Question

why is upper surface of dicot leaf much darker than the lower surface

Answer 1

because the upper part will recive more sunlight

Answer 2

simply , the upper part of the leaf is exposed to the sunlight which contain vast palisadecells 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