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Simply, the upper part of the leaf is exposed to the sunlight which contains vast palisade cells with huge accommodation of chloroplast in order to trap sunlight to perform photosynthesis where the lower side is nothing but spongy parenchyma with less number of chloroplast.
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The top surface of dicot leaf is darker than the lower part because the upper part is in front of sunlight, so it receive more sunlight than the lower part.
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