Veins - mesophyll cells are in direct contact withor at...

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Veins (vascular tissue) Veins penetrate all parts of the leaf, forming a network that connects the leaf through the petiole to  the vasculature of the stem and thereby to the root as well.  Primary xylem  cells occupy the upper  part of the vein and  phloem  cells the lower. The vascular tissues are surrounded by a  bundle  sheath  one or two layers thick, composed of fibers in the smaller veins and parenchyma in the  larger.  Fibers and collenchyma are present in and around the veins and give strength to them and to the  leaf as a whole. Bundle sheath extensions connect the bundle sheaths to either or both epidermises  giving added stability to the blade. The large veins branch repeatedly becoming smaller each time  they divide until they ultimately end with only one or two tracheids at the vein ending. Here the 
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Unformatted text preview: mesophyll cells are in direct contact withor at most one or two cells away fromthe raw materials carried in the xylem and used for photosynthesis. The phloem is equally convenient for export of photosynthetates. The bundle sheaths insulate the conducting cells and ensure the retention of materials in the pipeline. The veins of tropical grasses and other plants with C4 photosynthesis are surrounded by two cylinders, the inner of thick-walled bundle sheath cells, the outer of thin-walled mesophyll cells. C4 plants are said to have a Kranz (from the German word for wreath) anatomy because of these. In addition, no distinct palisade or spongy mesophyll zones are present in the C4 leaves....
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