leaf vein, near the palisade cells at the top of the leaf. These cells carry out photosynthesis fastest, so water is delivered close to where it is used. Phloem below is close to the cells that need sugars or produce them. Leaf xylem is uppermost in the vein. Leaf phloem is on the lower side of the vein.
Explain arrangements by the force they resist
To explain the position of xylem, connect the arrangement to the force the organ must withstand. Saying that a position is simply stronger does not explain why it helps. Root: central xylem resists tension along the root. Stem: bundles near the outside resist bending. Leaf: upper xylem delivers water near the palisade cells.