Mesophyll - does the spongy mesophyll The mesophyll...

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Mesophyll The mesophyll tissue forms the bulk of most leaves and the chloroplasts in its cells are the principal  sites of photosynthesis. The mesophyll is sandwiched between the epidermal layers. In leaves held  horizontally on stems and in which there is a discernable top and bottom, the upper and lower  mesophyll cells have different shapes whereas in leaves held vertically, the mesophyll is uniformly  the same throughout. If the mesophyll is differentiated, the upper layer is called the  palisade mesophyll  and consists of  closely packed columnar cells with their long axis at right angles to the leaf surface. The lower  tissue, called  spongy mesophyll , is made of irregularly shaped cells, loosely arranged with much  intercellular space. While both mesophyll types contain chloroplasts, the palisade has more than 
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Unformatted text preview: does the spongy mesophyll. The mesophyll, therefore, is a type of chlorenchyma—chloroplast-containing parenchyma. The spongy mesophyll with its air spaces is, additionally, an aerenchyma. The wet surfaces of the mesophyll cells are the sites of water loss and gas exchange; the stomata are merely the gates through which the water and gases pass to the outside. The mesophyll contains strengthening tissues, primarily around the veins, but also in scattered batches throughout the mesophyll. Sclereids are especially common and almost always collenchyma cells are used to strengthen veins. Fibers are common in the leaves of monocots....
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