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Unit 1: Section 6: Transport in plants
Xylem and Phloem
Be able to explain the need for transport systems in multicellular plants in terms
of size and surface area: volume ratio.
Be able to describe, with the aid of diagrams and photographs, the distribution of
xylem and phloem tissue in roots, stems and leaves of dicotyledonous plants.
Be able to describe, with the aid of diagrams and photographs, the structure and
function of xylem vessels, sieve tube elements and companion cells.
Why do plants need transport systems?
Plants are multicellular so they have a small surface area: volume ratio. Plants could
exchange substances by direct diffusion (from the outer surface to the cells), but that
would be too slow. So plants need transport systems to move substances to and from
individual cells quickly.
In plants there are two types of vascular tissue:
Xylem carries water from the roots, up the plant to the aerial parts.
Phloem carries sugars produced by leaves to other parts of the plant.
The two tissues occur together throughout the plant, sometimes with associated
tissues, such as sclerenchyma (plant tissue whose cells have become rigid due to the
presence of cell walls thickened with lignin. The cells are dead and function to provide
support), to form discrete areas, known as vascular bundles.
Distribution of vascular tissues in a leaf
The vascular tissues in a dicotyledonous leaf (dicotyledonous plants features: two seed
leaves, broad leaves, flower parts in rings of four or five, and vascular tissue arranged in a
ring in stems) form a network of tiny vascular bundles throughout the blade, or lamina,
of the leaf. These tiny bundles fuse to give a series of side veins that run parallel with
one another. These side veins then merge into the main vein. The main vein's diameter
increases towards the petiole, or leaf stalk. Within each vein there is an area of xylem
towards the upper surface of the leaf and an area of phloem towards the lower surface.
Distribution of vascular tissue in a stem
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The xylem and phloem are arranged towards the outside of the stem. This is because the
vascular bundles, along with associated sclerenchyma, not only transport material but
also provide support in herbaceous stems. Being discontinuous, this ring of supporting
tissues allows the stem to be flexible and to bend in the wind. Within the vascular
bundles, the xylem is to the inside of the stem and the phloem towards the outside.…read more