Plant systems A: Transport and support
Most large land plants belong to the Phylum Tracheophyta which have a system of internal tubes to transport water and nutrients around the plant . These tubes - called vascular bundles - also provide support for the plant, allowing the green leaves maximum exposure to sunlight. Xylem A medium-size tree (e.g. beech) can lose about 500 kg of water a day through transpiration. Desert plants reduce this loss by closing off the stomata in the heat of the day, having smaller leaves, or having the stomata embedded in pits, or displaying the leaves vertically to reduce the amount of sunlight falling directly on them. The xylem vessels become thickened and woody as the plant grows, thus providing support for the stem so that maximum display of leaves to sunlight is possible. The bundles of xylem tissue can easily be seen in a cut stick of celery
placed in coloured water. Phloem Alongside the xylem, in the vascular bundles, is the phloem tissue. The cylindrical-shaped cells of the phloem lie end-to-end forming long tubes transporting sugars and proteins. "Ringbarking" large trees involves cutting through the bark deep enough to damage the phloem tissue preventing food transport throughout the plant. The tree therefore slowly dies so it can no longer take up water which is then available for use by surrounding crops. The tree does, however, remain intact providing hollows and nesting sites for animals. Because water provides buoyancy, large marine kelp do not need vascular tissue (xylem and phloem) to support the plant or hold the photosynthesising parts up to the light. Water and minerals can pass directly into all parts of the plant as it is in direct contact with the surrounding water. Therefore complex transport and support systems are not necessary.
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